Host-Pathogen Interactions: Basic Concepts of Microbial Commensalism, Colonization, Infection, and Disease

Most of the terminology used to define the host-microbe interaction has been in use for nearly a century.Early in this period, microbes were thought to be primary aggressors that governed the host-pathogen interaction, resulting in disease.Later, new information about the attributes of microbes and their hosts resulted in the understanding that the hostpathogen interaction does not always result in disease.This recognition, in turn, led to the introduction of terms to explain states in which microbes exist within hosts without causing overt disease and why some microbes only cause disease in certain hosts.Commensal, carrier state, and opportunist were terms put forth to account for microbes and conditions that were sometimes associated with disease but for which Koch's postulates could not be fulfilled for one reason or another.Most of these terms were originally proposed to describe the behavior of particular microbes, rather than to define a more general host-microbe relationship.Recently, we reviewed the concepts of virulence and pathogenicity and described how the definitions for these terms changed over the years as microbiologists tried to find ways to convey that microbial pathogenesis reflects an interaction between two entities, host and pathogen (7).Based on the concept that host damage was the most relevant outcome of the host-pathogen interaction, we proposed revisions to the definitions of the terms pathogen, pathogenicity, and virulence (7).However, the proposed framework suggested a need to reexamine the terms used to define the outcomes of host-microbe interactions.Here, we critically review the origin and historical evolution of key concepts used to describe the outcome of host-microbe interactions, namely, infection, commensalism, colonization, persistence, infection, and disease.We propose that the meaning of these terms can be clarified by placing them in the context of the damage framework put forth previously (7). LEXICON OF MICROBIAL PATHOGENESISOnce the germ theory of disease was accepted, microbes were considered to be pathogens if they met the stipulations of Koch's postulate.However, it rapidly became apparent that (i) although there are many microbes, most human infections were caused by only a few; (ii) some microbes were classified as pathogens although they did not cause disease in every host; and (iii) some microbes were classified as nonpathogens, although they did cause disease in certain hosts (for an early review, see reference 56).In addition, it became evident that