Figure 4: Pathogen virulence and fitness.

(a) Virulence of ancestral and evolved S. aureus populations in C. elegans hosts after 24 h of exposure. Quasibinomial GLM: F=25.7, df=2, P=4.627e−05. Tukey contrasts: co-evolution versus Ancestor: P<0.001; co-evolution versus Single evolution: P<0.001; Ancestor versus Single evolution: P=0.035. Sample size for each treatment: five biological replicates (average of two technical replicates). (b) Plot of virulence against the in vitro growth rate of ancestral and evolved S. aureus populations. Pearson’s product-moment correlation: t=3.38, df=9, P=0.008, R²=0.56. Sample size for virulence data: as a but all ancestral replicates averaged to one biological replicate; Sample size for growth rate data: five biological replicates (average of three technical replicates) for evolved bacteria. One biological replicate for ancestral bacteria (average of eight technical replicates). (c) Bacterial fitness (CFU per host) of ancestral and evolved S. aureus populations under co-colonization with ancestral E. faecalis. ANOVA: F=6.55, df=2, P=0.012. Tukey contrasts: Ancestor versus co-evolution: P=0.035; Single evolution versus co-evolution: P=0.015; Single evolution versus Ancestor: P=0.883. Sample size for each treatment: five biological replicates (one technical replicate). *P<0.05, **P<0.01, ***P<0.001.