Normal Flora as a Resistance to Pathogenic Disease

Normal Flora as a Resistance to Pathogenic Disease

A diverse microbial flora is associated with the human body (skin and mucous membrane) of every human being from shortly after birth until death. The human body, which contains about 1013 cells, routinely harbours about 1014 bacteria (Pappas, 2009). This bacterial population constitutes the normal microbial flora (Humam Microbiome Project, 2012).

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The normal microbial flora is relatively stable, with specific genera populating various body regions during particular period in an individual life. Microorganism of the normal flora may aid the host (by competing for microenvironments more effectively than such pathogens as salmonella sp. or by producing nutrients that the host can use), may harm the host (by causing dental caries, abscesses) or may exist as commensals (inhabiting the host for long periods without causing detectable harm or benefit (Beaugerie and Petit 2004).

Even though most elements of the normal microbial flora inhabiting the human skin, nails eyes, oropharynx, genitalia, and gastrointestinal tract are harmless in healthy individual, these organisms frequently cause disease in compromised hosts (Eckburg et al., 2003). Viruses and parasites are not considered members of the normal microbial flora by most investigators because they are not commensalism and donot aid the host.

The fact that the normal flora substantially influences the well being of the host was not well understood until germ-free animals become available. Germ free animals were obtained by cesarean section and maintained in special isolators, this allowed the investigator to raise them in an environment free from detectable viruses, bacteria and other organisms. Two interesting observations were made about animals raised under germ free conditions. First, the germ free animals lived almost twice as long as their conventionally maintained counterparts, and second, the major causes of death were different in the two groups. Infection often caused death in conventional animals but intestinal atonia frequently killed germ free animals. Other investigations showed that germ free animals have anatomic, physiologic, and immunologic features not shared with conventional animals (Katherine, 2009).

For example, in germ-free animals, the alimentary lamina propria is underdeveloped, little or no immunoglobulin is present in sera or secretions, intestinal motility is reduced and the intestinal epithelial cell renewal rate is approximately one half that of normal animals (4 rather than 2 days) (MacDougall and Raymond, 2012). Although the forgoing indicates that bacterial flora may be undesirable, studies with antibiotic treated animals suggests that the flora protects individuals from pathogens (Wexier, 2007). Investigators have used streptomycin to reduce the normal flora and have then infected animals with streptomycin- resistant salmonella. Further studies suggested that fermentation products (acetic and butyric acids) produced by the normal flora inhibited salmonella growth in the gastro intestinal tract (Gibson, 2004). The normal flora in humans usually develops in an orderly sequence, or succession, after birth, leading to the stable population of bacteria that make up the normal adult flora.

The main factor determining the composition of the normal flora in a body region is the nature of the local environment, which is determined by PH, temperature, re-dox potential, and oxygen, water and nutrient levels. Other factors such as peristalsis, saliva, lysozyme secretion, and secretion of immunoglobulins also play roles in flora control (Sears, 2009). The local environment is like a concerto in which one principle instrument usually dominates. For example, an infant begins to contact organisms as it moves through the birth canal. A Gram positive (bifidobacteria and Lactobralli,) predominate in the gastrointestinal tract early in life if the infant is breast fed. The bacteria population is reduced and displaced some what by Gram negative flora/Enterobacteriaceae when the baby is bottle fed. The type of liquid that provided to the infant is the principal instrument of this flora control, immunoglobulin and perhaps, other elements in breast milk may also be important (Harnsen et al., 2000).
Bacteria make up host of the flora in the colon and up to 60% of the dry mass of faeces.

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The human microbiota is the aggregate of microorganisms, a microbiome that resides on the surface, and in deep players of skin (including mammary glands) in the saliva and oral mucosa, in the conjunctival and in the gastrointestinal tracts (Human Microbiome Project, 2012)). They include bacteria, fungi and archaea. Micro-animals which live on human body are excluded. One study indicated they out number human cells. Some of these organisms perform tasks that are useful for the human host (Karherine 2009).

