Evolution of increased virulence is associated with decreased spite in the insect-pathogenic bacterium Xenorhabdus nematophila

Abstract

Disease virulence may be strongly influenced by social interactions among pathogens, both during the time course of an infection and evolutionarily. Here, we examine how spiteful bacteriocin production in the insect-pathogenic bacterium Xenorhabdus nematophila is evolutionarily linked to its virulence. We expected a negative correlation between virulence and spite owing to their inverse correlations with growth. We examined bacteriocin production and growth across 14 experimentally evolved lineages that show faster host-killing relative to their ancestral population. Consistent with expectations, these more virulent lineages showed reduced bacteriocin production and faster growth relative to the ancestor. Further, bacteriocin production was negatively correlated with growth across the examined lineages. These results strongly support an evolutionary trade-off between virulence and bacteriocin production and lend credence to the view that disease management can be improved by exploiting pathogen social interactions.

Keywords: bacteriocins; spite; virulence.

Figure 1.

Increased virulence (a) of the 14 evolved lineages (light grey bars) relative to the ancestor (dark grey bar) is shown as a lower mean (±s.e.) time to host death. The evolved, more virulent lineages show (b) reduced bacteriocin production and (c) faster growth relative to the ancestor. Reduced bacteriocin production is shown as a lower mean (±s.e.) relative lag time, indicating that the bacteriocin extracted from the evolved lineages was less inhibitory than the bacteriocin extracted from the ancestor. Growth is shown as mean (±s.e.) OD₆₀₀ after 4.5 h culturing in vitro.

Figure 2.

Time to host death (a) and bacteriocin production (b) are negatively correlated with growth (OD₆₀₀ after 4.5 h culturing in vitro). Shorter time to host death indicates greater virulence. Bacteriocin production is shown as the relative lag time value exhibited by all evolved and ancestral lineages against both sensitive strains.