Abstract
Most opportunistic pathogens transit in the environment between hosts and the environment plays a significant role in the evolution of protective traits. The coincidental evolution hypothesis suggests that virulence factors arose as a response to other selective pressures rather for virulence per se. This idea is strongly supported by the elucidation of bacterial-protozoal interactions. In response to protozoan predation, bacteria have evolved various defensive mechanisms which may also function as virulence factors. In this review, we summarize the dual role of factors involved in both grazing resistance and human pathogenesis, and compare the traits using model intracellular and extracellular pathogens. Intracellular pathogens rely on active invasion, blocking of the phagosome and lysosome fusion and resistance to phagocytic digestion to successfully invade host cells. In contrast, extracellular pathogens utilize toxin secretion and biofilm formation to avoid internalization by phagocytes. The complexity and diversity of bacterial virulence factors whose evolution is driven by protozoan predation, highlights the importance of protozoa in evolution of opportunistic pathogens.
Highlights
Many bacterial pathogens are able to survive in non-clinical environments, where environmental conditions play an important role in the persistence and infectivity of those bacteria
The C. jejuni RND-type efflux pump, CmeABC which is associated with multidrug resistance, may be involved in virulence as well as survival in A. polyphaga (Vieira et al, 2017). These data highlight the role of these “virulence factors” in persistence and survival in the environment and likely evolved to protect bacteria from predation rather than for protection against antibiotics, given the long history of co-evolution of protozoa and bacteria
Horizontal gene transfer (HGT) is involved in resistance to antibiotics and virulence during infection (Juhas, 2015) and protozoan grazing has been shown to play an important role in HGT
Summary
Many bacterial pathogens are able to survive in non-clinical environments, where environmental conditions play an important role in the persistence and infectivity of those bacteria. Some mechanisms providing resistance to protozoan grazing may provide advantages during infection of human and animal hosts. As proposed in the review by Erken et al (2013), protozoan grazing is “a factor driving the evolution of human pathogens in the environment.”. Examples Supporting Coincidental Evolution pathogens involved in both resistance to predation and human pathogenesis, with an emphasis on advances made in the past 5 years. We compare these mechanisms using examples of intracellular and extracellular pathogens
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