· The
In a healthy animal, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. However, the surface tissues, i.e., skin and mucous membranes, are constantly in contact with environmental organisms and become readily colonized by various microbial species. The mixture of organisms regularly found at any anatomical site is referred to as the normal flora("Bacteriology": Chapter 6. Normal Flora. University of Texas Medical Branch at Galveston,1996.),except by researchers in the field who prefer the term "indigenous microbiota"(Todar,2007). The normal flora of humans consists of a few eukaryotic fungi,archeaens and protists, but bacteria are the most numerous and obvious microbial components of the normal flora. Normal flora are extremely abundant in terms of absolute numbers. A normal human has approximately 1013 body cells and 1014 individual normal flora.However, microorganisms also tend to be very small, bacteria especially are much smaller.The normal flora are not expected to cause diseases in a normal host.Transient microbiota are the normal flora that are not always present or are present for only a few days,weeks or months,before disappearing
- Associations Between Humans and the
Not much is known about the nature of the associations between humans and their normal flora, but they are thought to be dynamic interactions. Both host and bacteria are thought to derive benefit from each other, and the associations are, for the most part, mutualistic. The normal flora derive from their host a steady supply of nutrients, a stable environment, and protection and transport. The host obtains from the normal flora certain nutritional and digestive benefits, stimulation of the development and activity of immune system, and protection against colonization and infection by pathogenic microbes.
While most of the activities of the normal flora benefit their host, some of the normal flora are parasitic (live at the expense of their host), and some are pathogenic (capable of producing disease). Diseases that are produced by the normal flora in their host may be called endogenous diseases. Most endogenous bacterial diseases are opportunistic infections, meaning that the the organism must be given a special opportunity of weakness or let-down in the host defenses in order to infect. An example of an opportunistic infection is chronic bronchitis in smokers wherein normal flora bacteria are able to invade the weakened lung. Sometimes the relationship between a member of the normal flora and its host cannot be deciphered. Such a relationship where there is no apparent benefit or harm to either organism during their association is referred to as a commensal relationship. Many of the normal flora that are not predominant in their habitat, even though always present in low numbers, are thought of as commensals. However, if a presumed commensal relationship is studied in detail, parasitic or mutualistic characteristics often emerge.
MICROORGANISMS THAT MAKE UP THE
The human flora consist of:
Fungi
Archaeans
Bacteeria
· Fungal flora
Fungi, particularly yeasts are present in the human gut. The best studied of these are Candida species, due to their ability to become pathogenic in immunocompromised hosts (Bernhardt H, Knoke M,1997). Yeasts are also present on the skin, particularly Malassezia species, where they consume oils secreted from the sebaceous glands.
- Archaean flora
Archaea are present in the human gut, but in contrast to the enormous variety of bacteria in this organ, the number of archaeal species are much more limited. The dominant group is the methanogens, particularly Methanobrevibacter smithii and Methanosphaera stadtmanae (Duncan SH, Louis P, Flint HJ,2007). However, colonization by methanogens is variable and only about 50% of humans have easily-detectable populations or these organisms (Florin TH, Zhu G, Kirk KM, Martin NG,2000)
· Bacterial flora
It is estimated that 500 to 100,000 species of bacteria live in the human body. Bacterial cells are much smaller than human cells, and there are about ten times as many bacteria as human cells in the body (1000 trillion or 1 quadrillion (1015) versus 100 trillion (1014)) (Sears CL,2005). Though normal flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose, small intestine, and colon), the vast majority of bacteria live in the large intestine.
Table 1. Bacteria commonly found on the surfaces of the human body.
| BACTERIUM | Skin | Conjunctiva | Nose | Pharynx | Mouth | Lower Intestine | Anterior urethra | Vagina |
| Staphylococcus epidermidis (1) | ++ | + | ++ | ++ | ++ | + | ++ | ++ |
| Staphylococcus aureus* (2) | + | +/- | + | + | + | ++ | +/- | + |
| Streptococcus mitis | | | | + | ++ | +/- | + | + |
| Streptococcus salivarius | | | | ++ | ++ | | | |
| Streptococcus mutans* (3) | | | | + | ++ | | | |
| Enterococcus faecalis* (4) | | | | +/- | + | ++ | + | + |
| Streptococcus pneumoniae* (5) | | +/- | +/- | + | + | | | +/- |
| Streptococcus pyogenes* (6) | +/- | +/- | | + | + | +/- | | +/- |
| Neisseria sp. (7) | | + | + | ++ | + | | + | + |
| Neisseria meningitidis* (8) | | | + | ++ | + | | | + |
| Enterobacteriaceae* (Escherichia coli) (9) | | +/- | +/- | +/- | + | ++ | + | + |
| Proteus sp. | | +/- | + | + | + | + | + | + |
| Pseudomonas aeruginosa* (10) | | | | +/- | +/- | + | +/- | |
| Haemophilus influenzae* (11) | | +/- | + | + | + | | | |
| Bacteroides sp.* | | | | | | ++ | + | +/- |
| Bifidobacterium bifidum (12) | | | | | | ++ | | |
| Lactobacillus sp. (13) | | | | + | ++ | ++ | | ++ |
| Clostridium sp.* (14) | | | | | +/- | ++ | | |
| Clostridium tetani (15) | | | | | | +/- | | |
| Corynebacteria (16) | ++ | + | ++ | + | + | + | + | + |
| Mycobacteria | + | | +/- | +/- | | + | + | |
| Actinomycetes | | | | + | + | | | |
| Spirochetes | | | | + | ++ | ++ | | |
| Mycoplasmas | | | | + | + | + | +/- | + |
++ = nearly 100 percent + = common (about 25 percent) +/- = rare (less than 5%) * = potential pathogen
Tissue specificity
Most members of the normal bacterial flora prefer to colonize certain tissues and not others. This "tissue specificity" is usually due to properties of both the host and the bacterium. Usually, specific bacteria colonize specific tissues by one or another of these mechanisms.
