Abstracts (first author)
Maintenance of co-operation by positive feedback in quorum sensing networks
Signal blind mutants are naturally occurring bacterial strains that are defective for cell to cell signalling. These mutants can produce signal but do not respond to it, so they have reduced expression of quorum sensing regulated genes. Public goods are often regulated by quorum sensing and so signal blind mutants act as defectors, exploiting signal competent co-operators. Critically, signal blind strains are unable to increase their signal output in line with co-operators, because signal production is increased in response to signal, via a positive feedback loop. Therefore signal concentration in the environment is expected to be reduced when signal blind defectors are common. When public goods are regulated by quorum sensing, lower signal concentrations may reduce the production of public goods, minimising exploitation. Using knock-out strains of Pseudomonas aeruginosa we demonstrate that signal in the environment decreases as signal competent co-operators become rare. We show that this translates to a reduced co-operator output of secreted protease. Using competition experiments we then demonstrate that this can lead to negative frequency dependence of co-operator fitness, because co-operators behave as phenotypic defectors when rare. Frequency dependence has important implications as socio-microbiology is applied to new fields, notably the emergence of drug resistance.