الفهرس 
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Abstract
The present study focused on screening for quorum sensing inhibition activity of marine bacterial crude extract that have the ability to attenuate the decanoyl-homoserine lactone (C-10 HSL) signal of the wild type strain chromobacterium violaceum ATCC12472. One hundred and eighty-eight bacterial isolates were recovered from corals, sediment and seawater with different salinities, only 35% of them showed complete degradation of the signal without affecting the bacterial growth. Lactonase genes were recorded in six bacterial isolates which represents only 9.2 % of the positive isolates. After molecular characterization of the positive isolates using 16S rRNA gene, most of the results showed affiliation to the genus Bacillus, Enterobacter and Nocardiopsis. Lactonase genes were sequenced and analyzed; moreover, their tertiary structures were predicted and confirmed the presence of dinuclear zinc metal site in three isolates (4C, 8AQ and 26SP) which were 99, 99 and 97% phylogenetically close to Bacillus cereus and Bacillus safinsis in the database respectively. Only one isolate (7C) which was 96% close to uncultured Bacillus in the database showed presence of mononuclear zinc metal site and this is the first time to be detected. The 236 amino acid sequence contained conserved region 89 SHLHFDH 95 which is characteristic of the metallo-beta-lactamase family. Homology comparison revealed that the deduced amino acid sequence had a high degree of similarity with those of the known AiiA proteins in the B. cereus group.