Antibacterial spectrum of diaphorin, a polyketide produced by a bacterial symbiont of the Asian citrus psyllid

Diaphorin is a polyketide synthesized by "Candidatus Profftella armatura" (Betaproteobacteria: Burkholderiales), an obligate symbiont of a devastating agricultural pest, the Asian citrus psyllid Diaphorina citri (Hemiptera: Psyllidae). Previous studies showed that physiological concentrations of diaphorin, which is present in D. citri at 2-20 mM, are inhibitory to various eukaryotes and Bacillus subtilis (Firmicutes: Bacilli) but promote the growth and metabolic activity of Escherichia coli (Gammaproteobacteria: Enterobacterales). However, bacteria examined for diaphorin activity were limited to these two model species, and little was known about the activity spectrum of diaphorin, which is essential for understanding its effects on the D. citri microbiota. As a first step to address this issue, this study investigated the effects of diaphorin on six bacterial species: Arsenophonus nasoniae, Photorhabdus luminescens, Serratia entomophila, Serratia symbiotica (all Gammaproteobacteria: Enterobacterales), and Micrococcus luteus and Kocuria rhizophila (both Actinobacteria: Micrococcales). The results revealed that five milimolar diaphorin promotes the growth of M. luteus but inhibits the growth of other bacterial species, showing that the spectrum of diaphorin is complex and not simply determined by the taxonomic group or the cell envelope composition of the target bacteria. To further assess whether differences in the susceptibility to diaphorin affect the suitability as a potential biopesticide, we analyzed the mortality of D. citri after treatment with these bacteria. This revealed that only S. entomophila significantly increases D. citri mortality, implying that when diaphorin is not inhibitory enough on bacteria, the innate bacterial growth speed and susceptibility to the D. citri immune system have a more significant impact on controlling D. citri.