Characteristics of normal flora organisms

Characteristics of normal flora organisms

Certain properties of the normal flora have given them a selective advantage over bacteria, allowing them to become residents of the body. These properties may include physical characteristics such as the ability to adhere to body surfaces or metabolic characteristics such as the production of antimicrobial substances.

Adherence to host cells

Many species of the normal flora can adhere to the surface of host epithelial cells, the tissue cells that cover body surfaces; thus these species have a selective advantage over non-adhering microorganisms in colonizing the host. Adherence is the result of an interaction between the microbial cell surface and a chemical receptor on the body cell. Proteins or polysaccharide on the surface of the microbial cells, as well as pili which extend out from the microbial cell, have been implicated in adherence. The particular means of adherence varies with the species. Adherence allows the bacteria to multiply while they avoid removal by the flushing effects of surface fluids and peristalsis, a progressive wave of contraction that forces contents toward an opening. Microorganisms may often adhere specifically to one body site. For example, Streptococcus, salivarius adheres mainly to the surface of the tongue, whereas Streptococcus mutants selectively bind to the smooth enamel surface of the teeth.

A phenomenon that affects microbial adherence is desquamation, the detachment of host epithelial cells from body surfaces and replacement of the lost cells by new cells. In somebody sites like the intestinal tract, the rate of desquamation may be very high. One result of desquamation is the elimination of microorganisms that are not part of the normal flora (are transients) and that are only weakly attached to the epithelial cells. Normal flora microorganisms, however, have the ability to reattach firmly to the fresh epithelial layer and thus persist at this body size.

Production of antimicrobial substances

Some resident microorganisms produce metabolic products that can inhibit other microbes. For example, in the large bowel, certain anaerobic bacteria produce organic acids such as acetic, lactic, or butyric acid as metabolic waste products, these can inhibit the growth of the other bacteria. Some strains of skin staphylococci produce antibiotics that inhibit a wide variety of other bacteria.

Distribution and occurrence of the normal flora because bacteria account for most of the normal flora of the human body, this chapter deals mainly with the distribution and occurrence of various bacterial genera and species. Some fungi (mainly yeasts) and protozoa may also inhabit the human body, but their numbers are usually very low compared with bacterial flora. Intracellular parasites such as viruses do not belong to the normal flora of the body. However, some viruses can replicate themselves in tissues for long periods without causing any apparent disease symptoms. For instance, certain human intestinal viruses were discovered only by noting their destructive effects on nonhuman animal cells in laboratory tissue cultures; thus they were termed echoviruses (enteric cytopathogenic/man orphans). Similarly, coxsackieviruses, which also occur in the gastrointestinal tract of human hosts, were initially discovered only because of their pathogenicity for suckling mice in the laboratory; they apparently were not associated with human disease. Many echoviruses and coxsackieviruses have since been found to cause a number of diseases in humans -such as diseases of the respiratory tract, intestinal tract, and central nervous system. However, it must be recognized that not all of these viruses have been shown to have a disease-causing ability, even though they are obligate intracellular parasites.

In another example, chronic adenovirus infections are known to occur without causing disease symptoms. The presence of certain adenoviruses in the lymphoid tissue of normal individuals may become evident only after the tissue cells have been cultured in vitro in the laboratory. These studies suggest that some viruses apparently cause chronic asymptomatic infections.

Some predominant microorganisms of the normal flora summarize the predominant normal flora found in body sites. The normal flora of the body comprises many distinct species of microorganisms that are harmless and may even be beneficial in their usual locations in the human body. However, if they are introduced into other locations of the body. However, if they are introduced into other locations of the body, they may produce disease, especially if predisposing factors are present (the transient bacteremia discussed in the next paragraph is a good example.) For this reason, members of the normal flora may be described as opportunistic microorganisms when they cause disease.

Blood, body fluids, and tissues

In a healthy individual, the blood, cerebrospinal fluid, other body fluids, and tissues are normally free of microorganisms. Occasional microbes may cross-protective epithelial barriers as a result of trauma, such as tooth extraction or childbirth. For a very brief time, such microbes may be found in the bloodstream before they are filtered out in the pulmonary capillaries or removed by the cells of the immune system. Such transient bacteremia may cause infection of damaged or abnormal heart valves, leading to subacute bacterial endocarditis (inflammation of the membrane lining the heart and its valves.)

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.