However, the majority have been too poorly reached for us to understand the role they play, however communities of microflora have been shown to change their behavior in diseased individuals (Medicalxpress). Those that are expected to be present and that under normal circumstance do not cause disease, but instead participate in maintaining health are deemed members of the normal flora (MacDougall 2012). Though are widely known as microflora, this is a misnomer in technical terms, since the root word flora pertains to plants, and biota refers to the total collection of organisms in a particular ecosystem. Recently the more appropriate term microbiota is applied, though its use has not eclipsed the entrenched use and recognition of flora with regard to bacteria and other microorganisms (Pappas, 2009). Both terms are being used in different literature studies in 2009 questioned whether the decline in biota (including microfauna) as a result of human intervention might impede human health.

Most of the microbes associated with human appear to be not harmful at all, but rather assist in maintaining processes necessary for a healthy body (Katherine 2009). A surprising finding was that at specific sites on the body, different sets of microbes may perform the same function for different people. For examples, on the tongues of two people, two entirely different sets of organisms will break down sugars in the same way. This suggests that medical science may be forced to abandon the “one only microbe of infections disease, and rather pay attention to functions of groups of microbes that have some how gone away (Human Microbiome Project, 2012).


Populations of microbes (such as bacteria and yeasts) inhabit the skin and mucosal surfaces in various parts of the body (Pappas 2009). Their role forms part of normal, healthy human physiology, however if microbes numbers grow beyond their typical ranges (often due to a compromised immune system) or if-microbes populate (such as through poorly hygiene or injury) areas of the body normally not colonized or sterile (such as blood, or the lower respiratory tract, or the abdominal cavity) disease can result (causing, respectively bacteremia/sepsis, pneumonia, and peritonitis). It is estimated that 500-1000 species of bacteria live in the human body (Sears, 2009; Pappas, 2009).Bacteria cells are much smaller than human cells and is often said that they are at least ten times as many as bacteria as human cells in the body.

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Though people can survive without gut flora, the microorganism perform a host of useful functions, such as fermenting unused energy substrate, training the immune system, preventing growth of harmful pathogenic bacteria, regulating the development of the gut, producing vitamins for the host, such as biotin and vitamin K, and producing hormones to direct the host to store fats In return, these microorganism procure within the host a protected, nutrient rich environment in which they can thrive (Katherine and James, 1988).There is a point of view that human body contains approximate 1014 bacteria cells; while the number of cells has been calculated to be at least 1013 (Bianconi, 2013).

The mass of microorganism are estimated to account for 1-3% total body mass. Many of the bacteria in the digestive tract, collectively referred to as the gut flora, are able to break down certain nutrients such as carbohydrate that human other wise could not digest. The majority of the commmensals bacteria are anaerobes meaning they survive in an environment with no oxygen. Normal flora bacteria can act as opportunistic pathogens at times of lowered immunity (Beaugerie and Petit 2004).A number of bacteria live in the mouth, where they are part of a sticky substance called plaque and if this is not removed by brushing, it hardens into calculus (also called tartar). The same bacteria also secrete acid that dissolve tooth enamel, causing tooth decay.

Archaea are present in the human gut, but in contrast to the enormously variety of bacteria in this organ, the number of archaea species are much more limited (Sutter, 1984).The dominant groups are methanogens and methanosphaera stadtmanae (Florin et al., 2000).However, colonization by methanogen is variable and only about 50% of humans have easily.

A study of the area between toes in hundred young adults found fourteen different genera of fungi. These include yeasts such as candida albicens, Rhodotorula rubra, Torulopsis and Trichosporon cutaneun, dermatophyte(skin living fungi) such as Microsporum gypsum, and Trochophyton rubrum and non dermatophyte fungi (opportunistic fungi that can live in skin) such as Rhizopus stohoinfer, Trichosporon cutaneum, Eusarium Seopulariopsis brevicaulis, curvulains, Alternania alternaria, paecilomyces, Aspergillus flavus and Pencillium species (Marcon and Powell, 1992).

