Abstract
Environment is recognized as a huge reservoir for bacterial species and a source of human pathogens. Some environmental bacteria have an extraordinary range of activities that include promotion of plant growth or disease, breakdown of pollutants, production of original biomolecules, but also multidrug resistance and human pathogenicity. The versatility of bacterial life-style involves adaptation to various niches. Adaptation to both open environment and human specific niches is a major challenge that involves intermediate organisms allowing pre-adaptation to humans. The aim of this review is to analyze genomic features of environmental bacteria in order to explain their adaptation to human beings. The genera Pseudomonas, Aeromonas and Ochrobactrum provide valuable examples of opportunistic behavior associated to particular genomic structure and evolution. Particularly, we performed original genomic comparisons among aeromonads and between the strictly intracellular pathogens Brucella spp. and the mild opportunistic pathogens Ochrobactrum spp. We conclude that the adaptation to human could coincide with a speciation in action revealed by modifications in both genomic and population structures. This adaptation-driven speciation could be a major mechanism for the emergence of true pathogens besides the acquisition of specialized virulence factors.
Highlights
Regarding their pathogenicity towards human beings, bacteria are commonly classified as true pathogens and opportunistic pathogens
This review describes the genomic evolution of Opportunistic Bacterial Pathogens (OBPs) of environmental origin, based on published studies as well as on original genome comparisons
In addition to xenobiotic transporters, the lifestyle of bacteria in the rhizosphere predisposes them to become OBPs, thanks to several properties: production of bio-surfactants, competition for nutrients and minerals and degradation of pathogenicity factors naturally produced by phytopathogenic bacteria, which could be similar to those produced by the human immune cells
Summary
Regarding their pathogenicity towards human beings, bacteria are commonly classified as true (or strict) pathogens and opportunistic (or facultative) pathogens. OBPs are generally considered as harmless bacteria devoid of specific virulence factors This is true for several mild OBPs that cause diseases only when patients are deeply immunocompromised, debilitated or subject to invasive procedures. Notorious OBPs such as Pseudomonas aeruginosa are well known over-armed pathogens that can exhibit a battery of virulence factors [5] On closer analysis, these so-called virulence factors primarily serve general adaptation purposes, and it is their association that enables OBPs to infect susceptible hosts. One can observe a continuum from bacterial adaptation to a host to non-specific virulence to specialization as strict pathogens. In this wide array of biodiversity—adaptation and virulence properties—a comprehensive understanding of OBPs is difficult to achieve.
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