Harrison Freya, Paul Jon, Massey Ruth C, Buckling Angus
Department of Zoology, University of Oxford, Oxford, UK.
ISME J. 2008 Jan;2(1):49-55. doi: 10.1038/ismej.2007.96. Epub 2007 Nov 1.
Both intra- and interspecific interactions between microbes are likely to play an important role in determining the severity of microbial infections. Here, we study the impact of interactions between coinfecting opportunistic pathogens Staphylococcus aureus and Pseudomonas aeruginosa on both phenotypic and genetic changes in a P. aeruginosa social trait, the production of iron-scavenging siderophores. Siderophores are facultatively upregulated in response to iron limitation and play a key role in determining the virulence of microbial infections. Siderophore production is metabolically expensive to individual producers but benefits the group as a whole because siderophores can be used by all cells in the vicinity with siderophore receptors. Hence, populations of siderophore producers can be invaded by nonproducing cheats. Previous work has shown that P. aeruginosa can lyse S. aureus, supplying a source of free iron. We therefore hypothesized that the presence of S. aureus might result in facultative downregulation of siderophore production, and in turn, reduced selection for siderophore cheats. We tested this hypothesis by evolving P. aeruginosa in the presence and absence of free iron and S. aureus, in a fully factorial design. Iron had the expected effect: siderophore production was downregulated and cheats evolved less readily, but the presence of S. aureus instead increased facultative siderophore production and selection for cheats. This is probably because the S. aureus had the net effect of competing for iron, rather than acting as an iron source. This study demonstrates that interspecific competition can have a marked effect on intraspecific social interactions.
微生物之间的种内和种间相互作用可能在决定微生物感染的严重程度方面发挥重要作用。在这里，我们研究了共感染的机会性病原体金黄色葡萄球菌和铜绿假单胞菌之间的相互作用对铜绿假单胞菌一种群体特征——铁载体产生的表型和遗传变化的影响。铁载体在铁限制条件下会被适应性上调，并且在决定微生物感染的毒力方面起着关键作用。铁载体的产生对单个生产者来说代谢成本高昂，但对整个群体有益，因为铁载体可以被附近所有带有铁载体受体的细胞利用。因此，铁载体生产者群体可能会被不产生铁载体的“欺骗者”入侵。先前的研究表明，铜绿假单胞菌可以裂解金黄色葡萄球菌，提供游离铁的来源。因此，我们假设金黄色葡萄球菌的存在可能导致铁载体产生的适应性下调，进而减少对铁载体“欺骗者”的选择。我们通过在有无游离铁和金黄色葡萄球菌的情况下，采用完全析因设计来培养铜绿假单胞菌，以验证这一假设。铁产生了预期的效果：铁载体的产生下调，“欺骗者”进化得较慢，但金黄色葡萄球菌的存在反而增加了铁载体的适应性产生以及对“欺骗者”的选择。这可能是因为金黄色葡萄球菌的净效应是竞争铁，而不是作为铁源。这项研究表明种间竞争可以对种内社会相互作用产生显著影响。