1. Tissue tropism is the bacterial preference or predilection for certain tissues for growth. One explanation for tissue tropism is that the host provides essential nutrients and growth factors for the bacterium, in addition to suitable oxygen, pH, and temperature for growth.
2. Specific adherence Most bacteria can colonize a specific tissue or site because they can adhere to that tissue or site in a specific manner that involves complementary chemical interactions between the two surfaces. Specific adherence involves biochemical interactions between bacterial surface components (ligands or adhesins) and host cell molecular receptors. The bacterial components that provide adhesins are molecular parts of their capsules, fimbriae, or cell walls. The receptors on human cells or tissues are usually glycoprotein molecules located on the host cell or tissue surface.
Figure 1. Specific adherence involves complementary chemical interactions between the host cell or tissue surface and the bacterial surface. In the language of medical microbiologist, a bacterial "adhesin" attaches covalently to a host "receptor" so that the bacterium "docks" itself on the host surface. The adhesins of bacterial cells are chemical components of capsules, cell walls, pili or fimbriae. The host receptors are usually glycoproteins located on the cell membrane or tissue surface.
Some examples of adhesins and attachment sites used for specific adherence to human tissues are described in the table below.
Table 2. Examples of bacterial specific adherence to host cells or tissue.
| Bacterium | Bacterial adhesion | Attachment site | |
| Streptococcus pyogenes | Cell-bound protein (M-protein) | Pharyngeal epithelium | |
| Streptococcus mutans | Cell- bound protein (Glycosyl transferase) | Pellicle of tooth | |
| Streptococcus salivarius | Lipoteichoic acid | Buccal epithelium of tongue | |
| Streptococcus pneumonia | Cell-bound protein (choline-binding protein) | Mucosal epithelium | |
| Staphylococcus aureus | Cell-bound protein | Mucosal epithelium | |
| Neisseria gonorrhoeae | N-methylphenyl- alanine pili | Urethral/cervical epithelium | |
| Enterotoxigenic E. coli | Type-1 fimbriae | Intestinal epithelium | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
| | | | |
3.Biofilm formation
Some of the indigenous bacteria are able to construct biofilms on a tissue surface, or they are able to colonize a biofilm built by another bacterial species. Many biofilms are a mixture of microbes, although one member is responsible for maintaining the biofilm and may predominate.
The classic biofilm that involves components of the normal flora of the oral cavity is the formation of dental plaque on the teeth. Plaque is a naturally-constructed biofilm, in which the consortia of bacteria may reach a thickness of 300-500 cells on the surfaces of the teeth. These accumulations subject the teeth and gingival tissues to high concentrations of bacterial metabolites, which result in dental disease.
The Composition of the Normal Flora
The normal flora of humans are exceedingly complex and consist of more than 200 species of bacteria. The makeup of the normal flora may be influenced by various factors, including genetics, age, sex, stress, nutrition and diet of the individual.
Three developmental changes in humans, weaning, the eruption of the teeth, and the onset and cessation of ovarian functions, invariably affect the composition of the normal flora in the intestinal tract, the oral cavity, and the vagina, respectively. However, within the limits of these fluctuations, the bacterial flora of humans is sufficiently constant to a give general description of the situation.
A human first becomes colonized by a normal flora at the moment of birth and passage through the birth canal. In utero, the fetus is sterile, but when the mother's water breaks and the birth process begins, so does colonization of the body surfaces. Handling and feeding of the infant after birth leads to establishment of a stable normal flora on the skin, oral cavity and intestinal tract in about 48 hours.
It has been calculated that a human adult houses about 1012 bacteria on the skin, 1010 in the mouth, and 1014 in the gastrointestinal tract. The latter number is far in excess of the number of eucaryotic cells in all the tissues and organs which comprise a human. The predominant bacteria on the surfaces of the human body are listed in Table 3. Informal names identify the bacteria in this table. Formal taxonomic names of organisms are given in Table 1.
Table 3. Predominant bacteria at various anatomical locations in adults.
| Anatomical Location | Predominant bacteria | |
| Skin | staphylococci and corynebacteria | |
| Conjunctiva | sparse, Gram-positive cocci and Gram-negative rods | |
| Oral cavity | | |
| Teeth | streptococci, lactobacilli | |
| mucous membranes | streptococci and lactic acid bacteria | |
| Upper respiratory tract | | |
| nares (nasal membranes) | staphylococci and corynebacteria | |
| pharynx (throat) | streptococci, neisseria, Gram-negative rods and cocci | |
| Lower respiratory tract | None | |
| Gastrointestinal tract | | |
| Stomach | Helicobacter pylori (up to 50%) | |
| small intestine | lactics, enterics, enterococci, bifidobacteria | |
| | bacteroides, lactics, enterics, enterococci, clostridia, methanogens | |
| Urogenital tract | | |
| Anterior urethra | sparse, staphylococci, corynebacteria, enterics | |
| Vagina | lactic acid bacteria during child-bearing years; otherwise mixed | |
Normal Flora of the Skin The adult human is covered with approximately 2 square meters of skin. The density and composition of the normal flora of the skin varies with anatomical locale. The high moisture content of the axilla, groin, and areas between the toes supports the activity and growth of relatively high densities of bacterial cells, but the density of bacterial populations at most other sites is fairly low, generally in 100s or 1000s per square cm. Most bacteria on the skin are sequestered in sweat glands.
