Rapid evolutionary adaptation to elevated salt concentrations in pathogenic freshwater bacteria Serratia marcescens

Abstract

Rapid evolutionary adaptions to new and previously detrimental environmental conditions can increase the risk of invasion by novel pathogens. We tested this hypothesis with a 133-day-long evolutionary experiment studying the evolution of the pathogenic Serratia marcescens bacterium at salinity niche boundary and in fluctuating conditions. We found that S. marcescens evolved at harsh (80 g/L) and extreme (100 g/L) salt conditions had clearly improved salt tolerance than those evolved in the other three treatments (ancestral conditions, nonsaline conditions, and fluctuating salt conditions). Evolutionary theories suggest that fastest evolutionary changes could be observed in intermediate selection pressures. Therefore, we originally hypothesized that extreme conditions, such as our 100 g/L salinity treatment, could lead to slower adaptation due to low population sizes. However, no evolutionary differences were observed between populations evolved in harsh and extreme conditions. This suggests that in the study presented here, low population sizes did not prevent evolution in the long run. On the whole, the adaptive potential observed here could be important for the transition of pathogenic S. marcescens bacteria from human-impacted freshwater environments, such as wastewater treatment plants, to marine habitats, where they are known to infect and kill corals (e.g., through white pox disease).

Keywords: Experimental evolution; fluctuating environment; harsh environment; niche expansion; pathogen invasions; tolerance curve.

Figure 1

Weekly population biomasses (optical density) during the selection experiment in stable environment treatments. Note that few missing data points are due to lost samples during the sampling process.

Figure 2

(A) Average biomass (optical density) ± SE in different salinity treatments during the selection experiment and (B) stability of the population biomass. Letters a–c indicate homogenous subsets that are not different from each other.

Figure 3

Evolutionary changes in tolerance to different levels of salinity measured by maximum obtained biomass (optical density) ± SE. Letters a–d indicate homogenous subsets based on pairwise comparisons of yield of differentially evolved clones (ancestral, 0, 80, 100 g/L and fluctuating salt concentrations) in different measurement salt concentrations. Note that all pairwise comparisons are corrected for multiple comparisons with Bonferroni correction.