Viscous medium promotes cooperation in the pathogenic bacteriumPseudomonas aeruginosa

Abstract

There has been extensive theoretical debate over whether population viscosity (limited dispersal) can favour cooperation. While limited dispersal increases the probability of interactions occurring between relatives, which can favour cooperation, it can also lead to an increase in competition between relatives and this can reduce or completely negate selection for cooperation. Despite much theoretical attention, there is a lack of empirical research investigating these issues. We cultured Pseudomonas aeruginosa bacteria in medium with different degrees of viscosity and examined the fitness consequences for a cooperative trait-the production of iron-scavenging siderophore molecules. We found that increasing viscosity of the growth medium (i) significantly limited bacterial dispersal and the diffusion of siderophore molecules and (ii) increased the fitness of individuals that produced siderophores relative to mutants that did not. We propose that viscosity favours siderophore-producing individuals in this system, because the benefits of siderophore production are more likely to accrue to relatives (i.e. greater indirect benefits), and, at the same time, bacteria are more likely to gain direct fitness benefits by taking up siderophore molecules produced by themselves (i.e. the trait becomes less cooperative). Our results suggest that viscosity of the microbial growth environment is a crucial factor determining the dynamics of wild-type bacteria and siderophore-deficient mutants in natural habitats, such as the viscous mucus in cystic fibrosis lung.

Examining the role of individual movement in promoting coexistence in a spatially explicit prisoner's dilemma

“…On the contrary, in this work we have demonstrated that, even if the motility of cooperators and defectors is the same and the spatial domain is homogeneous -so that there are no physical barriers -the natural tendency of cooperators to aggregate into clusters is su cient to preserve cooperation. These findings confirm the results presented in previous theoretical studies by Allison (2005), Durrett and Levin (1994), Fletcher and Doebeli (2009), Hauert and Doebeli (2004), Korolev and Nelson (2011), Nowak and May (1992), Ohtsuki et al (2006), Perc and Szolnoki (2008), Santos and Pacheco (2005), Szabó and Fath (2007) and Taylor et al (2007), and experimental works by Gri n et al (2004), Julou et al (2013), Kümmerli et al (2009) and Momeni et al (2013), and provide further evidence that spatial clustering enhances the benefits of mutual cooperation and plays a crucial role in preserving cooperative behaviours. The spiral structures created by defectors share some striking similarities with spiral waves arising in models of host-parasitoid dynamics, such as those reported by Comins et al (1992), May (1995) and Solé et al (1992).…”

Altruism can evolve when relatedness is low: evidence from bacteria committing suicide upon phage infection

“…In both types of competition assays, costs of suicide should manifest in E. coli HK97 outcompeting E. coli l. Finally, we manipulated relatedness at the altruistic trait by competing E. coli l against E. coli HK97 in medium containing different amounts of agarose. Bacterial dispersal is limited in more solid medium [15,16], which increases relatedness among interacting individuals (i.e. Rex-carriers interact more often among themselves than expected based on their relative frequency in the population).…”

Evolutionary dynamics ofn-player games played by relatives

“…Another plausible mechanism of assortative group formation is population viscosity often via limited dispersal [58,59,[74][75][76]]. Wright's island model [77] and its variants are typical ones in population genetics [30,44,45,78,79], and others prefer network formulations [80][81][82].…”

“…An essential feature is that those microbes often clonally reproduce and therefore they are inevitably genetically highly related to each other. The reason for such an assortment is mainly due to spatial structure and limited dispersal [58,59]. As another instance, in larger animals such as primates, we often find nepotistic cooperation among kin [60][61][62].…”