Parasites and competitors suppress bacterial pathogen synergistically due to evolutionary trade-offs

Abstract

Parasites and competitors are important for regulating pathogen densities and subsequent disease dynamics. It is, however, unclear to what extent this is driven by ecological and evolutionary processes. Here, we used experimental evolution to study the eco‐evolutionary feedbacks among Ralstonia solanacearum bacterial pathogen, Ralstonia‐specific phage parasite, and Bacillus amyloliquefaciens competitor bacterium in the laboratory and plant rhizosphere. We found that while the phage had a small effect on pathogen densities on its own, it considerably increased the R. solanacearum sensitivity to antibiotics produced by B. amyloliquefaciens. Instead of density effects, this synergy was due to phage‐driven increase in phage resistance that led to trade‐off with the resistance to B. amyloliquefaciens antibiotics. While no evidence was found for pathogen resistance evolution to B. amyloliquefaciens antibiotics, the fitness cost of adaptation (reduced growth) was highest when the pathogen had evolved in the presence of both parasite and competitor. Qualitatively similar patterns were found between laboratory and greenhouse experiments even though the evolution of phage resistance was considerably attenuated in the tomato rhizosphere. These results suggest that evolutionary trade‐offs can impose strong constraints on disease dynamics and that combining phages and antibiotic‐producing bacteria could be an efficient way to control agricultural pathogens.

Rapid evolution of generalized resistance mechanisms can constrain the efficacy of phage–antibiotic treatments

“…PAS is thought to occur because two sufficiently different selective pressures are more likely to kill both nonresistant (susceptible to antibiotics) and antibiotic‐resistant (susceptible to phages) pathogen genotypes (Torres‐Barceló & Hochberg, 2016). Moreover, two concurrently acting selection pressures might impose evolutionary trade‐offs for the pathogen, which could constrain the evolution of resistance to antibiotics, phage or both (Chan et al., 2016; Torres‐Barceló & Hochberg, 2016; Wang et al., 2017). Here, we studied this using a model system where we exposed P. aeruginosa pathogen to both phage and sublethal concentration of gentamycin and monitored ecological and evolutionary dynamics in both homogenous (planktonic) and spatial (biofilm) dimensions of the microcosms.…”

“…Mutations that change the structure of these receptors has been shown to confer P. aeruginosa resistance to phages, but at the same time increase its susceptibility to antibiotics due to its less functional efflux pump (Chan et al., 2016). In the same way, it has recently been reported that phage selection can make Ralstonia solanacearum plant pathogenic bacterium more susceptible to antibiotics produced by Bacillus amyloliquefaciens biocontrol bacterium even though the mechanism has not yet been described in detail (Wang et al., 2017). Third, selection by phages and antibiotics could lead to increased costs of resistance, which could limit bacterial growth and infectiveness (Andersson & Hughes, 2010; Friman et al., 2016; Mumford & Friman, 2017).…”

“…Third, selection by phages and antibiotics could lead to increased costs of resistance, which could limit bacterial growth and infectiveness (Andersson & Hughes, 2010; Friman et al., 2016; Mumford & Friman, 2017). In support for this, it has been shown that phage selection can reduce P. aeruginosa and R. solanacearum growth and potential competitive ability most when bacteria evolved in the presence of both phage and antibiotics (Torres‐Barceló, Franzon, Vasse, & Hochberg, 2016; Wang et al., 2017), while phage resistance mutations have been shown to impair genes that are used for virulence leading to weaker infections (Addy, Askora, Kawasaki, Fujie, & Yamada, 2012; Chaturongakul & Ounjai, 2014; Torres‐Barceló et al., 2016). …”

Phage resistance evolution in vitro is not reflective of in vivo outcome in a plant‐bacteria‐phage system*

“…; Meaden and Koskella ; Wang et al. ; De Sordi et al. ), and none have directly compared the outcome of serially passaged bacteria and phage in vitro and in vivo .…”

Digest: Beating pathogens at their own game*

“…Wang et al. () propose that this pattern could be explained by an evolutionary trade‐off where the combination of the phage and B. amyloliquefaciens reduces population densities of R. solanacearum more than either antagonist individually. This trade‐off occurs because phage‐treated R. solanacearum populations become more susceptible to antimicrobial metabolites produced by B. amyloliquefaciens .…”