The membrane that forms the lining of the eyelids and eyeball is called the conjunctiva. It is a very delicate membrane that contains a sparse microflora due to the following factors: The skin offers good examples of different microenvironments. Skin regions were compared to geographical regions of the earth: forearm desert, cool scalp forests, and armpit rainforest. The composition of dermal microflora varies from site to site depending on the character of the microenvironment. A different bacterial flora characterizes each of the three skin regions: (1) the armpits, perineum and nets; (2) the hand, face and torso; and (3) upper arms and legs. Skin areas with partial occlusion (armpits, perineum and toe nets) harbor more microorganisms than less closed areas (legs, arms and trunk). These quantitative differences may be related to an increased amount of moisture, higher body temperature, and higher concentrations of skin surface lipids. The armpits, perineum and toe nets are more frequently colonized by gram-negative bacilli than by areas of drier skin. Disturbances in normal flora make the host cell susceptible to pathogens. Therefore, the study of normal microflora allows us to better understand infections at the specific site. Normal flora of the gastrointestinal tract The bacterial flora of the gastrointestinal (GI) tract of animals has been studied more thoroughly than that of any other site. The composition differs between different animal species and within an animal species. In humans, there are differences in the composition of flora, which are influenced by age, diet, cultural conditions and the use of antibiotics.
The latter strongly disrupts the composition of the intestinal microflora. Although the above suggests that bacterial flora may be undesirable, studies on animals treated with antibiotics suggest that the flora protects individuals from pathogens. The researchers used streptomycin to reduce normal flora and then infected the animals with streptomycin-resistant salmonella. Normally, about 106 organisms are needed to establish gastrointestinal infection, but in streptomycin-treated animals with altered flora, fewer than 10 organisms were needed to cause infectious diseases. Other studies have suggested that fermentation products (acetic acid and butyric) produced by normal flora inhibit the growth of salmonella in the gastrointestinal tract. Figure 6-2 shows some of the factors that are important in the competition between normal flora and pathogenic bacteria. This area is free of microorganisms. The trachea and bronchi have no normal flora due to the upward movement of mucus driven by the cilia. However, microorganisms that enter the alveoli are destroyed by alveolar macrophages via phagocytosis. Eikenella corrodens, which is also part of the normal oral flora, causes skin and soft tissue infections associated with human bites and “clenched fist” injuries (i.e. hand injuries that occur during fights).
The nose and perineum are the most common sites for colonization of S. aureus, which is present in 10 percent to more than 40 percent of normal adults. S. aureus is prevalent on vulvar skin (67%). Its appearance in the nasal passages varies with age, is greater in newborns, less in adults. S. aureus is extremely common (80-100%) on the skin of patients with certain dermatological conditions such as atopic dermatitis, but the reason for this finding is unclear. S. epidermidis is a major inhabitant of the skin, accounting for more than 90 percent of the resident aerobic flora in some areas.
Normal skin flora The adult human is covered with about 2 square meters of skin. The density and composition of normal skin flora varies depending on the anatomical location. The high moisture content of the armpit, groin and the areas between the toes promotes the activity and growth of relatively high densities of bacterial cells, but the density of bacterial populations in most other places is quite low, usually in the 100s or 1000s per square centimeter. Most of the bacteria on the skin are bound in the sweat glands.