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Black Band Disease (BBD) is a well-described disease plaguing corals worldwide. It has been established that ecological and environmental stress factors contribute to the appearance and progression of the disease, believed to be caused by a diverse microbial consortium. We have identified and characterized Vibrio sp. associated with BBD in Eilat reef corals using both culture-dependent and -independent methods. Direct sampling using 16S rRNA gene clone libraries showed seasonal dynamics in the diversity of BBD-associated Vibrios. In the two sampling periods, BBD-associated Vibrio clones showed similarities to different groups: October samples were similar to known pathogens, while December samples were similar to general aquatic Vibrio sp. Cultured bacterial isolates of Vibrio sp. were highly homologous (>or=99%) to previously documented BBD-associated bacteria from the Caribbean, Bahamas and Red Seas, and were similar to several known coral pathogens, such as Vibrio coralliilyticus. The proteolytic activity of Vibrio sp., as measured using casein- and azocasein-based assays, directly correlated with temperature elevation and peaked at 26-28 degrees C, with the microorganisms producing more proteases per bacterial cell or increasing the rate of proteolytic activity of the same proteases (potentially metalloproteases). This activity may promote coral tissue necrosis and aid in ensuing progression of the coral BBD.