Even though some parts of the human body are free from microorganisms (axenic state), the body harbours millions of mutualistic and commensal symbionts. Each of us unwittingly carries a population of bacterial cells on our skin and in our mouth and digestive tract that outnumbers cells carrying our own genome by approximately ten to one.
THE HUMAN MICROBIOME
Even though some parts
of the human body are free from microorganisms (axenic state), the body harbours millions of mutualistic and
commensal symbionts. Each of us unwittingly carries a population of bacterial
cells on our skin and in our mouth and digestive tract that outnumbers cells
carrying our own genome by approximately ten to one. Mutualistic relationships occur when both organisms (microbial and
host cell) benefit from the interactions, whereas commensalism is a state in which one member of the relationship benefits
without significantly affecting the other. Microorganisms that colonize body
surfaces (internal and external) without normally causing disease constitute
the microbiome. The microbiome
comprises a number of distinct microbiotas which are characteristic of the
region of the body colonized.
The microbiome is
present throughout life and may comprise predominantly bacteria, with some
fungi and protozoa, the majority of which are commensal. The microbiome begins
to develop when the amniotic membrane surrounding the unborn child ruptures,
allowing contact with vaginal, faecal and skin associated microorganisms from
the mother during childbirth. Microorganisms enter the infant’s mouth and nose
during passage through the birth canal, colonization of the upper respiratory
tract occurs with the first breath of air, and the beginning of the colon microbiota
occurs during feeding. The development of the resident microbiota is therefore
initiated during the first few months of life. By comparison, transient
microbiotas remain in the body for only a few hours, days or months. They are
found in the same locations as the resident flora, but cannot persist because
of their inability to compete with the microbiome, to resist elimination by the
body’s defence mechanisms or to tolerate the chemical and physical changes
encountered.
Changes in relative abundance
of normal microbiota, for whatever reason (e.g. major changes to diet,
antibiotic treatment, hormonal changes, chemo or radiotherapy), may allow one
or more members of the microbiome to become opportunistic pathogens. For
example, reductions in numbers of protective lactobacilli within the vagina
brought about through antibiotic use can allow Candida albicans, a minority member of the normal microbiota, to grow more prolifically,
resulting in an opportunistic vaginal yeast infection such as vaginal candidiasis.
Following appropriate treatment, the normal flora is typically re-established
in adults. In some areas of the body, such as the gastrointestinal tract, it is
claimed that the recolonization by desired species can be encouraged by
administration of probiotics. These are live cultures of intestinal bacteria
that are marketed as conferring a health benefit and preventing digestive problems.
Prebiotics, (normally fibre like carbohydrates) that represent preferred or
selective growth substrates for probiotic bacteria already present within the
digestive tract are also of demonstrable benefit.
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