Microbial Wars: Mucus-Associated Bacteria Fend off Coral Pathogens
[John Lisle]

Most coral-disease pathogens that have been identified are bacteria (reviewed by Rosenberg and Loya, 2004; Richardson, 1998). Although some microbes have been successfully linked to specific coral diseases, the mechanisms by which they establish infections are not understood. One of the recent investigations of bacterially induced coral diseases concluded that a bacterium commonly found in aquatic environments and human feces, Serratia marcescens, is the cause of white-pox disease in the elkhorn coral Acropora palmata (Patterson, 2002). Previous studies that have isolated alleged coral disease-causing microbes from infected tissues have done so from the perspective of the pathogen being introduced from the sea water, with no consideration of the role played in the infection process by the native bacteria within the coral's mucus layer. This aspect is important to consider because the mucus layer provides a barrier between the water and coral tissues.

Left: A bacterial-inhibition experiment. Serratia marcescens is spread over an agar plate (its growth is visible as a orange haze on the colorless agar); then native bacteria are spotted on top of it (visible as the circles on the plate). Native bacteria that do not resist S. marcescens are grown over (orange circles), but white circles indicate antibacterial activity. Those spots are colorless because the S. marcescens could not grow there. Photo credit: John Lisle, USGS. [larger version]

To determine if native bacteria living in coral mucus are capable of providing an anti-bacterial defense against outsider microbes, mucus samples were collected from healthy corals in the Florida Keys. Bacterial isolates recovered from these mucus samples were screened for antibacterial activity against a strain of S. marcescens (a known coral pathogen), and Escherichia coli (a bacterium commonly found in human feces). Approximately 5 percent of the screened isolates exhibited antibacterial activity, where inhibition of growth was the criterion. These native bacterial isolates were characterized by biochemical tests and gene sequencing in collaboration with University of South Florida microbiologist Valerie J. Harwood. All of the isolates were identified as belonging to the genus Vibrio. These data indicate that coral-associated bacteria residing in the mucus layer provide a first line of defense against the establishment of potentially pathogenic bacteria in corals.