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.