Spite and virulence in the bacterium Pseudomonas aeruginosa

Abstract

Social interactions within populations of pathogenic microbes may play an important role in determining disease virulence. One such ubiquitous interaction is the production of anticompetitor toxins; an example of a spiteful behavior, because it results in direct fitness costs to both the actor and recipient. Following from predictions made by mathematical models, we carried out experiments using the bacterium Pseudomonas aeruginosa to test under what social conditions toxin (bacteriocin) production is favored and how this in turn affects virulence in the larvae of the wax moth Galleria mellonella. Consistent with theory, we found that the growth of bacteriocin producers relative to sensitive non-producers is maximized when toxin producers are at intermediate frequencies in the population. Furthermore, growth rate and virulence in caterpillars was minimized when bacteriocin producers have the greatest relative growth advantage. These results suggest that spiteful interactions may play an important role in the population dynamics and virulence of natural bacterial infections.

Fig. 1.

Modeling the relative growth and virulence of a bacteriocin producer. Output from our mathematical models across a range of resource competition (a), when c = 0.1 and k = 0.5, showing that producers growth (Upper) is maximized at intermediate frequencies and at these frequencies virulence (Lower) is attenuated.

Fig. 2.

Relative growth of bacteriocin producer. Relative growth rate of PAO1 vs. O:9. (producer, black circle) compared to relative growth rate of PAO1150-2 vs. O:9 (control, white triangle) along a range of different starting frequencies used to manipulate relatedness. PAO1 vs. O:9 shows a distinct peak in relative growth at intermediate frequencies (linear term, F_1,32 = 20.76, P < 0.001; quadratic term, F _1,31 = 29.64, P < 0.001).

Fig. 3.

Virulence and density affected by frequency of bacteriocin producer. (A) Time to death of caterpillars inoculated with PAO1/O:9 mixtures. Initial starting frequencies of PAO1 are indicated on the graph and correspond to the adjacent line. At the intermediate starting frequency death is significantly delayed (linear term, F_1,58 = 52.47, P < 0.001; quadratic term, F_1,57 = 55.85, P < 0.001). (B) The average total bacterial density of PAO1 and O:9 is indicated for the 3 different starting frequencies of the bacteriocin producer. A significant reduction in overall density occurs after 8 h of growth in the intermediate frequency treatment of PAO1 vs. O:9, where bacteriocin producers and sensitive non-producers are inoculated at initially near equal densities.

Fig. 4.

Virulence and density are unaffected by frequency when bacteriocins are not produced. (A) Time to death of caterpillars inoculated with PAO1150-2/O:9 mixtures. Initial starting frequencies of PAO1150-2 are again indicated on the graph and correspond to the adjacent line, but in this case there is no significant difference delay in time to death (linear term, F_1,58 = 1.28, P > 0.263; quadratic term, F_1,57 = 0.65, P < 0.422) (B) The average total bacterial density PAO1150-2 and O:9 is indicated for the 3 different starting frequencies of the bacteriocin-negative mutant. There is no significant difference in overall density after 8 h of growth in the caterpillars between the different starting frequencies.