Human Microbiota: The Residential Microbes of Human Body and Their Potential Pathogenicity

By Bunmi and Tayo Fasuan

Microbiota are the collection of microorganisms that live in peaceful coexistence with their hosts (Gill et al., 2006). They are also referred to as microflora, or normal flora of the body, though technically, microbiota is regarded as the more appropriate term. Microbiota colonize virtually every surface of the human body that is in contact with the external environment, including the skin, genitourinary, gastrointestinal, and respiratory tracts. According to Kunz et al., (2009), it is estimated that the human microbiota contains as many as 10¹⁴ bacterial cells, a number that is 10 times greater than the number of human cells present in our bodies. The human microbiota are influenced by various factors, like genetics, age, sex, stress, nutrition and diet of the individual.

Microbiota play a significant role in the balance between disease and health. They provide protection against incoming pathogens, making their immediate environment inhospitable and antagonistic against incoming pathogens. They achieve this by producing biosulphactants, competing for nutrients and sites of attachment, and producing antimicrobial substances.

They also promotes maturation of secondary lymphoid organs in the intestine which are the organs involved in the first line of defence against foreign materials in the intestine. The intestinal microbiota synthesize vitamin K and also help in the absorption of nutrients.

Despite all these beneficial sides, human microbiota may cause endogenous diseases if they reach sites or tissues where their growth cannot be restrained or where they cannot be tolerated by the host defence. For instance, Streptococcus spp of the viridians groups are the most common residents of the upper respiratory tracts. If they are incidentally or accidentally introduced into the bloodstream following, for example, tooth extraction or tonsillectomy, they may settle on deformed prosthetic heart valves and cause infective endocarditic.

Also, Bacteriodes spp are the normal resident bacteria of the large intestine and are quite harmless in that location. If introduced into the free peritoneal cavity or into pelvic tissues along with other bacteria as a result of trauma, they cause suppuration (formation of pus) and bacteraemia (presence of free bacterial cells in the blood stream).

Overgrowth of the human microbiota can also lead to systemic diseases. When the immune systems are not able to control the microbiota to prevent its overgrowth and translocation to systemic sites, diseases such as small intestine bacterial overgrowth (SIBO) occur. Most gut microbiota are in the colon, with relatively few in the small intestine. But due to changes in the intestinal anatomy, gastric acid secretion and motility of the microorganisms, microbiota may migrate from the colon up into the small intestine, resulting into SIBO. This makes many of the intestinal microbiota potential, opportunistic pathogens because when they gain access to and invade an immune-compromised tissue, they cause disease.

The issue of human microbiota, however, is a double edge sword. Resultant imbalances between the resident microbiota, external pathogens and the immune system have been associated with pathological conditions such as inflammatory bowel disease (IBD), diabetes, obesity and others. These imbalances may be due to suppression of the internal environment through the use of antibiotics, which makes the body conducive and receptive for opportunistic external pathogens to enter, colonise, proliferate and cause diseases.

Other factors include altered diets, accidents (burn), lifestyle change (e.g. tattooing), legal and illegal uses of immuno-suppressive drugs and many others. For instance, a breach on the protective wall of the skin can lead to the normal skin flora, Staphylococcus epidermidis to get into the blood system and cause infection. This is what usually leads to the wrong assumption that Staphylococcus is a causative agent of sexually transmitted infection. Also, burned patients will experienced loss of Staphylococcus epidermidis and will be exposed to host of external microbes, especially the well-known pathogenic Pseudomonas aeromonas.

Tattooing can introduce pathogenic organisms deep into the body during the tattooing process and suppression of the immune system illegally (using steroids) or legally (during organ transplants) is also a well-known means of allowing non-resident bacteria to gain entry into the body thrive at the expense of the residential ones. Excessive growth of the normal skin flora, Propionibacterium acnes, which is usually present on the face, can lead to serious acne infection if there is also a corresponding sebum production under the skin.

However, the indiscriminate use of antibiotics is perhaps the major factor that can alter the equilibrium between resident microorganisms and external microbes. For example, antibiotics abuse can reduce the number of normal residential Escherichia coli in the gut, paving way for colonization of the gut by the more pathogenic Clostridium difficile. Once this organism colonizes the gut, it produces endotoxins which can cause colitis and severe diarrhoea.

Furthermore, reduction in the population of lactobacilli in the virginal wall/epithelium through the indiscriminate use, abuse or misuse of antibiotics or pH fluctuation, allows more resistant pathogenic microbes to gain foothold and multiply. This can lead to bacterial vaginosis by Gardnerella vaginalis, Mobiluncus spp, Bacteriodes spp and Mycoplasma spp. The absence of Lactobacilli may also lead to the proliferation of the pathogenic fungi Candida albicans, which causes the yeast infection popularly called Candidiasis.

Conclusively, the human microbiota are important part of our defense system, creating opposition to colonization from external microbes. However, the right balance must be maintained as either their increase or reduction in population can lead to adverse effect on the human body. Indiscriminate use of drugs and other lifestyles which may alter this equilibrium must be avoided so that these microorganisms can function well in their capacity to keep us healthy.