Production of exotoxins by two luminous Vibrio harveyi strains known to beprimary pathogens of Penaeus monodon larvae

Two luminous strains of Vibrio harveyi, previously demonstrated to be virul ent to Penaeus monodon larvae were shown to produce proteinaceous exotoxins capable of causing mortality in mice and P, monodon at relatively small co ncentrations. These toxins were isolated from cell-free supernatants of mid -exponential phase broth cultures. Toxin T1, produced by V. harveyi strain 47666-1, had an LD50 Of 2.1 mu g g(-1) by intra-peritoneal injection in CBA mice and 1.8 mu g g(-1) by intra-muscular injection in juvenile P. monodon . This protein comprised 2 subunits of approximately 55 and 45 kDa, giving the native protein a weight of approximately 100 kDa. Toxin T2, produced by V. harveyi strain 642, had an LD50 of 3.1 mu g g(-1) by intra-peritoneal i njection in CBA mice and 2,2 mu g g(-1) by intra-muscular injection in juve nile P, monodon. Partial sequencing of both toxins did not reveal definitiv e matches to other known bacterial proteins. However, some partial homology to toxin or toxin-associated proteins was found. These toxins are likely t o be important virulence factors for these strains. The quantity, type and mode of action of toxins such as these, when expressed by virulent V. harve yi strains, may provide an explanation for the variability in virulence whi ch has been observed for this species.