The role of microbes in chronic inflammatory diseases
As many of the more obvious effects of microbes in disease have already
been elucidated, the more subtle, delayed, and chronic effects are
increasingly being studied. Methodological advances have led to the
detection of microbial communities in tissues previously thought to be
sterile in healthy individuals, e.g., the blood[4], the
lungs[5], and possibly even the brain[6]. Despite these recent
advances, a problem that has become increasingly recognized is the
difficulty of detecting rare species/strains. The situation has been
compared to an iceberg, with much of the microbial diversity under the
surface still undetected[7].
This limited attention to rare species may seem justified, since
abundant species may be considered to be more important. However, the
low abundance oral bacteria and likely PHM, Poryphyromonas gingivalis,
has been implicated in varied diseases. Also, antifungal therapy has
shown benefit in asthma even when fungi have been too low in abundance
to be detected[8].
Presumably at least some of these rare microbes come from the
environment; however, for various reasons, they do not become abundant
in the human body. The less abundant and often unknown microbes that are
changing with human cultural practices may play a key role in disease.
It is known from ecological studies of diverse environments that there
are typically many rare species, often with patchy distributions.