Quorum Sensing Signals Produced by Heterotrophic Bacteria in Black Band Disease (BBD) of Corals and Their Potential Role in BBD Pathogenesis

Black band disease (BBD) of corals is a temperature dependent, highly virulent, polymicrobial disease affecting reef-building corals globally. The microbial consortium of BBD is primarily comprised of functional physiological groups that include photosynthetic cyanobacteria, sulfate reducers, sulfide oxidizers and a vast repertoire of heterotrophic bacteria. Quorum sensing (QS), the cell-density dependent communication phenomenon in bacteria, is known to induce expression of genes for a variety of virulence factors in diseases worldwide. Microbes capable of QS release signals such as acyl homoserine lactones (AHLs) and autoinducer-2 (AI-2), which coordinate microbial interaction. The focus of the present study was to investigate the presence and potential role of QS in BBD pathogenicity, utilizing culture dependent and independent methodologies. Isolates across coral health states including BBD, were screened for production of QS signals, and AHL and AI-2 production capabilities were analyzed via LC-MS/MS. The effect of temperature on AHLs was also examined. Additionally, antimicrobial production capabilities of isolates were tested. BBD metagenomes were utilized to screen for sequences related to QS, antimicrobial synthesis, and antimicrobial resistance genes. BBD isolates represented a significantly higher proportion of isolates capable of producing QS signals in comparison to healthy coral isolates. Several AHLs produced by coral derived bacterial cultures were identified, and three AHLs, specifically 3OHC4, 3OHC5 and 3OHC6, showed a significant increase in production at an elevated temperature of 30 °C, which correlates with increased BBD incidence on reefs with increasing water temperature. Most of the BBD cultured isolates were identified as vibrios. Several sequences related to QS, antimicrobial synthesis and resistance genes were detected in the BBD metagenomes. Based on the findings of this study, a model for potential microbial interactions amongst BBD heterotrophs, centered around QS, is proposed. Taken together, the findings from this study provide a clearer understanding of the potential role of QS in BBD, and serve as the basis for further studies aimed at elucidating the pathogenesis of an intricate coral disease