The skin microbes found in the most superficial layers of the epidermis and the upper parts of the hair follicles are Gram-positive cocci (Staphylococcus epidermidis and Micrococcus sp.) and corynebacteria such as Propionibacterium sp. These are generally nonpathogenic and considered to be commensal, although mutualistic and parasitic roles have been assigned to them. For example, staphylococci and propionibacteria produce fatty acids that inhibit the growth of fungi and yeast on the skin. But, if Propionibacterium acnes, a normal inhabitant of the skin, becomes trapped in hair follicle, it may grow rapidly and cause inflammation and acne.
Sometimes potentially pathogenic Staphylococcus aureus is found on the face and hands in individuals who are nasal carriers. This is because the face and hands are likely to become inoculated with the bacteria on the nasal membranes. Such individuals may autoinoculate themselves with the pathogen or spread it to other individuals or foods.
Normal Flora of the Conjunctiva A variety of bacteria may be cultivated from the normal conjunctiva, but the number of organisms is usually small. Staphylococcus epidermidis and certain coryneforms (Propionibacterium acnes) are dominant. Staphylococcus aureus, some streptococci, Haemophilus sp. and Neisseria sp. are occasionally found. The conjunctiva is kept moist and healthy by the continuous secretions from the lachrymal glands. Blinking wipes the conjunctiva every few seconds mechanically washing away foreign objects including bacteria. Lachrymal secretions (tears) also contain bactericidal substances including lysozyme. There is little or no opportunity for microorganisms to colonize the conjunctiva without special mechanisms to attach to the epithelial surfaces and some ability to withstand attack by lysozyme.
Pathogens which do infect the conjunctiva (e.g. Neisseria gonorrhoeae and Chlamydia trachomatis) are thought to be able to specifically attach to the conjunctival epithelium. Newborn infants may be especially prone to bacterial attachment. Since Chlamydia and Neisseria might be present on the cervical and vaginal epithelium of an infected mother, silver nitrate or an antibiotic may be put into the newborn's eyes to avoid infection after passage through the birth canal.
Normal Flora of the Respiratory Tract A large number of bacterial species colonize the upper respiratory tract (nasopharynx). The nares (nostrils) are always heavily colonized, predominantly with Staphylococcus epidermidis and corynebacteria, and often (in about 20% of the general population) with Staphylococcus aureus, this being the main carrier site of this important pathogen. The healthy sinuses, in contrast are sterile. The pharynx (throat) is normally colonized by streptococci and various Gram-negative cocci. Sometimes pathogens such as Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae and Neisseria meningitidis colonize the pharynx.
The lower respiratory tract (trachea, bronchi, and pulmonary tissues) is virtually free of microorganisms, mainly because of the efficient cleansing action of the ciliated epithelium which lines the tract. Any bacteria reaching the lower respiratory tract are swept upward by the action of the mucociliary blanket that lines the bronchi, to be removed subsequently by coughing, sneezing, swallowing, etc. If the respiratory tract epithelium becomes damaged, as in bronchitis or viral pneumonia, the individual may become susceptible to infection by pathogens such as H. influenzae or S. pneumoniae descending from the nasopharynx.
Normal Flora of the Urogenital Tract Urine is normally sterile, and since the urinary tract is flushed with urine every few hours, microorganisms have problems gaining access and becoming established. The flora of the anterior urethra, as indicated principally by urine cultures, suggests that the area may be inhabited by a relatively consistent normal flora consisting of Staphylococcus epidermidis, Enterococcus faecalis and some alpha-hemolytic streptococci. Their numbers are not plentiful, however. In addition, some enteric bacteria (e.g. E. coli, Proteus) and corynebacteria, which are probably contaminants from the skin, vulva or rectum, may occasionally be found at the anterior urethra.
The vagina becomes colonized soon after birth with corynebacteria, staphylococci, streptococci, E. coli, and a lactic acid bacterium historically named "Doderlein's bacillus" (Lactobacillus acidophilus). During reproductive life, from puberty to menopause, the vaginal epithelium contains glycogen due to the actions of circulating estrogens. Doderlein's bacillus predominates, being able to metabolize the glycogen to lactic acid. The lactic acid and other products of metabolism inhibit colonization by all except this lactobacillus and a select number of lactic acid bacteria. The resulting low pH of the vaginal epithelium prevents establishment by most other bacteria as well as the potentially-pathogenic yeast, Candida albicans. This is a striking example of the protective effect of the normal bacterial flora for their human host.
Normal Flora of the Oral Cavity The presence of nutrients, epithelial debris, and secretions makes the mouth a favorable habitat for a great variety of bacteria. Oral bacteria include streptococci, lactobacilli, staphylococci and corynebacteria, with a great number of anaerobes, especially bacteroides.
The mouth presents a succession of different ecological situations with age, and this corresponds with changes in the composition of the normal flora. At birth, the oral cavity is composed solely of the soft tissues of the lips, cheeks, tongue and palate, which are kept moist by the secretions of the salivary glands. At birth the oral cavity is sterile but rapidly becomes colonized from the environment, particularly from the mother in the first feeding. Streptococcus salivarius is dominant and may make up 98% of the total oral flora until the appearance of the teeth (6 - 9 months in humans). The eruption of the teeth during the first year leads to colonization by S. mutans and S. sanguis. These bacteria require a nondesquamating (nonepithelial) surface in order to colonize. They will persist as long as teeth remain. Other strains of streptococci adhere strongly to the gums and cheeks but not to the teeth. The creation of the gingival crevice area (supporting structures of the teeth) increases the habitat for the variety of anaerobic species found. The complexity of the oral flora continues to increase with time, and bacteroides and spirochetes colonize around puberty.
