Normal microbial flora, also called normal microbiota, refers to all of the beneficial bacteria that live in the mucous membranes and on the skin. They are considered beneficial because they defend against harmful bacteria, help destroy toxins, aid digestion and help the immune system to mature. Although beneficial, they are capable of causing disease in people with impaired immune systems, or if they reach an area of the body they do not normally inhabit.
Staphylococcus Epidermidis
This bacteria is one of the beneficial bacteria, as it is one of the normal microbial flora of the skin and being on the skin, it provides an important first line of defense against harmful bacteria. There are approximately 10,000 to 100,000 Staphylococcus epidermidis on each square centimeter of skin, according to Warren Levinson, M.D., Ph.D., Professor of Microbiology at the University of California in "Review of Medical Microbiology and Immunology." Some of them are on the outermost layer of the skin, while others are in the hair follicles and will travel to the outer skin layer after people wash their hands. This bacteria is also found in the mucous membranes of the throat.
Lactobacilli
Not long after a female infant is born, the Lactobacilli bacteria are found in the vagina. They stay there for several weeks, making the environment of the vagina acidic while they make acid from stored glucose, as explained in "Jawetz, Melnick, & Adelberg's Medical Microbiology" by George Brooks, M.D., Chief of the Microbiology Section at Clinical Laboratories. A mixture of various bacteria then inhabit the vagina and it stops being acidic. When she reaches puberty, however, so many Lactobacilli are now in the vagina, that it becomes acidic once again. The acidic environment created by the Lactobacilli protects the vagina from being inhabited by harmful microorganisms.
Viridans Streptococcus
Dr. Brooks explains in "Jawetz, Melnick, & Adelberg's Medical Microbiology" that the mucous membranes that line the throat and mouth do not usually have any bacteria in them initially, but they may get bacteria as the infant passes through the birth canal of the mother. By four to 12 hours after an infant is born, Viridans streptococcus is the main microbial flora of the mouth and throat, and it stays that way for life. Both S. epidermidis and V. streptococcus can cause endocarditis, or the inflammation of the inner layer of the heart muscle, if they gain access to the bloodstream and reach the heart. Just as S. epidermidis protects against harmful bacteria that may contact the skin, V. streptococcus is beneficial because it protects against harmful bacteria that may try to invade the mouth and throat.
Neisseria
Neisseria gonorrhoeae causes the sexually transmitted disease named gonorrhea and Neisseria meningitidis causes meningitis, or the inflammation and infection of the meninges membrane that covers the brain. Yet, there are other Neisseria bacteria that are part of the normal flora of the respiratory tract. Like Lactobacillus, they hardly ever cause any disease. Examples of this group include N. mucosa, N. flavescens, N. sicca, N. cinerea, N. lactamica and N. subflava. From three to 40 percent of people have N. lactamica, especially children, per Dr. Brooks in "Jawetz, Melnick, & Adelberg's Medical Microbiology." While this beneficial group of Neisseria protect against any harmful bacteria that may enter the respiratory tract, N. lactamica has proteins that specifically help the immune system to destroy the N. meningitidis bacteria that causes meningitis.
References
- "Jawetz, Melnick, & Adelberg's Medical Microbiology"; George Brooks, M.D., Karen Carroll, M.D., Janet Butel, Ph.D., Stephen Morse, Ph.D.; 2007
- "Review of Medical Microbiology and Immunology," Warren Levinson, M.D., Ph.D.; 2008
