Induction of antibiotic production and its influence on interactions in marine Vibrionaceae

Abstract

The alarming trend of pathogenic microorganisms developing resistance against currently used antibiotics has reinvigorated the search for new antimicrobial compounds. Secondary metabolites produced by microorganisms are the most important source of antibiotics, and thus also for the discovery of novel antimicrobial compounds. Genome mining of biosynthetic gene clusters (BGCs) has revolutionized the field of natural product discovery and identified a large potential for the production of secondary metabolites in a wide range of microbial taxa, including marine Vibrionaceae . The purpose of this PhD project was to study cues and stimuli that activate secondary metabolism in two antibiotic producing marine  Vibrionaceae  strains, Vibrio coralliilyticus and Photobacterium galatheae , producing the antibiotics andrimid and holomycin, respectively. Previous studies have shown that sub inhibitory concentrations of antibiotics can induce the production of secondary metabolites in microorganisms. Using a genetic reporter strain, we identified that andrimid, produced by V. coralliilyticus , is an elicitor of a silent BGC. Andrimid was used to activate secondary metabolism in  P. galatheae , and this resulted in a 4.0-fold overproduction of the antibiotic holomycin and significant upregulation of BGCs for which the chemical compounds remain to be identified. Furthermore, high performance liquid chromatography - mass spectrometry analyses demonstrated induction of six unknown compounds, but unfortunately we were not able to identify the structure of these. Secondary metabolites are important mediators of microbial interactions and their production can be stimulated by the presence of other microorganisms. In light of this, our previous results motivated us to study the interactions between the ecological neighbors V. coralliilyticus  and  P. galatheae  in co-culture and the effect of co-culture on secondary metabolite production. Cocultivation interactions led to increased production of andrimid by  V. coralliilyticus  (4.3-fold) and holomycin by P. galatheae (2.7-fold) compared to mono-cultures. Moreover,  V. coralliilyticus  detoxified the antibiotic holomycin by sulphur methylation. This thesis concludes that competitive interaction is an ecologically important cue for the production of antibiotic secondary metabolites in  Vibrionaceae . This insight can be leveraged to unlock the remaining bioactive potential of this diverse group of marine bacteria towards finding new antibiotics.