Search

The main goal of this study was to find bacterial isolates with the ability to inhibit the growth of the fish pathogens Aeromonas hydrophila, Vibrio anguillarum, and Flavobacterium psychrophilumand to inhibit the blockage of the quorum-sensing (QS) system. A total of 80 gram-negative strains isolated from various freshwater Chilean salmonid farms were studied. We determined that 10 strains belonging to the genus Pseudomonasinhibited at least one of the assayed fish pathogens. Of these, nine strains were able to produce siderophores and two strains were able to inhibit the growth of all assayed pathogenic species. When the 80 strains were examined for QS-blocking activity, only the strains Pseudomonassp. FF16 and Raoultella planticolaR5B1 were identified as QS blockers. When the QS-blocker strains were analyzed for their ability to produce homoserine lactone (HSL) molecules, thin-layer chromatography analysis showed that both strains were able to produce C6-HSL– and C8-HSL–type molecules. Strain R5B1 did not show growth inhibition properties, but strain FF16 also led to inhibition of growth in A. hydrophilaand F. psychrophilumas well as to siderophore production. Pseudomonassp. FF16 exhibited potentially useful antagonistic properties and could be a probiotic candidate for the salmon farming industry. Received July 31, 2014; accepted December 17, 2014

AbstractThe main goal of this study was to find bacterial isolates with the ability to inhibit the growth of the fish pathogens Aeromonas hydrophila, Vibrio anguillarum, and Flavobacterium psychrophilumand to inhibit the blockage of the quorum-sensing (QS) system. A total of 80 gram-negative strains isolated from various freshwater Chilean salmonid farms were studied. We determined that 10 strains belonging to the genus Pseudomonasinhibited at least one of the assayed fish pathogens. Of these, nine strains were able to produce siderophores and two strains were able to inhibit the growth of all assayed pathogenic species. When the 80 strains were examined for QS-blocking activity, only the strains Pseudomonassp. FF16 and Raoultella planticolaR5B1 were identified as QS blockers. When the QS-blocker strains were analyzed for their ability to produce homoserine lactone (HSL) molecules, thin-layer chromatography analysis showed that both strains were able to produce C6-HSL– and C8-HSL–type molecules. Strain R5B1 did not show growth inhibition properties, but strain FF16 also led to inhibition of growth in A. hydrophilaand F. psychrophilumas well as to siderophore production. Pseudomonassp. FF16 exhibited potentially useful antagonistic properties and could be a probiotic candidate for the salmon farming industry.Received July 31, 2014; accepted December 17, 2014

ABSTRACTTwenty-five distinct tetracycline-resistant gram-negative bacteria recovered from four Chilean fish farms with no history of recent antibiotic use were examined for the presence of tetracycline resistance (tet) genes. Sixty percent of the isolates carried 1 of the 22 known tetgenes examined. The distribution was as follows. The tet(A) gene was found in six isolates. The tet(B) gene was found in two isolates, including the first description in the genus Brevundimonas. Two isolates carried the tet(34) and tet(B) genes, including the first description of the tet(34) gene in Pseudomonasand Serratiaand the first description of the tet(B) gene in Pseudomonas. The tet(H) gene was found in two isolates, which includes the first description in the genera Moraxellaand Acinetobacter. One isolate carried tet(E), and one isolate carried tet(35), the first description of the gene in the genus Stenotrophomonas. Finally, one isolate carried tet(L), found for the first time in the genus Morganella. By DNA sequence analysis, the two tet(H) genes were indistinguishable from the previously sequenced tet(H) gene from Tn5706found in Pasteurella multocida. The Acinetobacter radioresistensisolate also harbored the Tn5706-associated 1,063-bp IS element IS1597, while the Moraxellaisolate carried a 1,026-bp IS-like element whose 293-amino-acid transposase protein exhibited 69% identity and 84% similarity to the transposase protein of IS1597, suggesting the presence of a novel IS element. The distribution of tetgenes from the Chilean freshwater ponds was different than those that have previously been described from other geographical locations, with 40% of the isolates carrying unidentified tetracycline resistance genes.