Cory J. Krediet

Microbial communities associated with coral surfaces are diverse and complex. They play key roles in nutrient acquisition by coral holobionts and in responses to stressors and diseases. Members of coral-associated microbiota produce antimicrobial compounds, inhibit cell-to-cell signaling, and disrupt virulence in opportunistic pathogens. Characterization of coral-associated microbial communities suggests that metabolic capabilities define the core members of the communities. However, some taxonomic conservation is becoming evident in microbial communities associated with the same coral species and genera in different geographic regions. Even though shifts in the composition of coral microbiota often correlate with the appearance of signs of diseases and/or bleaching, it is not yet clear to what extent these shifts are a cause or a consequence of diseases. This chapter focuses on interactions within coral-associated microbial communities and suggests potentially interesting directions for future research.

Sensitive, rapid and specific methods for monitoring development and turnover are extremely important for a comprehensive understanding of biofilms. The first part of this chapter describes methods of Multimodal Laser Scanning Microscopy (ML-SM) for in situ identification of biofilm components and monitoring of biofilm development, pattern of gene expression, and visualization of dynamic molecular processes in biofilms. The second part gives an overview of designing and characterizing luminescent reporter systems for high-throughput screening of bioactive molecules. This part specifically addresses the GacS/GacA two-component regulatory system, which is central to biofilm formation in all γ-proteobacteria, and any well understood regulatory cascade can be targeted with a similar approach by targeting important promoters in the pathway. Methods described in this chapter are useful in screening compounds and their libraries for antibacterial, quorum sensing and biofilm inhibitory compounds.

The coral holobiont is a complex symbiotic association between the polyp animal, photosynthetic dinoflagellates, and their associated microbes. During colonization of coral mucus, opportunistic pathogens, including the white pox pathogen Serratia marcescens PDL100, compete with native bacteria for available nutrients in coral mucus and rely on catabolic enzymes, such as beta -galactosidase and chitinase. This study tested the hypothesis that specific glycosidases and chitinase were critical for the growth of S. marcescens on mucus, and that their inhibition by native coral microbiota reduces fitness of the pathogen. Consistent with this hypothesis, a S. marcescens transposon mutant defective in glycosidase and chitinase activities was unable to compete with the wild type on the mucus of the host coral Acropora palmata. A survey revealed that approximately 8% of culturable coral commensal bacteria have the ability to inhibit glycosidases in the pathogen. In an attempt to understand how coral pathogens infect their hosts, the hypothesis that in the necrotizing coral pathogen S. marcescens PDL100, gacA is involved in the interactions of the pathogen with the polyp hosts was examined. A disruption of the S. marcescens gacA resulted in an increased competitive fitness of the mutant on crude mucus of the host coral Acropora palmata and on the high molecular weight fraction of the mucus, whereas the mutant was as competitive as the wild type on the low molecular weight fraction of the mucus. This indicates a critical role for the gacA-mediated phenotypes in the efficient utilization of coral mucus and establishment within the surface mucopolysaccharide layer. The susceptibility of the sea anemone Aiptasia pallida to common coral pathogens (Serratia marcescens, Vibrio coralliilyticus, and V. shiloi) was also tested. A. pallida responded to the pathogens with symptoms that closely resemble the progression of the coral diseases caused by necrotizing pathogens. Infection studies with this model polyp will elucidate some virulence mechanisms used by coral pathogens to infect and degrade polyps. These results demonstrate, that indeed, coral pathogens rely on specific regulated behaviors to infect their coral hosts and that coral commensal bacteria show the potential to disrupt these early infection strategies.