Aims:  Several Gram‐negative bacterial species use N‐acyl homoserine lactone (AHL) molecules as quorum‐sensing (QS) signals to regulate various biological functions. Similarly, various bacteria can stimulate, inhibit or inactivate QS signals in other bacteria by producing molecules called as quorum‐sensing inhibitors (QSI). Our aim was to screen and identify the epibiotic bacteria associated with brown algae for their ability of producing QS‐inhibiting activity. Methods and Results:  QSI screenings were conducted on several epibiotic bacteria isolated from a marine brown alga Colpomenia sinuosa, using Serratia rubidaea JCM 14263 as an indicator organism. Strain JCM 14263 controls the production of red pigment, prodigiosin by AHL QS. Out of 96 bacteria, which were isolated from the surface of the brown alga, 12% of strains showed the ability to produce QSI, which was observed from the pigmentation inhibition on Ser. rubidaea JCM 14263 without affecting its growth. Phylogenetic analysis using 16S rRNA gene sequencing method demonstrated bacterial isolates showing QS inhibition‐producing bacteria belonging to the Bacillaceae (Firmicutes), Pseudomonadaceae (Proteobacteria), Pseudoalteromonadaceae (Proteobacteria) and Vibrionaceae (Proteobacteria). Conclusion:  An appreciable percentage of bacteria isolated from the brown alga produced QSI‐like compounds. Significance and Impact of the Study:  The screening method using Ser. rubidaea described in this report will facilitate the rapid identification of QSI‐producing bacteria from marine environment. This study reveals new avenue for future environmental applications. This study also suggests that these algal epibiotic bacteria may play a role in the defensive mechanism for their host by producing QSI or QSI‐like compounds to suppress the settlement of other competitive bacteria.

We investigated the diversity of epibiotic bacteria possessing antimicrobial activity isolated from nine species of red algae, and identified their phylogenetic position. For the isolation of epibiotic bacteria, nine species of red algae, Pachymeniopsis lauceolata, Plocamium telfairiae, Gelidium amansii, Chondrus oncellatus, Grateloupia filicina, Ceramium kondoi, Lomentaria catenata, Schizymenia dubyi and Porphyra yezoensis, were collected from the intertidal zone of Awaji Island, Japan. In total 92 bacteria were collected from the above red algal species. Primary screening results using disc diffusion assay revealed that 33% of bacteria possess antibacterial activity. Ten bacteria that showed high antibacterial activity were further studied for their ability to inhibit a set of fouling bacteria, some luminescent Vibrio and Photobacterium species and a panel of pathogenic bacteria. In general, the inhibitory activities were high against fouling and luminescent bacteria, while low against various pathogenic bacteria tested. These results suggest that some epibiotic bacteria have adapted to defend their position in their surface environment through the production of antibacterial metabolites giving defense against a broad spectrum of bacterial competitors. The phylogenetic analysis using 16 S rRNA sequences identified 7 of the 10 strains as belonging to the genus Bacillus, and other strains each 1 belonging to genus Microbacterium, Psychrobacter, and Vibrio species.

One hundred and sixteen epibiotic bacteria were isolated from the surface of nine species of brown algae Sargassum serratifolium, S. fusiforme, S. filicinum, Padina arborescens, Undaria pinnatifida, Petalonia fascia, Colpomenia sinuosa, Scytosiphon lomentaria and Ecklonia cava which were collected at Awaji Island, Japan. Primary screening results using disc-diffusion assay revealed that, among the bacteria isolated 20% of epibiotic bacteria exhibited antibacterial activity. Among them, 10 bacteria which showed high antibacterial activity were further studied for their ability against (i) a set of fouling bacteria isolated from marine natural biofilm, (ii) some luminescent Vibrio and Photobacterium species and (iii) a panel of pathogenic bacteria. In general, inhibitory activities were high or moderate against fouling bacteria, Vibrio and Photobacterium species, while they were found to be low against pathogenic bacteria tested. The phylogenetic analysis using 16S rRNA sequencing revealed that all of the bacteria with high antibacterial activity showed a close affiliation with genus Bacillus. This result suggested that the genus Bacillus are efficient producers of antibacterial compounds and these epibiotic bacteria isolated are highly successful colonizers on macroalgal surfaces.

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Serratia rubidaea N-1, which was isolated from the Ariake Sea, a bay located in the Kyusyu region, Japan, is a producer of red pigment prodigiosin. Using this N-1 strain, we attempted to examine the effect of salt stress on the prodigiosin production and the usefulness of the strain as an indicator bacterium for the detection of quorum sensing inhibitors (QSIs) from marine microbes.

The effects of four strains of Gram positive bacteria (PS2, PS9, PS11, and PS79) isolated from the sponge Pseudoceratina pupurea on the growth of a set of fouling bacteria and standard strains of genera Vibrio-Photobacterium were examined. Using a paper disc diffusion method, 4 sponge isolates were found to have inhibitory activity against 4 of 12 isolates of the fouling bacteria. All of the four bacterial supernatants also showed inhibitory activity against Vibrio alginolyticus and V. fischeri, while sponge extracts inhibited every strain tested. Phylogenetic analysis using 16S rRNA gene sequences revealed that three of the sponge isolates (PS2, PS9 and PS79) belonged to the genus Bacillus and one (PS11) belonged to the genus Virgibacillus. The isolation of strains possessing inhibitory activity against fouling bacteria suggests that sponge-associated bacteria could be potential sources of novel natural products.

Four antibiotic producing bacteria were isolated from the surface of the marine sponge Pseudoceratina purpurea and exposed to living cells of two human pathogenic bacteria as well as some marine fouling bacteria to induce the production of antimicrobial activity. Experimental results showed that these four marine epibiotic bacteria enhanced their antibacterial production, when exposed to these test strains. The highest induction was exhibited by the sponge isolate PS79 against fouling bacterium FB-9 (from 3 mm to 7 mm inhibition zone). All the four strains were induced and showed increased activity specifically against the challenged pathogenic or fouling bacteria tested. Specific induction by these species suggests that the induction might be attributed to the response to the chemical signals received from potential challenger strains.

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