The Rise of Pathogens: Predation as a Factor Driving the Evolution of Human Pathogens in the Environment

Abstract

Bacteria in the environment must survive predation from bacteriophage, heterotrophic protists, and predatory bacteria. This selective pressure has resulted in the evolution of a variety of defense mechanisms, which can also function as virulence factors. Here we discuss the potential dual function of some of the mechanisms, which protect against heterotrophic protists, and how predation pressure leads to the evolution of pathogenicity. This is in accordance with the coincidental evolution hypothesis, which suggests that virulence factors arose as a response to other selective pressures, for example, predation rather than for virulence per se. In this review we discuss some of those environmental factors that may be associated with the rise of pathogens in the marine environment. In particular, we will discuss the role of heterotrophic protists in the evolution of virulence factors in marine bacteria. Finally, we will discuss the implications for expansion of current pathogens and emergence of new pathogens.

The predatory life cycle of Myxococcus xanthus

“…The full impact of myxobacteria on microbial communities is difficult to assess, but they may be a resource for production of novel chemical structures. Secondary metabolite production for predation by myxobacteria in general is likely to be important for the evolution of attack and defence mechanisms that can specifically act as virulence factors in pathogens (Erken et al, 2013;Pukatzki & Provenzano, 2013).…”

Bacterial Sphingomyelinases and Phospholipases as Virulence Factors

“…Due to the striking similarities between the phagocytic and microbicide molecular machineries in amoebae and macrophages, these mechanisms are also effective for escape or survival within macrophages (324). Thus, virulence traits that help bacteria thrive within macrophages, likely emerged as adaptations for intraamoebal survival and multiplication (305,306,320,325). Therefore, obligate and facultative intracellular bacterial pathogens of animals and humans could have evolved after surviving phagocytosis by free-living amoebae and adapting in them to an intracellular lifestyle (305,306,320,326).…”

“…However, bacteria have existed for more than 3 billion years and adapted to a huge diversity of environments long before multicellular organisms appeared, about 600 million years ago (304,305). Thus, for bacterial pathogens that have a free-living state, selection of traits associated with virulence may have occurred in settings outside infection if they provide an advantage for survival in their natural environment, and virulence to multicellular organisms could be just coincidental (303)(304)(305)(306).…”

“…Bacterial survival in diverse ecosystems has been constrained for more than 800 million years by the pressure of predation by heterotrophic protists, and this is now considered a major force in the selection of traits serving as antipredator adaptations (316,317). Those traits might have evolved before the emergence of multicellularity, played a role in bacterial persistence, and only by chance provided enhanced fitness for pathogenicity in metazoans (304,306). Phagocytosis evolved in an ancestral unicellular eukaryote as a way to acquire food by predation of other microorganisms and was then maintained as a core function of the innate immune response in metazoans (317)(318)(319)(320).…”

“…Thus, virulence traits that help bacteria thrive within macrophages, likely emerged as adaptations for intraamoebal survival and multiplication (305,306,320,325). Therefore, obligate and facultative intracellular bacterial pathogens of animals and humans could have evolved after surviving phagocytosis by free-living amoebae and adapting in them to an intracellular lifestyle (305,306,320,326). A large number of bacterial species from different phylogenetic groups can withstand predation by protozoa and persist as intracellular parasites, suggesting that the capability to thrive within these microorganisms evolved several times independently (320,325).…”