Spite versus cheats: competition among social strategies shapes virulence in Pseudomonas aeruginosa

Abstract

Social interactions have been shown to play an important role in bacterial evolution and virulence. The majority of empirical studies conducted have only considered social traits in isolation, yet numerous social traits, such as the production of spiteful bacteriocins (anticompetitor toxins) and iron-scavenging siderophores (a public good) by the opportunistic pathogen Pseudomonas aeruginosa, are frequently expressed simultaneously. Crucially, both bacteriocin production and siderophore cheating can be favored under the same competitive conditions, and we develop theory and carry out experiments to determine how the success of a bacteriocin-producing genotype is influenced by social cheating of susceptible competitors and the resultant impact on disease severity (virulence). Consistent with our theoretical predictions, we find that the spiteful genotype is favored at higher local frequencies when competing against public good cheats. Furthermore, the relationship between spite frequency and virulence is significantly altered when the spiteful genotype is competed against cheats compared with cooperators. These results confirm the ecological and evolutionary importance of considering multiple social traits simultaneously. Moreover, our results are consistent with recent theory regarding the invasion conditions for strong reciprocity (helping cooperators and harming noncooperators).

Figure 1

Mutant invasion conditions as a function of local competition a, and local frequency, p. Regions favoring investment in spite only (dashed line, eq. A5), cooperation only (dotted line, eq. A7), and spite given cooperation (solid line, eq. A8). Models are derived in the Appendix. Parameters are c = 0.4, d= 0.2, k = 2, b = 2.

Figure 2

Fitness of spiteful strains as a function of initial frequency p and scale of competition a. (A) Fitness of spiteful strain competing against a sensitive strain, no difference in cooperative investments (eq. A4). Solid line = purely local competition (a= 1), dotted line = largely local competition (a = 0.7). Parameters are c = 0.4, d= 0.2, k = 2, b = 2. (B) Fitness of spiteful cooperator competing against sensitive cheat (eq. A3). Solid lines, a= 1. dotted line, a= 0.7. Parameters are c = 0.4, d= 0.2, k = 2, b = 2.

Figure 3

Virulence (mean within-host growth rate) as a function of frequency of focal strain. For all lines, the resident strain displays neither social trait, and has virulence = 1. Dotted line = a focal cooperative strain. Dashed line = a focal spiteful strain. Solid line = a focal cooperative and spiteful strain. Other parameters, b = 2, c = 0.4, d = 0.2, k = 2.

Figure 4

Relative growth of spiteful compared to nonspiteful bacteria when competed against sensitive cooperators (i.e., all strains produce siderophores) in iron-limited media. Spiteful bacteriocin production (PAO1: siderophore and bacteriocin producer vs. O:9: siderophore producer sensitive to bacteriocin) is favored at intermediate starting frequencies as previously described in noniron-limited media. Error bars represent ± 1 SE of the mean.

Figure 5

Relative growth of spiteful compared to nonspiteful bacteria when competed against sensitive public goods cheats (i.e., bacteria that do not produce siderophores). Spiteful bacteriocin production (PAO1: siderophore and bacteriocin producer vs. O:9 cheats: siderophore nonproducer sensitive to bacteriocin) is now favored at intermediate and high starting frequencies, whereas cooperators (PAO1150–2: siderophore producer vs. O:9 cheats: siderophore nonproducer) display a strong negative frequency dependent relationship with starting frequency. Error bars represent ± 1 SE of the mean.

Figure 6

Bacterial density and virulence in a caterpillar host. (A) Bacterial density affected by starting frequency of spiteful and nonspiteful cooperators. Lowest densities were observed in low and intermediate spiteful cooperator treatments. High starting frequencies of spiteful cooperators reached similar densities to that of low and intermediate nonspiteful cooperators. The highest density was observed in the high nonspiteful cooperator frequency. Error bars represent ± 1 SE of the mean. (B) Average time to death of caterpillars infected with different starting frequencies of spiteful and nonspiteful cooperators. Lowest virulence (as measured by time to death) was observed in the low and intermediate spiteful cooperator treatments. The highest virulence is found in the high starting frequency of nonspiteful cooperator, with similar virulence occurring in the intermediate and low nonspiteful cooperator treatments and high starting frequencies of spiteful cooperators.