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Diaphorin, a polyketide produced by a bacterial symbiont of the Asian citrus psyllid, inhibits the growth of Bacillus subtilis but promotes the growth of Escherichia coli

Diaphorin is a polyketide produced by Candidatus Profftella armatura (Gammaproteobacteria: Burkholderiales), an obligate symbiont of a notorious agricultural pest, the Asian citrus psyllid Diaphorina citri (Hemiptera: Psyllidae). Diaphorin belongs to the pederin family of bioactive agents found in various host-symbiont systems, including beetles, lichens, and sponges, harboring phylogenetically diverse bacterial producers. Previous studies showed that diaphorin has inhibitory effects on various eukaryotes, including the natural enemies of D. citri. However, little is known about its effects on prokaryotic organisms. To address this issue, the present study assessed the biological activities of diaphorin on two model prokaryotes, Escherichia coli (Gammaproteobacteria: Enterobacterales) and Bacillus subtilis (Firmicutes: Bacilli). The analyses revealed that diaphorin inhibits the growth of B. subtilis but moderately promotes the growth of E. coli. This finding implies that diaphorin functions as a defensive agent of the holobiont (host + symbionts) against some bacterial lineages but is beneficial for others, which potentially include obligate symbionts of D. citri. Importance Certain secondary metabolites, including antibiotics, evolve to mediate interactions among organisms. These molecules have distinct spectra for microorganisms and are often more effective against Gram-positive bacteria than Gram-negative ones. However, it is rare that a single molecule has completely opposite activities on distinct bacterial lineages. The present study revealed that a secondary metabolite synthesized by an organelle-like bacterial symbiont of psyllids inhibits the growth of Gram-positive Bacillus subtilis but promotes the growth of Gram-negative Escherichia coli. This finding not only provides insights into the evolution of symbiosis between animal hosts and bacteria but may also potentially be exploited to promote the effectiveness of industrial material production by microorganisms.