Utilization of Mucus from the Coral Acropora palmata by the Pathogen Serratia marcescens and by Environmental and Coral Commensal Bacteria

In recent years, diseases of corals caused by opportunistic pathogens have become widespread. How opportunistic pathogens establish on coral surfaces, interact with native microbiota, and cause disease is not yet clear. This study compared the utilization of coral mucus by coral-associated commensal bacteria (“Photobacterium mandapamensis” and Halomonas meridiana) and by opportunistic Serratia marcescens pathogens. S. marcescens PDL100 (a pathogen associated with white pox disease of Acroporid corals) grew to higher population densities on components of mucus from the host coral. In an in vitro coculture on mucus from Acropora palmata, S. marcescens PDL100 isolates outgrew coral isolates. The white pox pathogen did not differ from other bacteria in growth on mucus from a nonhost coral, Montastraea faveolata. The ability of S. marcescens to cause disease in acroporid corals may be due, at least in part, to the ability of strain PDL100 to build to higher population numbers within the mucus surface layer of its acroporid host. During growth on mucus from A. palmata, similar glycosidase activities were present in coral commensal bacteria, in S. marcescens PDL100, and in environmental and human isolates of S. marcescens. The temporal regulation of these activities during growth on mucus, however, was distinct in the isolates. During early stages of growth on mucus, enzymatic activities in S. marcescens PDL100 were most similar to those in coral commensals. After overnight incubation on mucus, enzymatic activities in a white pox pathogen were most similar to those in pathogenic Serratia strains isolated from human mucosal surfaces.Serratia is a gammaproteobacterium frequently isolated from waters, plants, and animals (7). Some isolates of Serratia are well-characterized symbionts of invertebrates. Serratia marcescens and Serratia liquefaciens have been identified as vertically transmitted symbionts of the sugar beet maggot (9). Serratia colonizes male and female reproductive tracts of the maggots, eggs, and pharyngeal filter. There, the bacteria are hypothesized to aid in metamorphosis by digesting chitinous puparial walls (9). In the gut of another insect, the diamondback moth, strains of S. marcescens appear to live as commensals capable of modestly (5 to 8%) increasing growth rates of the host (8). Serratia strains have also been isolated from feces and cloacal swabs from clinically normal captive birds, but not from organs or carcasses of sick or diseased animals housed within the same facility (3, 20). Serratia spp. have also been linked to diseases of invertebrate animals and their larvae (for reviews, see references 7, 15, and 21). To cause diseases in nematodes and flies, S. marcescens first colonizes the intestines, degrades cells of the alimentary tract and then spreads to other organs (14, 21). There are, however, exceptions to this mode of infection. Serratia entomophila, the causal agent of amber disease in grubs, grows within the alimentary tract of the animal to >10⁶ CFU. However, bacteria neither attach to nor colonize surfaces of the gut; rather, they adhere to gut contents (10) and cause the appearance of signs by producing the Sep toxin that inhibits accumulation of the insect''s digestive serine proteases and disrupts the cytoskeletal network (6). It appears, therefore, that various isolates of Serratia are capable of entering into a full range of interactions (from mutualistic to commensal to pathogenic) with their animal hosts (for reviews, see references 7, 15, and 21).A strain of S. marcescens, PDL100, was shown to be associated with white pox disease of the threatened Caribbean coral Acropora palmata (22, 27). White pox disease results in coral tissue necrosis, exposing carbonate skeleton at a rate of 2.5 cm² day⁻¹ (22). It is not yet clear how S. marcescens PDL100 colonizes and infects corals. It is likely that to cause disease, the pathogen first needs to colonize and establish within the coral surface mucus layer.The coral surface mucus layer contains polymers of mixed origin. Coral mucus is made in the mucocytes of the polyp, where the photosynthate produced by the coral symbiotic dinoflagellate Symbiodinium spp. is converted into polymers that are excreted onto the coral surface (for a review, see reference 2). A glycoprotein is the major component of coral mucus from both hard and soft corals (16, 17, 19). The composition of the glycoprotein differs among coral species (4, 17). The mucus polymer of Acropora formosa, for example, contains 36 to 38% of neutral sugars, 18 to 22% of amino sugars, and 19 to 30% of amino acids; lipids make up 4.2% of the polymer (17). In the mucus of A. formosa, the oligosaccharide decorations (two to four sugar residues long) are attached to the polypeptide backbone by an O-glycosidic link to serine or threonine through the carbon 1 of mannose (16). The glycoproteins from A. formosa and Pseudopterogorgia americana corals contain terminal arabinose residues linked by a β1→2 or β1→3 bond. In the mucus of acroporid corals, arabinose, N-acetyl-glucosamine, mannose, glucose, galactose, N-acetyl-galactosamine, and fucose were the major sugars; serine and threonine were the major amino acids (4, 17). The elucidation of the chemical structure of coral mucus is complicated by the fact that the mucus contains excretions of coral mucocytes, extracellular substances produced by the associated microbiota as well as oligomers that may result from the degradation of these polymers (for reviews, see references 2 and 24).In this study, we tested the hypothesis that S. marcescens PDL100 is capable of a more extensive utilization of A. palmata mucus than other environmental or pathogenic isolates of S. marcescens. This hypothesis is based on the recent discoveries that pathogenic and commensal host-associated bacteria differ in their patterns of carbon source utilization during growth on components of the mucus that lines host surfaces (5, 26). These different strategies of mucus utilization may allow pathogenic bacteria to outcompete native residents and establish within the host''s mucosa (5, 13, 26). To test this hypothesis, growth of the strain PDL100 on coral mucus and enzymatic activities induced during growth on mucus were assayed and compared to those of pathogenic and environmental isolates of S. marcescens and three native coral-associated bacteria.