The normal bacterial flora of the oral cavity clearly benefit from their host who provides nutrients and habitat. There may be benefits, as well, to the host. The normal flora occupy available colonization sites which makes it more difficult for other microorganisms (nonindigenous species) to become established. Also, the oral flora contribute to host nutrition through the synthesis of vitamins, and they contribute to immunity by inducing low levels of circulating and secretory antibodies that may cross react with pathogens. Finally, the oral bacteria exert microbial antagonism against nonindigenous species by production of inhibitory substances such as fatty acids, peroxides and bacteriocins.
On the other hand, the oral flora are the usual cause of various oral diseases in humans, including abscesses, dental caries, gingivitis, and periodontal disease. If oral bacteria can gain entrance into deeper tissues, they may cause abscesses of alveolar bone, lung, brain, or the extremities. Such infections usually contain mixtures of bacteria with Bacteroides melaninogenicus often playing a dominant role. If oral streptococci are introduced into wounds created by dental manipulation or treatment, they may adhere to heart valves and initiate subacute bacterial endocarditis.
Normal Flora of the Gastrointestinal Tract The bacterial flora of the gastrointestinal (GI) tract of animals has been studied more extensively than that of any other site. The composition differs between various animal species, and within an animal species. In humans, there are differences in the composition of the flora which are influenced by age, diet, cultural conditions, and the use of antibiotics. The latter greatly perturbs the composition of the intestinal flora.
In the upper GI tract of adult humans, the esophagus contains only the bacteria swallowed with saliva and food. Because of the high acidity of the gastric juice, very few bacteria (mainly acid-tolerant lactobacilli) can be cultured from the normal stomach. However, at least half the population in the
The proximal small intestine has a relatively sparse Gram-positive flora, consisting mainly of lactobacilli and Enterococcus faecalis. This region has about 105 - 107 bacteria per ml of fluid. The distal part of the small intestine contains greater numbers of bacteria (108/ml) and additional species, including coliforms (E. coli and relatives) and Bacteroides, in addition to lactobacilli and enterococci.
The flora of the large intestine (colon) is qualitatively similar to that found in feces. Populations of bacteria in the colon reach levels of 1011/ml feces. Coliforms become more prominent, and enterococci, clostridia and lactobacilli can be regularly found, but the predominant species are anaerobic Bacteroides and anaerobic lactic acid bacteria in the genus Bifidobacterium (Bifidobacterium bifidum). These organisms may outnumber E. coli by 1,000:1 to 10,000:1. Sometimes, significant numbers of anaerobic methanogens (up to 1010/gm) may reside in the colon of humans. This is our only direct association with archaea as normal flora. The range of incidence of certain bacteria in the large intestine of humans is shown in Table 4 below.
Table 4. Bacteria found in the large intestine of humans.
| BACTERIUM | |
| Bacteroides fragilis | 100 |
| Bacteroides melaninogenicus | 100 |
| Bacteroides oralis | 100 |
| Lactobacillus | 20-60 |
| Clostridium perfringens | 25-35 |
| Clostridium septicum | 5-25 |
| Clostridium tetani | 1-35 |
| Bifidobacterium bifidum | 30-70 |
| Staphylococcus aureus | 30-50 |
| Enterococcus faecalis | 100 |
| Escherichia coli | 100 |
| Salmonella enteritidis | 3-7 |
| Klebsiella sp. | 40-80 |
| Enterobacter sp. | 40-80 |
| Proteus mirabilis | 5-55 |
| Pseudomonas aeruginosa | 3-11 |
| Peptostreptococcus sp. | |
| Peptococcus sp. | |
At birth the entire intestinal tract is sterile, but bacteria enter with the first feed. The initial colonizing bacteria vary with the food source of the infant. In breast-fed infants, bifidobacteria account for more than 90% of the total intestinal bacteria. Enterobacteriaceae and enterococci are regularly present, but in low proportions, while bacteroides, staphylococci, lactobacilli and clostridia are practically absent. In bottle-fed infants, bifidobacteria are not predominant. When breast-fed infants are switched to a diet of cow's milk or solid food, bifidobacteria are progressively joined by enterics, bacteroides, enterococci lactobacilli and clostridia. Apparently, human milk contains a growth factor that enriches for growth of bifidobacteria, and these bacteria play an important role in preventing colonization of the infant intestinal tract by non indigenous or pathogenic species.
For example, The growth of Clostridium difficile in the intestinal tract is normally held in check by other members of the normal flora. When antibiotics given for other infections cause collateral damage to the normal intestinal flora, the Clostridium may be able to "grow out" and produce a serious diarrheal syndrome called pseudo membranous colitis. This is an example of an "antibiotic induced diarrheal disease".
The composition of the flora of the gastrointestinal tract varies along the tract (at longitudinal levels) and across the tract (at horizontal levels) where certain bacteria attach to the gastrointestinal epithelium and others occur in the lumen. There is frequently a very close association between specific bacteria in the intestinal ecosystem and specific gut tissues or cells (evidence of tissue tropism and specific adherence). Gram-positive bacteria, such as the streptococci and lactobacilli, are thought to adhere to the gastrointestinal epithelium using polysaccharide capsules or cell wall teichoic acids to attach to specific receptors on the epithelial cells. Gram-negative bacteria such as the enterics may attach by means of specific fimbriae which bind to glycoproteins on the epithelial cell surface.
It is in the intestinal tract that we see the greatest effect of the bacterial flora on their host. This is due to their large mass and numbers. Bacteria in the human GI tract have been shown to produce vitamins and may otherwise contribute to nutrition and digestion. But their most important effects are in their ability to protect their host from establishment and infection by alien microbes and their ability to stimulate the development and the activity of the immunological tissues.
On the other hand, some of the bacteria in the colon (e.g. Bacteroides) have been shown to produce metabolites that are carcinogenic, and there may be an increased incidence of colon cancer associated with these bacteria. Alterations in the GI flora brought on by poor nutrition or perturbance with antibiotics can cause shifts in populations and colonization by nonresidents that leads to gastrointestinal disease.
