Before we jump into our topic, it’s better to know a little bit about microbes i.e. bacteria, viruses, and fungi etc. Microbes are tiny living things that are found all around us and are too small to be seen by the naked eye. They are found in almost every habitat present in nature, including hostile environments such as the North and South poles, deserts, geysers, and rocks. They are present inside and outside of our body. This association with our body is also known as normal flora. The normal flora influences the anatomy, physiology, susceptibility to pathogens, and morbidity of the host. Below are some associations with human body
SKIN FLORA
The varied environment of the skin results in locally dense or sparse populations, with Gram-positive organisms (e.g., staphylococci, micrococci, diphtheroids) usually predominating.
Enterobacter, Klebsiella, Escherichia coli, and Proteus spp. are the predominant among Gram-negative organisms found on the skin. Acinetobacter spp also occurs on the skin of normal individuals and, like other Gram-negative bacteria, is more common in the moist intertriginous areas (where skin folds).
NAIL FLORA
The microbiology of a normal nail is generally similar to that of the skin. Dust particles and other extraneous materials may get trapped under the nail, depending on what the nail contacts. In addition to resident skin flora, these dust particles may carry fungi and bacilli. Aspergillus, Penicillium, Cladosporium, and Mucor are the major types of fungi found under the nails.
ORAL AND UPPER RESPIRATORY TRACT FLORA
A varied microbial flora is found in the oral cavity, and streptococcal anaerobes inhabit the gingival crevice. The pharynx can be a point of entry and initial colonization for Neisseria, Bordetella, Corynebacterium, and Streptococcus spp.
The pharynx and trachea contain primarily those bacterial genera found in the normal oral cavity (for example, α-and β-hemolytic streptococci); however, anaerobes, staphylococci, neisseriae, diphtheroids, and others are also present. Potentially pathogenic organisms such as Haemophilus, mycoplasmas, and pneumococci may also be found in the pharynx. Anaerobic organisms also are reported frequently. The upper respiratory tract is so often the site of initial colonization by pathogens (Neisseria meningitides, C. diphtheriae, Bordetella pertussis, and many others) and could be considered the first region of attack for such organisms. In contrast, the lower respiratory tract (small bronchi and alveoli) is usually sterile, because particles the sizes of bacteria do not readily reach it. If bacteria do reach these regions, they encounter host defence mechanisms, such as alveolar macrophages, that are not present in the pharynx.
GASTROINTESTINAL TRACT FLORA
Organisms in the stomach are usually transient, and their populations are kept low (103 to 106/g of contents) by acidity. In normal hosts the duodenal flora is sparse (0 to 103/g of contents). The ileum contains a moderately mixed flora (106 to 108/g of contents). The flora of the large bowel is dense (109 to 1011/g of contents) and is composed predominantly of anaerobes.
Coliforms are bacteria that are always present in the digestive tracts of animals, including humans, and are found in their wastes. Typical genera include Citrobacter, Enterobacter,Hafnia, Klebsiella and Escherichia.
Concentrations of 109 to 1011 bacteria/g of contents are frequently found in human colon and faeces. This flora includes a bewildering array of bacteria (more than 400 species have been identified); nonetheless, 95 to 99 percent belong to anaerobic genera such as Bacteroides, Bifidobacterium, Eubacterium, Peptostreptococcus, and Clostridium.
UROGENITAL FLORA
The vaginal flora changes with the age of the individual, the vaginal pH, and hormone levels. Transient organisms (e.g., Candida spp.) frequently cause vaginitis. The distal urethra contains a sparse mixed flora; these organisms are present in urine specimens (104/ml) unless a clean-catch, midstream specimen is obtained.
The type of bacterial flora found in the vagina depends on the age, pH, and hormonal levels of the host. Lactobacillus spp. predominates in female infants (vaginal pH, approximately 5) during the first month of life. Glycogen secretion seems to cease from about I month of age to puberty. During this time, diphtheroids, S. epidermidis, streptococci, and E. coli predominate at a higher pH (approximately pH 7). At puberty, glycogen secretion resumes, the pH drops, and women acquire an adult flora in which L. acidophilus, corynebacteria, peptostreptococci, staphylococci, streptococci, and Bacteroides predominate. After menopause, pH again rises, less glycogen is secreted, and the flora returns to that found in prepubescent females. Yeasts (Torulopsis and Candida) are occasionally found in the vagina (10 to 30 percent of women); these sometimes increase and cause vaginitis.
In the anterior urethra of humans, S. epidermidis, enterococci, and diphtheroids are found frequently; E. coli, Proteus, and Neisseria (non-pathogenic species) are reported occasionally (10 to 30 percent). Because of the normal flora residing in the urethra, care must be taken in clinically interpreting urine cultures; urine samples may contain these organisms at a level of 104/ml if a midstream (clean-catch) specimen is not obtained.
CONJUNCTIVAL FLORA
The conjunctiva harbors few or no organisms. Haemophilus and Staphylococcus are among the genera most often detected.