The Role of Antibiotics in Microbial Host Colonization

Abstract

Surface colonization is ubiquitous in marine environments, and bacteria are one of the first, or primary colonizers, to attach to newly formed surfaces. Marine algae are host to a plethora of such bacterial colonizers, one of the most abundant being roseobacters. Most of known pathogenic or mutualistic interactions between algal hosts and roseobacters are mediated by bioactive molecules produced by the organisms. Tropodithietic acid (TDA), an antibiotic made by the roseobacter Phaeobacter inhibens, is hypothesized to contribute to the chemical defense for Emiliania huxleyi, a globally abundant microalga, against colonization of roseobacter Ruegeria italica, a known opportunistic pathogen of E. huxleyi. To investigate the role of TDA in the chemical defense, a multi-species culturing system was established with E. huxleyi, R. italica and P. inhibens, as well as a P. inhibens transposon mutant lacking TDA production. This system was monitored, with the bacterial-algal surface association visualized via fluorescence microscopy, population densities of the bacteria were measured by colony forming unit and all algal populations monitored using pulse-amplitude modulated fluorometry. Within 72 hours of colonization, algal health was not affected by bacteria inoculated. R. italica gained higher density than P. inhibens during simultaneous primary colonization of E. huxleyi by these two bacteria, though P. inhibens can inhibit R. italica growth on solid media in a diffusion-dependent manner. If R. italica was added as a secondary colonizer after P. inhibens primary colonization of E. huxleyi, R. italica growth was significantly inhibited by 98%. In conclusion, P. inhibens was successful at inhibiting the secondary colonization of pathogen R. italica, in part due to its production of TDA, which acts as a chemical defense for E. huxleyi.