Beneficial Effects of the
The effects of the normal flora are inferred by microbiologists from experimental comparisons between "germ-free" animals (which are not colonized by any microbes) and conventional animals (which are colonized with a typical normal flora). Briefly, some of the characteristics of a germ-free animals that are thought to be due to lack of exposure to a normal flora are:
1. vitamin deficiencies, especially vitamin K and vitamin B12
2. increased susceptibility to infectious disease
3. poorly developed immune system, especially in the gastrointestinal tract
4. lack of "natural antibody" or natural immunity to bacterial infection
Because these conditions in germ-free mice and hamsters do not occur in conventional animals, or are alleviated by introduction of a bacterial flora (at the appropriate time of development), it is tempting to conclude that the human normal flora make similar contributions to human nutrition, health and development. The overall beneficial effects of microbes are summarized below.
1. The normal flora synthesize and excrete vitamins in excess of their own needs, which can be absorbed as nutrients by their host. In humans, enteric bacteria secrete Vitamin K and Vitamin B12, and lactic acid bacteria produce certain B-vitamins. Germ-free animals may be deficient in Vitamin K to the extent that it is necessary to supplement their diets.
2. The normal flora prevent colonization by pathogens by competing for attachment sites or for essential nutrients. This is thought to be their most important beneficial effect, which has been demonstrated in the oral cavity, the intestine, the skin, and the vaginal epithelium. In some experiments, germ-free animals can be infected by 10 Salmonella bacteria, while the infectious dose for conventional animals is near 106 cells.
3. The normal flora may antagonize other bacteria through the production of substances which inhibit or kill nonindigenous species. The intestinal bacteria produce a variety of substances ranging from relatively nonspecific fatty acids and peroxides to highly specific bacteriocins, which inhibit or kill other bacteria.
4. The normal flora stimulate the development of certain tissues, i.e., the caecum and certain lymphatic tissues (Peyer's patches) in the GI tract. The caecum of germ-free animals is enlarged, thin-walled, and fluid-filled, compared to that organ in conventional animals. Also, based on the ability to undergo immunological stimulation, the intestinal lymphatic tissues of germ-free animals are poorly-developed compared to conventional animals.
5. The normal flora stimulate the production of natural antibodies. Since the normal flora behave as antigens in an animal, they induce an immunological response, in particular, an antibody-mediated immune (AMI) response. Low levels of antibodies produced against components of the normal flora are known to cross react with certain related pathogens, and thereby prevent infection or invasion. Antibodies produced against antigenic components of the normal flora are sometimes referred to as "natural" antibodies, and such antibodies are lacking in germ-free animals.
6.The normal flora is used to produce probiotics. A probiotic is a substance containing live microorganisms that are beneficial to humans and animals. They help restore balance of the microbiota. The use of probiotics is of great interest to microbioloical researches and they have been found to incur more benefits than harm. They are also easy to use, and their activity is fast because the live organisms are already present in them. This is unlike the prebiotics, which favour the growth of these desirable microbes.
Probiotics, in details.
Probiotics are dietary supplements containing potentially beneficial bacteria or yeasts. According to the currently adopted definition by FAO/WHO, probiotics are: ‘Live microorganisms which when administered in adequate amounts confer a health benefit on the host’(FAO/WHO,2001)
Lactic acid bacteria (LAB) are the most common type of microbes used. LAB have been used in the food industry for many years, because they are able to convert sugars (including lactose) and other carbohydrates into lactic acid. This not only provides the characteristic sour taste of fermented dairy foods such as yogurt, but also by lowering the pH may create fewer opportunities for spoilage organisms to grow, hence creating possible health benefits on preventing gastrointestinal infections(Nichols,Andrews W.,2007) Strains of the genera Lactobacillus and Bifidobacterium, are the most widely used probiotic bacteria(Tannock,G.,2005).
Probiotic bacterial cultures are intended to assist the body's naturally occurring gut flora, an ecology of microbes, to re-establish themselves. They are sometimes recommended by doctors, and, more frequently, by nutritionists, after a course of antibiotics, or as part of the treatment for gut related candidiasis. Claims are made that probiotics strengthen the immune system to combat allergies, excessive alcohol intake, stress, exposure to toxic substances, and other diseases(Nichols,Andrews W.,2007). In these cases, the bacteria that work well with our bodies (see symbiosis) may decrease in number, an event which allows harmful competitors to thrive, to the detriment of our health.
Maintenance of a healthy gut flora is, however, dependent on many factors, especially the quality of food intake including a significant proportion of prebiotic foods in the diet has been demonstrated to support a healthy gut flora and may be another means of achieving the desirable health benefits promised by probiotics.
History of probiotics
Probiotics, which means "for life", have been used for centuries as natural components in health-promoting foods. The original observation of the positive role played by certain bacteria was first introduced by Russian scientist and Nobel laureate Eli Metchnikoff, who in the beginning of the 20th century suggested that it would be possible to modify the gut flora and to replace harmful microbes by useful microbes. Metchnikoff produced the notion that the ageing process results from the activity of putrefactive (proteolytic) microbes producing toxic substances in the large bowel. Proteolytic bacteria such as clostridia, which are part of the normal gut flora, produce toxic substances including phenols, indols and ammonia from the digestion of proteins. According to Metchnikoff these compounds were responsible for what he called “intestinal auto-intoxication”, which caused the physical changes associated with old age. It was at that time known that milk fermented with lactic-acid bacteria inhibits the growth of proteolytic bacteria because of the low pH produced by the fermentation of lactose. Metchnikoff had also observed that certain rural populations in Europe, for example in Bulgaria and the Russian Steppes who lived largely on milk fermented by lactic-acid bacteria were exceptionally long lived. Based on these facts, Metchnikoff proposed that consumption of fermented milk would “seed” the intestine with harmless lactic-acid bacteria and decrease the intestinal pH and that this would suppress the growth of proteolytic bacteria. Metchnikoff himself introduced in his diet sour milk fermented with the bacteria he called “Bulgarian Bacillus” and found his health benefited. Friends in
Henry Tissier, also from the Pasteur Institute, was the first to isolate a Bifidobacterium. He isolated the bacterium from a breast-fed infant and named it Bacillus bifiduscommunis(Tissier,H.,1900). This bacterium was later renamed Bifidobacterium bifidum. Tissier showed that bifidobacteria are predominant in the gut flora of breast-fed babies, and he recommended administration of bifidobacteria to infants suffering from diarrhea. The mechanism claimed was that bifidobacteria would displace the proteolytic bacteria that cause the disease.
