The Human Body as an Ecosystem

From a microbes “point of view” the human body is a vast array of surfaces some internal, others external that are continuously colonized by endemic and exotic microbes beginning from the time of our birth and continuing throughout life up until and even beyond the point of death.  The surfaces on which these organisms grow and are constantly being sloughed off as new tissues form beneath existing ones resulting in the loss of established biofilms but providing new and uncolonized, or at least less colonized, cell surfaces on which new biofilm communities can be come established. 

These interconnected ecosystems vary from highly abraded, exceptionally dry habitats on the surface of the skin (e.g. the elbow) to constantly moist, highly nutritious membranous surfaces (the conjunctiva of the eye), from niches of low population density and a pH of 3 (stomach) to habitats with a hydrogen ion concentration a million fold lower and a population density of 10¹¹ cells /gram (gut).  Then, within the same body, there are vast areas which, in a state of health, remain sterile  (blood) or which have a very low biologically controlled  microbial population (lungs).

Occasionally, due to preexisting pathology (Cystic fibrosis, diabetes), anatomical anomaly (middle ear orientation in children), injury (either accidental or surgical), or the implantation of some foreign object (again either accidental or surgical), novel niches are presented capable of being colonized.  These biofilm populations may result in pathology and due to their antimicrobial resistence, these “misplaced” biofilms are often intractable and may result in dire, even lethal, consequences for the host and the expenditure of billions of dollars in added medical costs.

Many volumes would be necessary to describe all the sorts of benefits and dangers that biofilms bring about.  Our goal here is to outline the major ways in which biofilms protect us under the best of circumstances and threaten us when we are at risk.  A description of the nature of biofilms, which offer us protection and the circumstances that place us at risk will be our emphasis.