Holomycin, an antibiotic secondary metabolite, is required for biofilm formation of the native producer Photobacterium galatheae S2753

Whilst the effects of antibiotics on microorganisms are widely studied, it remains less well-understood how antibiotics affect the physiology of the native producing organisms. Here, using a marine bacterium Photobacterium galatheae S2753 that produces the antibiotic holomycin, we generated a holomycin deficient strain by in-frame deletion of hlmE, the core gene responsible for holomycin production.

Mass spectrometry analysis of cell extracts confirmed that ΔhlmE did not produce holomycin and that the mutant was devoid of antibacterial activity. Biofilm formation of ΔhlmE was significantly reduced compared to that of the wild-type S2753 and was restored in an hlmE complementary mutant. Consistently, exogenous holomycin, but not its dimethylated and less antibacterial derivative, S,S'-dimethyl holomycin, restored the biofilm formation of ΔhlmE Furthermore, zinc starvation was found essential for both holomycin production and biofilm formation of S2753, although the molecular mechanism remains elusive.

Collectively, these data suggest that holomycin promotes biofilm formation of S2753 via its ene-disulfide group. Lastly, the addition of holomycin in sub-inhibitory concentrations also enhanced the biofilm of four other Vibrionaceae strains. P. galatheae likely gains an ecological advantage from producing holomycin as both an antibiotic and a biofilm stimulator, which facilitates the nutrition acquisition and protects P. galaltheae from environmental stresses.

Studying the function of antibiotic compounds in the native producer will shed light on their role in nature and could potentially point to novel bioprospecting strategies. Importance  Despite the societal impact of antibiotics, their ecological functions remain elusive and have mostly been studied by exposing non-producing bacteria to sub-inhibitory concentrations.

Here, we studied the effects of the antibiotic holomycin on its native producer, Photobacterium galatheae S2753, a Vibrionaceae bacterium. Holomycin provides a distinct advantage to S2753 both as an antibiotic and by enhancing biofilm formation in the producer. Vibrionaceae species successfully thrive in global marine ecosystems, where they play critical ecological roles as free-living, symbiotic, or pathogenic bacteria.

Genome mining has demonstrated that many have the potential to produce several bioactive compounds, including P. galaltheae To unravel the contribution of the microbial metabolites to the development of marine microbial ecosystems, better insight into the function of these compounds in the producing organisms is needed.

Our finding provides a model to pursue this and highlights the ecological importance of antibiotics to the fitness of the producing organisms.