German professor Alfred Nissle, in 1917 isolated a strain of Escherichia coli from the feces of a First World War soldier who did not develop enterocolitis during a severe outbreak of shigellosis(Nissle,A.,1918). In those days, antibiotics were not yet discovered, and Nissle used the strain with considerable success in acute cases of infectious intestinal diseases (salmonellosis and shigellosis). Escherichia coli Nissle 1917 is still in use and is one of the few examples of a non-LAB probiotic.
In 1920 Rettger demonstrated that Metchnikoff’s “Bulgarian Bacillus”, later called Lactobacillus bulgaricus, could not live in the human intestine(Cheplin,H.A.,et al,1920) and the fermented food phenomena petered out. Metchnikoff’s theory was disputable (at this stage), and people doubted his theory of longevity.
After Metchnikoff’s death in 1916, the centre of activity moved to the US. It was reasoned that bacteria originating from the gut were more likely to produce the desired effect in the gut, and in 1935 certain strains of Lactobacillus acidophilus were found to be very active when implanted in the human digestive tract(Rettger,L.F.,et al,1935). Trials were carried out using this organism, and encouraging results were obtained especially in the relief of chronic constipation.
The term “probiotics” was first introduced in 1953 by Kollath. Contrasting antibiotics, probiotics were defined as microbially derived factors that stimulate the growth of other microorganisms. In 1989 Roy Fuller suggested a definition of probiotics which has been widely used: “A live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance”(Fuller,R.,1989). Fuller’s definition emphasizes the requirement of viability for probiotics and introduces the aspect of a beneficial effect on the host.
In the 1960s the dairy industry began to promote fermented milk products containing Lactobacillus acidophilus. In subsequent decades other [Lactobacillus] species have been introduced including Lactobacillus rhamnosus, Lactobacillus casei, and Lactobacillus johnsonii, because they are intestinal species with beneficial properties(Tannock,G.W.,2003).
There is no published evidence that probiotic supplements are able to completely replace the body’s natural flora when these have been killed off; indeed bacterial levels in faeces disappear within days when supplementation ceases. While the oral use of probiotics is considered safe and even recommended by World Health Organization under specific guidelines(FAO/WHO,2001), in some specific situations (such as critically ill patients) they could be potentially harmful. In one therapeutic clinical trial, a probiotic cocktail have been shown to increase the death rates of patients with acute pancreatitis(Baselink,2008), but was given through tube feeding directly in the intestine instead of the usual oral way. Some other therapeutic use of probiotics have been shown to be beneficial for other types of patients(Nichols Andrew,W,2007),(Tannock,G,2005).
Potential benefits
Experiments into the benefits of probiotic therapies suggest a range of potentially beneficial medicinal uses for probiotics. For many of the potential benefits, research is limited and only preliminary results are available. It should be noted that the general effects of probiotics are not described. All effects can only be attributed to the strain(s) tested, not to the species, nor to the whole group of LAB (or other probiotics)(Gilliland,S.E,et al,1990).
· Managing Lactose Intolerance
As lactic acid bacteria actively convert lactose into lactic acid, ingestion of certain active strains may help lactose intolerant individuals tolerate more lactose than what they would have otherwise(Beselink,2008). In practice probiotics are not specifically targeted for this purpose, as most are relatively low in lactase activity as compared to the normal yoghourt bacteria.
In laboratory investigations, some strains of LAB have demonstrated anti-mutagenic effects thought to be due to their ability to bind with heterocyclic amines, which are carcinogenic substances formed in cooked meat(Wollowski,I.,2001). Animal studies have demonstrated that some LAB can protect against colon cancer in rodents, though human data is limited and conflicting(Brady,L.J.,et al,2000).
First human trials have found that the strains tested may exert anti-carcinogenic effects by decreasing the activity of an enzyme called β-glucuronidase (which can generate carcinogens in the digestive system). Lower rates of colon cancer among higher consumers of fermented dairy products have been observed in some population studies(Sanders,M.E.,2000).
Animal studies have demonstrated the efficacy of a range of LAB to be able to lower serum cholesterol levels, presumably by breaking down bile in the gut, thus inhibiting its reabsorption (which enters the blood as cholesterol). Some, but not all human trials have shown that dairy foods fermented with specific LAB can produce modest reductions in total and LDL cholesterol levels in those with normal levels to begin with, however trials in hyperlipidemic subjects are needed(Sanders,M.E.,2000).
· Lowering Blood Pressure
Several small clinical trials have shown that consumption of milk fermented with various strains of LAB can result in modest reductions in blood pressure. It is thought that this is due to the ACE inhibitor-like peptides produced during fermentation(Sanders,M.E.,2000).
