Isolation of quorum quenching bacteria and their impact on virulence of Vibrio harveyi

Abstract

Quorum Sensing (QS) is a bacterial cell to cell communication mechanism, which is responsible for regulation of gene expression that mediates the production of virulence factors in a cell density-dependent manner. Inactivation of quorum sensing signal molecules of pathogenic bacteria like acylhomoserine lactones (AHLs) has been proposed as a novel biotherapeutic method to fight against bacterial diseases in aquaculture. In this study, potential quorum quenching property of virulence factor production of shrimp pathogen V. harveyi by Bacillus spp. was determined. One hundred and twenty two isolates of Bacillus spp. isolated from aquaculture ponds and mangrove soil were screened for their ability to degrade synthetic AHLs; Nbutyryl dl homoserine lactone, Nhexanoyl DL homoserine lactone , N-octanoyl DL homoserine lactone, N decanoyl DL homoserine lactone, and N dodecanoyl DL homoserine lactone using agarplate well diffusion assays with reporter strain, Chromobacterium violaceum CV026. Based on the ability to degrade all the five synthetic AHLs tested, seventeen Bacillus spp. isolates were selected for further study. None these isolates showed direct inhibitory effect on the growth of V. harveyi when cocultured. Quantification of AHLs degradation activity evaluated using microplate assay revealed that of the seventeen isolates tested, AHL degradation potential was highest in B. subtilis MFB 10, B. lentus MFB 2 and B. firmus MFB 7 and among these B. subtilis MFB 10 exhibited maximum activity. In addition to the inhibition of hemolytic activity and biofilm formation in V. harveyi, supernatant of the B. subtilis MFB 10 suppressed the production of various virulence factors such as protease, lipase, phospholipase, caseinase, gelatinase and chitinase. Furthermore this isolate was able to survive at a wide range of temperatures, pH values, and NaCl levels. Thus the study suggests the potential of AHLs degrading bacteria as an alternative for antibiotics in aquaculture for controlling bacterial diseases.