Antagonistic interactions subdue inter‐species green‐beard cooperation in bacteria

Cooperation can be favoured through the green‐beard mechanism, where a set of linked genes encodes both a cooperative trait and a phenotypic marker (green beard), which allows carriers of the trait to selectively direct cooperative acts to other carriers. In theory, the green‐beard mechanism should favour cooperation even when interacting partners are totally unrelated at the genome level. Here, we explore such an extreme green‐beard scenario between two unrelated bacterial species—Pseudomonas aeruginosa and Burkholderia cenocepacia, which share a cooperative locus encoding the public good pyochelin (an iron‐scavenging siderophore) and its cognate receptor (green beard) required for iron–pyochelin uptake. We show that pyochelin, when provided in cell‐free supernatants, can be mutually exchanged between species and provide fitness benefits under iron limitation. However, in co‐culture we observed that these cooperative benefits vanished and communities were dominated by P. aeruginosa, regardless of strain background and species starting frequencies. Our results further suggest that P. aeruginosa engages in interference competition to suppress B. cenocepacia, indicating that inter‐species conflict arising from dissimilarities at the genome level overrule the aligned cooperative interests at the pyochelin locus. Thus, green‐beard cooperation is subdued by competition, indicating that interspecific siderophore cooperation is difficult to evolve and to be maintained.     

Viscous medium promotes cooperation in the pathogenic bacterium Pseudomonas aeruginosa

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.     

Unraveling the mechanism of the farnesyltransferase enzyme

Farnesyltransferase enzyme (FTase) is currently one of the most fascinating targets in cancer research. Studies in other areas, namely in the fight against parasites and viruses, have also led to very promising results. However, in spite of the thrilling achievements in the development of farnesyltransferase inhibitors (FTIs) over the past few years, the farnesylation mechanism remains, to some degree, a mystery. This review tries to shed some light on this puzzling enzyme by analyzing seven key mechanistic dilemmas, based on recent studies that have dramatically changed the way this enzyme is currently perceived.