· Improving Immune Function and Preventing Infections
LAB are thought to have several presumably beneficial effects on immune function. They may protect against pathogens by means of competitive inhibition (i.e., by competing for growth) and there is evidence to suggest that they may improve immune function by increasing the number of IgA-producing plasma cells, increasing or improving phagocytosis as well as increasing the proportion of T lymphocytes and Natural Killer cells(Reid,G.,et al,2003). Clinical trials have demonstrated that probiotics may decrease the incidence of respiratory tract infections(Hatakka,K.,et al,2001) and dental caries in children(Nase,L.,et al,2001). LAB foods and supplements have been shown to be effective in the treatment and prevention of acute diarrhea, and in decreasing the severity and duration of rotavirus infections in children and travelers' diarrhea in adults(Reid,G.,et al,2003).
LAB are also thought to aid in the treatment of Helicobacter pylori infections (which cause peptic ulcers) in adults when used in combination with standard medical treatments(Hamilton-Miller,J.M.,2003).
· Antibiotic-associated diarrhea
A meta-analysis suggested probioticsmay reduce antibiotic-associated diarrhea(Cremonini,F,et al,2002). A subsequent randomized controlled trial also found benefit in elderly patients(Hickson,M,et al,2007).
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Pharmaceutical Probiotics for the Treatment of Anaerobicand Other Infections
Pharmaceutical probiotics have been used as alternative treatments or preventative therapies for a variety of clinical diseases. The overuse of antibiotics and emergence of multiple-antibiotic resistant pathogens has refocused clinical attention on the field of probiotics. Anaerobic infections which seem to respond well to probiotics are infections which involve the disruption of normal microbial flora. Gastrointestinal infections (travelers' diarrhea, antibiotic-associated diarrhea,Clostridium difficiledisease, rotavirus diarrhea) have been studied using the following pharmaceutical probiotics:Saccharomyces boulardii, Lactobacillus caseiGG,Lactobacillus acidophilus, Lactobacillus bulgaricus, Bifidobacterium bifidum, Streptococcus thermophilusandEnterococcus faecium. Vaginitis has been experimentally studied usingL. acidophilusandL. caseiGG. The efficacy, safety and mechanisms of action of these various probiotics are reviewed. Requirements for drug approval are similar for biologic probiotics and new drug entities and these requirements involve preclinical tolerability studies, pharmacokinetic studies and large, well-controlled blinded clinical trials.
· Reducing Inflammation
LAB foods and supplements have been found to modulate inflammatory and hypersensitivity responses, an observation thought to be at least in part due to the regulation of cytokine function(Reid,G.,et al,2003). Clinical studies suggest that they can prevent reoccurrences of inflammatory bowel disease in adults, as well as improve milk allergies and decrease the risk of atopic eczema in children
· Improving Mineral Absorption
It is hypothesized that probiotic lactobacilli may help correct malabsorption of trace minerals, found particularly in those with diets high in phytate content from whole grains, nuts, and legumes(Famularo,G.,et al,2005).
· Prevents Harmful Bacterial Growth Under Stress
In a study done to see the effects of stress on intestinal flora, rats that were fed probiotics had little occurrence of harmful bacteria latched onto their intestines compared to rats that were fed sterile water(Hitti,M.,2006).
· Irritable Bowel Syndrome (IBS) and Colitis
Bifidobacterium. infantis 35624, sold as Align, was found to improve some symptoms of irritable bowel syndrome in women in a recent study. Another probiotic bacterium, Lactobacillus plantarum 299V, was also found to be effective in reducing IBS symptoms. Additionally, a probiotic formulation, VSL3, was found to be effective in treating ulcerative colitis(Kerr,M.,2003). Bifidobacterium animalis DN-173 010 may help.
- Synbiotics It is also possible to increase and maintain a healthy bacterial gut flora by increasing the amounts of prebiotics in the diet such as inulin, raw oats, and unrefined wheat.
As probiotics are mainly active in the small intestine and prebiotics are only effective in the large intestine, the combination of the two may give a synergistic effect. Appropriate combinations of pre- and probiotics are synbiotics.
Synbiotics have also been defined as metabolites produced by ecoorgan or by synergistic action of prebiotics and probiotics e.g. short chain fatty acids, other fatty acids, amino acids, peptides, polyamines, carbohydrates, vitamins, numerous antioxidants and phytosterols, growth factors, coagulation factors, various signal molecules Such as cytokine-like bacteriokines.
Probiotic Strains The most common form for probiotics are dairy products and probiotic fortified foods. However, tablets, capsules, powders and sachets containing the bacteria in freeze dried form and live probiotics are also available.
