The human body is a host to a number of bacteria that are beneficial, and sometimes even necessary, to human life, called commensal bacteria. These useful bacteria are often overlooked due to the negative effects associated with other bacterial species, as well as to the problems associated with their overgrowth. A normal healthy person has bacteria living in their digestive tract, on their skin, and often eat it in their food.
Digestive tract flora
The most well known bacterium in the gastrointestinal tract is Lactobacillus acidophilus, which aids in digestion of lactose, the synthesis of vitamins (niacin, folic acid, and vitamin K), conversion of dietary fiber, recycling of amino acids from bile, and degradation of toxins. The byproducts of acidophilus-assisted digestion create an acidic environment, which competes out much more virulent microbes such as fungi and non-commensal bacteria. In women, this bacterium is also found in the vagina.
Another commensal bacterium is Streptococcus mutans. This strain is present in the mouth and converts the sugar sucrose to lactic acid. Though this species of bacteria is considered helpful, it can cause dental plaque and tooth decay it oral hygiene is not up to the standard of care. Similar to acidophilus, this bacterium competes out fungal infections and virulent Streptococcus and Staphylococcus strains.
The bacteria in the digestive tract also aid in developing a vigorous immune response; more than half of the body's immune tissue is located in the lining of the small intestine. The bacteria are discharged from the body daily and renewed without causing disease.
Staphylococcus epidermidis and Propionibacterium acnes (better known as P. acnes) are two bacterial species that are normally present on the skin. P. acnes lives off fatty acids and the oils secreted through the pores of the skin. S. epidermidis is generally not virulent, except in the case of patients with suppressed immune systems or catheters. These two bacteria species can cause infection, acne, or dental plaques if not kept in check, but they also compete out more virulent species of bacteria. Eliminating these colonizing species altogether would leave a person vulnerable to more severe infections.
Staphylococcus aureus is also found on the skin of some people, though more commonly in the nose (10-20% of people according to Sharing in Health), but doesn’t cause disease unless there is a break in the skin it can enter. This is the bacterial strain associated with MRSA (methicillin-resistant Staph. aureus).
People inhale and ingest virulent bacteria on a daily basis. Without the normal oral and intestinal flora, disease would be much more common and severe. Antibiotics actually end up depleting the intestinal flora leaving a person susceptible to other infections. This also occurs when antibiotic lotions or creams are used in excess upon the skin. Though the full effects are not yet known.
To counter the effects of antibiotics and infection on the flora of the human body, probiotics come into use in the 1990s. Bacteria being added to foods, including yogurt, which is itself a probiotic (live bacteria) culture. These cultures are expected to aid in immune responses and digestion as well as alleviate lactose intolerance. Many yogurt brands tout such "live and active cultures" of useful bacteria.
A number of bacterial species live on or in humans, playing roles that are helpful to the function of the human body. However, these microbes have to be kept in check to avoid infection. But they also must be retained to avoid non-beneficial microbes from taking up residence.