Normal Human Microbial Flora PDF

# LESSON 11 Normal Human Microbial Flora ## COURSE MATERIAL The human body is a haven for myriads of microbes, associated with the skin and mucus membranes from birth until death. The normal microbial flora affects the anatomy, physiology, susceptibility to pathogens, and morbidity of the host. Na...

# LESSON 11 Normal Human Microbial Flora ## COURSE MATERIAL The human body is a haven for myriads of microbes, associated with the skin and mucus membranes from birth until death. The normal microbial flora affects the anatomy, physiology, susceptibility to pathogens, and morbidity of the host. Natural microflora may aid host such as competing for microenvironment more effectively or by producing nutrients the host can use. The normal flora in humans usually develop in an orderly sequence, or succession, after birth, leading to the stable populations of bacteria that make up the normal adult flora. Environmental factors such as temperature, pH, redox potential, and oxygen, water, and nutrients levels, determine the composition of the normal flora in a body region. Other factors such as peristalsis, saliva, lysozyme secretion, and secretion of immunoglobulins also play role in floral control. A good example is that, an infant is exposed to the organisms as it moves through the birth canal. A gram-positive bacterial population (Bifidobacteria and Lactobacilli) predominates in the gastrointestinal tract early in life if the infant is breast-fed. This bacterial population is reduced and displaced somewhat by a Gram-negative flora (Enterobacteriaceae) when the baby if bottle fed. The type of liquid diet provided to the infant is the principal instrument of this floral control; immunoglobulins and, perhaps, other elements in breast milk may also be important. ## Normal Flora of the Skin The dermal flora varies from site to site, according to the character of the microenvironment. Skin sites with partial occlusion or partially blocked, such as the axilla, perineum, and toe webs, harbor more microorganisms that do less occluded areas like the legs, arms and trunk. The difference is due to the amount of moisture, higher body temperature, and greater concentrations of skin surface lipids. The normal microbiota of the skin tends to inhibit transient microbe colonization by producing antimicrobial substances and outcompeting other microbes that land on the surface of the skin. It helps to protect the skin from pathogenic infection. The partially occluded areas are more frequently colonized by Gram-negative bacilli than drier areas of the skin. Generally, nonpathogenic bacteria are commensal but if, a normal inhabitant of the skin like Propionibacterium becomes trapped in hair follicle, it may grow rapidly and cause inflammation and acne. Examples of normal skin flora are species of: Staphylococci epidermidis, Micrococci sp., and Corynebacterium. ## Normal Flora of the Conjunctiva The conjunctiva is kept moist and healthy by the secretions of the tears by the lachrymal glands. Blinking wipes, the conjunctiva every few seconds mechanically washing away foreign objects including bacteria. There is a little opportunity for microorganisms to colonize the eyes. Still a variety of bacteria may be cultivated from the normal conjunctiva, but the number of organisms is usually few. Staphylococcus epidermidis, and Propionibacterium acnes are dominant. Staphylococcus aureus, some streptococci, Hemophilus sp. and Neisseria sp. are occasionally found. Pathogens which infect the conjunctiva like the Neisseria gonorrhoeae and Chlamydia trachomatis are thought to be able to attach to the conjunctival epithelium. ## Normal Oral cavity and Upper Respiratory Tract Flora The upper respiratory tract is practically in direct contact with the external environment. The oral cavity is secondary opening for the respiratory tract. The upper respiratory tract and the oral cavity contains an abundant and diverse microbiota. The most common bacteria identified in the area includes Staphylococcus epidermis, Viridans Group Streptococci (VGS), Corynebacterium sp. (diphtheroids), Propionibacterium spp., and Haemophillus spp. Variable number of bacterial species of Prevotella, Fusobacterium, Moraxella, and Eikenella as well as Candida fungal isolates are present in the oropharynx area. The pharynx, can be colonized with pathogenic strains of Streptococcus, Haemophilus, and Neisseria. The lower respiratory tract, have fewer microbes where the most common are species of Pseudomonas, Streptococcus, Prevotella, Fusobacterium and Veillonella. But it is not clear if they constitute the normal microbiota or they are transients only. ## Normal Flora of the Gastrointestinal Tract Population of organisms in the stomach is kept low by the acids. The duodenal flora is relatively sparse, consisting mainly of Gram-positive lactobacilli and Enterococcus faecalis. Ileum, the distal part of the small intestine contains greater number of bacteria. It contains moderately mixed flora, consisting of same species plus coliforms, or Escherichia coli and relatives, and Bacteriodes. While the large intestine is thickly populated by mostly anaerobic bacteria. The species coliforms, enterococci, clostridia and lactobacilli can be regularly found, but the predominant species are anaerobic Bacteriodes and anaerobic lactic acid bacteria in the genus Bifidobacterium. Sometimes, significant numbers of anaerobic methanogens_may reside in the colon of humans. This is the only direct association with archaea as normal flora. These organisms play a role in bile acid conversion and in vitamin K and ammonia production. But their most important effects are in their ability to protect their host from establishment and infection. On the other hand, some of the bacteria in the colon have evidences of production of metabolites that are carcinogenic, and may have increase the incidence of colon cancer. Poor nutrition and or alteration of gut flora with antibiotics can cause shift in populations and colonization by nonresidents that leads to gastrointestinal disease. ## Normal Flora of the Urogenital Tract Staphylococcus epidermidis, Enterococcus faecalis and some alpha-hemolytic streptococci are relatively normal microflora of the anterior urethra. In addition, contaminants from the skin, vulva, or rectum like E.coli, Proteus and corynebacteria, are occasionally found in urine samples. Urine is sterile and urination flush the urinary tract thus few microorganisms can thrive the interior of the urethra. Corynebacteria, staphylococci, streptococci, E. coli, and lactic acid bacterium Lactobacillus acidophilus, colonized the vagina soon after birth. Due to the actions of circulating estrogen from puberty to menopause, the vaginal epithelium contain glycogen which in turn metabolized by L. acidophilus and produce lactic acid, resulting to low pH of the vaginal epithelium. This inhibits the growth of potentially pathogenic yeast, Candida albicans. ## READINGS: **Normal Flora** [https://www.ncbi.nlm.nih.gov/books/NBK7617/](https://www.ncbi.nlm.nih.gov/books/NBK7617/) [The Normal Bacterial Flora of Humans](http://www.textbookofbacteriology.net/normalflora_3.html) [Watch: Microorganisms and Humans: Commensal and Pathogenic Flora](https://www.youtube.com/watch?v=JYRkXhT1XEs&t=7s)