A number of gram-negative bacteria have a quorum-sensing system and produce N-acyl-L-homoserine lactone (AHL) that they use them as a quorum-sensing signal molecule. Pantoea ananatis is reported as a common colonist of wheat heads at ripening and causes center rot of onion. In this study, we demonstrated that P. ananatis SK-1 produced two AHLs, N-hexanoyl-L-homoserine lactone (C6-HSL) and N-(3-oxohexanoyl)-Lhomoserine lactone (3-oxo-C6-HSL). We cloned the AHL-synthase gene (eanI) and AHL-receptor gene (eanR) and revealed that the deduced amino acid sequence of EanI/EanR showed high identity to those of EsaI/EsaR from P. stewartii. EanR repressed the ean box sequence and the addition of AHLs resulted in derepression of ean box. Inactivation of the chromosomal eanI gene in SK-1 caused disruption of exopolysaccharide (EPS) biosynthesis, biofilm formation, and infection of onion leaves, which were recovered by adding exogenous 3-oxo-C6-HSL. These results demonstrated that the quorum-sensing system involved the biosynthesis of EPS, biofilm formation, and infection of onion leaves in P. ananatis SK-1.
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.
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.
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