A study by the national Human Genome Research Institute in Bethesda, Maryland, researched the DNA of human skin fungi at fourteen different locations on the body. These were the ear canal, between the eyebrows, the back of the head, behind the ear, the heel, toenails, groin, nostrils, chest, palm, and the crook of the elbow. The study showed a large fungal diversity across the body, the richest habitat being the heel which host about 80 species of fungi. By way of contrast there are some sixty (60) specie in the nail clippings, and fourty (40) between the toes. Other rich areas are palms, forearm and inside the elbow, with values from eighteen (18) to thirty two (32) species. The head and trunk host between two (2) and ten (10) each (Eckburg et al., 2003).

The microflora can be commensals, mutualistic or pathogens. Often they can be all three depending upon the strength of the persons immune syste (Lee 2003). Research upon the immune system in the gut and lungs has shown that microflora aids immunity development (Rogers 2013): however such research has only started upon whether this is the case with the skin. Pseudomonas aeruginosa is an example of a mutualistic bacterium that can turn into a pathogen and cause disease. If it gains entry into the blood system it can result in infection in bone joint, gastrointestinal tract, and respiratory systems (Beaugerie and Petit, 2004). It can also cause dermatitis.

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However, pseudomonas aeruginosa produces antimicrobial substances such as pseudomonic acid (that are exploited commercially such as mupirocin). This works against Staphylococcal and streptococcal infections. Pseudomonas aeruginosa also produced substances that inhibit the growth of fungus species such as Candids krusel, Cadida albicans, Torulopssi glabrata, Saccharomyies cerevisiae and Aspergillus fumigates (Todar 2012).It can also inhibit the growth of Helicobacter pylori. So important is its anti microbial action that it has been noted that “removing E.aeruginosa from the skin, through use of oral or topical antibiotics, may inversely allow for aberrant yeast colonization and infection.

Another aspect of bacteria is the generation of body odour. Sweat is odourless, however, several bacteria may consume It and create by products which may be considered putrid by man as in contrast to flies, for example, that may find them attractive/appealing). Several examples are Propionibacteria in adolescence and adult sebaceous glands can turn its amino acid into propionic acid. Staphylococcus epidermidis create body odour by breaking sweat into isovaleric acid (3-methyl butanoic acid) Bacillus subtilis creates strong foot odour.

Skin provides good examples of various microenvironments. Skin regions have been compared to geographic regions of Earth.The desert of the forearm, the cool woods of the scalp, and the tropical forest of the armpit. The composition of the dermal microflora varies from site to site according to the character of the microenvironment (MacDougall, 2012).

A different bacterial flora characterizes each of the three regions of the skin.
• Axilla, perineum, and toe webs
• Hand, face, and trunk and
• Upper arms and legs. Skin sites with partial occlusion (axilla, perineum, and toe webs) harbor more microorganisms than do less occluded area (legs, arms, and trunk).

These quantitative differences may relate to increased amount of moisture higher body temperature and greater concentrations of skin surface lipids. The axilla, perineum and toe webs are more frequently colonized by Gram negative bacilli than are drier areas of the skin (Bianconi, 2013). The number of bacteria on an individuals’ skin remain relatively constant; bacterial survival and the extent of colonization propably depend partly on the exposure of the skin to a particular environment and partly on the innate and species –specific bactericidal activity in skin.

Also, high degree of specificity is involved in the adherence of bacteria to epithelial surfaces (Human Microbiome Project, 2012). The microbiology literature is inconsistent about the density of bacteria on skin, one reason for this is the variety of methods used to collect skin bacteria (Roth and Petit 1988). The scrub method yields the highest and most accurate count for a given skin area.

Most microorganisms live is the superficial layers of the stratum corneum and in the upper parts of the hair follicles (Roth and James, 1988). Some bacteria, however, reside in the deeper area of the hair follicles and are beyond the reach of ordinary disinfection procedures. These bacteria are a reservoir for re colonization after surface bacteria are removed (Pappas, 2009).

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