Human Microbiome - Bacteria

Bacteria

Populations of microbes (such as bacteria and yeasts) inhabit the skin and mucosa. Their role forms part of normal, healthy human physiology, however if microbe numbers grow beyond their typical ranges (often due to a compromised immune system) or if microbes populate atypical areas of the body (such as through poor hygiene or injury), disease can result.

In 2012 some 200 researchers from some 80 research institutions comprising the Human Microbiome Project (HMP) Consortium have used advanced DNA-sequencing to identify and catalogue the thousands of microorganisms co-existing with humans. This study examined, amongst other things, the carbohydrate active enzymes from microbial populations from twelve sites on and in the human body, and concluded that microbes colonise each site to utilise the available sugars. Considerable variation was found in the enzymes for carbohydrate metabolism from site to site, and the researchers suggested that the composition of local carbohydrate metabolites may be the most important factor shaping the composition of microbial sub-communities of the human microbiome.

The same project examined the diversity of microbial communities present in multiple sites on the human body, using some 200 healthy persons and examining 18 sites on the body. Healthy individuals were found to host thousands of bacterial types, different body sites having their own distinctive communities. Skin and vaginal sites showed smaller diversity than the mouth and gut, these showing the greatest richness. The bacterial makeup for a given site on a body, varies from person to person, not only in type, but also in abundance. Bacteria of the same species found throughout the mouth, are of multiple subtypes preferring to inhabit distinctly different locations in the mouth. Even the enterotypes in the human gut, previously thought to be well-understood, are from a broad spectrum of communities with blurred taxon boundaries.

It is estimated that 500 to 1000 species of bacteria live in the human gut and a roughly similar number on the skin. Bacterial cells are much smaller than human cells, and there are at least ten times as many bacteria as human cells in the body (approximately 1014 versus 1013). The mass of microorganisms are estimated to account for 1-3% total body mass. Though members of the flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose, small intestine), the vast majority of bacteria live in the large intestine.

Many of the bacteria in the digestive tract, collectively referred to as the gut flora, are able to break down certain nutrients such as carbohydrates that humans otherwise could not digest. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen. Normal flora bacteria can act as opportunistic pathogens at times of lowered immunity.

Escherichia coli (a.k.a. E. coli) is a bacterium that lives in the colon; it is an extensively studied model organism and probably the best-understood bacterium of all. Certain mutated strains of these gut bacteria do cause disease; an example is E. coli O157:H7.

A number of types of bacteria, such as Actinomyces viscosus and A. naeslundii, live in the mouth, where they are part of a sticky substance called plaque. If this is not removed by brushing, it hardens into calculus (also called tartar). The same bacteria also secrete acids that dissolve tooth enamel, causing tooth decay.

The vaginal microflora consist mostly of various lactobacillus species. It was long thought that the most common of these species was Lactobacillus acidophilus, but it has later been shown that the most common one is L. iners followed by L. crispatus. Other lactobacilli found in the vagina are L. jensenii, L. delbruekii and L. gasseri. Disturbance of the vaginal flora can lead to bacterial vaginosis.