| Table 5: Proven probiotic strains. (Sanders,M.E.,2007) | |||
| Strain | Brandname | Producer | Proven effect in humans |
| Bifidobacterium animalis subsp. lactis BB-12 | | Immune stimulation, improves phagocytic activity, alleviates atopic eczema, prevents diarrhoea in children and traveller's diarrhea | |
| Bifidobacterium breve Yakult | | ||
| Bifidobacterium infantis 35624 | Align | Irritable Bowel Syndrome (IBS) | |
| Bifidobacterium lactis HN019 (DR10) | Immune stimulation] | ||
| Bifidobacterium longum BB536 | | positive effects against allergies | |
| Escherichia coli Nissle 1917 | Mutaflor | Ardeypharm | Immune stimulation |
| | | ||
| Lactobacillus acidophilus NCFM | | reduces symptoms of lactose intolerance, prevents bacterial overgrowth in small intestine | |
| Lactobacillus casei DN114-001 (Lactobacillus casei Immunitas(s)/Defensis) | Diarrhea and allergy reduction, immune stimulation, reduction of duration of winter infections, H. pylori eradication, antibiotic associated diarrhea & C. difficile infections | ||
| Lactobacillus casei CRL431 | | | |
| Lactobacillus casei F19 | Cultura | Improves digestive health, immune stimulation, reduces antibiotic-associated diarrhoea, induces satiety, metabolizes body fat, reduces weight gain | |
| Lactobacillus casei Shirota | Yakult | Maintenance of gut flora, immune modulation, bowel habits and constipation | |
| Lactobacillus johnsonii La1 (= Lactobacillus LC1) | | Immune stimulation, active against Helicobacter pylori | |
| | Immune stimulation, improves digestive health, reduces antibiotic-associated diarrhea | ||
| GoodBelly ProViva | IBS, used post-operative | ||
| Lactobacillus reuteri ATTC 55730 (Lactobacillus reuteri SD2112) | | Immune stimulation, against diarrhea | |
| Lactobacillus rhamnosus ATCC 53013 (discovered by Gorbach & Goldin(=LGG)) | Vifit and others | Immune stimulation, alleviates atopic eczema, prevents diarrhoea in children and many other types of diarrhea | |
| Lactobacillus rhamnosus LB21 | Verum | Immune stimulation, improves digestive health, reduces antibiotic-associated diarrhea | |
| Lactobacillus salivarius UCC118 | | | positive effects with intestinal ulcers and inflammation |
| Saccharomyces cerevisiae (boulardii) lyo | DiarSafe and others | Wren Laboratories and others | against antibiotic-associated diarrhoea and Clostridium difficile infections; to treat acute diarrhoea in adults & children. |
| tested as mixture: | Bion Flore Intime Jarrow Fem-Dophilus | Oral ingestion results in vaginal colonisation and prevention of vaginitis | |
| tested as mixture: | VSL#3 | Sigma-Tau Pharmaceuticals, Inc. | positive effects with intestinal ulcers and inflammation |
| tested as mixture: | | | reduction of Cl. difficile in faeces |
| tested as mixture: | A'Biotica and others | prevents diarrhoea in children, prevents upset stomachs for patients on antibiotics, active against Helicobacter pylori | |
Some commonly used bacteria in products, but without probiotic effect (yoghurt bacteria):
Some other bacteria mentioned in probiotic products:
Lactobacillus bifidus - became new genus Bifidobacterium
Some fermented products containing similar lactic acid bacteria include:
Pickled vegetables
Fermented bean paste such as tempeh, miso and doenjang
Some strains found in live probiotics are:
- Lactobacillus acidophilus
- Lactobacillus bulgaricus
- Lactobacilus casei
- Lactobacillus fermentum
- Lactobacillus plantarum – 2 species
- Lactobacillus brevis
- Lactobacillus amylovorus
- Lactobacillus buchneri
- Lactobacillus acetotolerans
- Bifidobacterium bifidum
- Bifidobacterium longum
- Pediococcus pentosaceus
- Pediococcus halophilus
- Pediococcus damnosus
- Streptococcus thermophilus
- Lactococcus lactis
The benefits of using live probiotics include:
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- treat and control fungal infections, including eradicating Candida, through the antifungal effects of LAB especially the LAB strains in LB17
- bring the balance of friendly flora in the digestive system back to at least 80% good bacteria
- assist in the digestion of food consumed and its subsequent absorption
- boost the immune system of the body
- in bringing the digestive functions back to good order, help to ease the stress on and cleanse the organs associated with digestion such as the liver, kidneys, pancreas, spleen, etc
- support the treatment of crohns and colitis
- study shows live probiotics protects intestinal cells from effects of infection by enteroinvasive Escherichia coli (EIEC)
- improve oxygen intake to the red cell platelets and thus improve the stamina and endurance of athletes from studies conducted by researchers at
- convert botancial lignans (SDG) in the bowel to mammalian lignans (ED and EL) to enable the body to absorb these powerful and amazingly beneficial substances. Lignans have been recognized and acknowledged by the medical fraternity as effective in treating and preventing cancers such as breast, bowel, colon and prostate
- significantly boost the healing powers of the skin. This is vital to individuals who are afflicted with Diabetes (Type-2) where healing properties of the skin have been impaired so that loss of limbs arising from serious infections may be avoided
- help treat psoriasis, a condition which is being recognized to be caused by a poor or inadequate immune system
- help treat asthma and mothers who use probiotics during pregnancy may ensure that the new born child do not develop asthmatic symptoms
- prevent colds and nasal congestions through elimination of excessive bad and harmful bacteria in the nasal passages
- ease and relief heart-burn, acid reflux and most other digestive disorders such as excessive intestinal gas.
Research about probiotics shows both benefits and harm.
A 2007 study at University College Cork in Ireland showed that a diet including milk fermented with Lactobacillus bacteria prevented Salmonella infection in pigs.
A 2007 clinical study at Imperial College London showed that preventive consumption of a commercially available probiotic drink containing L casei DN-114001, L bulgaricus, and S thermophilus can reduce the incidence of antibiotic-associated diarrhea and C difficile-associated diarrhea.
In a double-blind, placebo-controlled therapeutic study on the effects of a probiotic cocktail on pancreatitis at University Medical Center Utrecht (UMC), 24 out of 296 patients died between 2004 and 2007, with more deaths among those receiving the probiotic cocktail directly in the intestine. According to the spokesman of UMC, it is likely that some of these deaths would not have occurred without the probiotics[49], although other therapeutic trials conducted on probiotics were more positively conclusive (Sanders,M.E.,2000).
Recommendation
Microbiologists have been able to identify the beneficial microbes that make up the human microbita only through researches. Therefore,if,as microbiologists,we divert more energy,time and resources to research work,more advantageous finding could be discovered.
Also,the human lifestyle could be adjusted in such a way as to favour the existence of their normal flora.
Conclusion
Clearly,the influence of the members of the normal human microbiota, inhabiting various sites in the body, on their host, is more beneficial than otherwise.
