Interkingdom responses of flies to bacteria mediated by fly physiology and bacterial quorum sensing

Insect location and utilization of a resource is influenced by a host of variables including nutrients acquired prior to encountering a stimulus and age of the individual. For the carrion system, we hypothesized that volatiles to which primary colonizers, such as blow flies, respond are the same signalling molecules produced and utilized for quorum sensing by bacteria found on the resource. We provided freshly emerged blow flies, Lucilia sericata, different diets (blood or powdered milk) and assessed their behaviour in a dual-choice assay based on sex and ovarian status of 7-day-old or 14-day-old adults. We determined their preference between wild-type Proteus mirabilis, which is able to swarm (a quorum-sensing response), or mutated (by transposon mutagenesis) P. mirabilis, which is unable to swarm. In most instances, an individual's sex did not significantly influence its response. Age and diet appeared to regulate fly motivation and preference. Seven-day-old flies had a significantly greater probability of responding to the wild type than to the mutant, regardless of diet, but the percentage of milk-fed flies that responded was significantly smaller (85% less) than the percentage of blood-fed flies that responded. Blood-fed flies oviposited, whereas milk-fed flies did not. Seven-day-old flies oviposited predominately on the wild type, whereas 14-day-old flies oviposited predominately on the mutant. Our results demonstrate that the mechanism used by L. sericata for detecting a resource can be associated with bacterial quorum sensing, and that the physiological state of the insect influences its response. We also identified several differences in volatile compounds produced by the bacteria that could explain blow fly response.