Virulence of Vibrio harveyi possessing a transferable chloramphenicol resistance determinant to larvae of Indian white shrimp Fenneropenaeus indicus (Decapoda)

Abstract

The virulence of luminous Vibrio harveyi possessing a transferable chloramphenicol-resistance determinant to Indian white shrimp, Fenneropenaeus indicus larvae was studied. The V. harveyi strains isolated from luminous F. indicus larvae produced extracellular virulence factors such as haemolysin, chitinase, proteinase, lipase and phospholipase were resistant to at least four of six antibiotics tested. The chloramphenicol-resistance determinant was transferred from V. harveyi to Escherichia coli at frequencies of 6.69  10⁻⁴ - 4.27  10⁻³. The concentration of V. harveyi cells capable of causing 50% mortality differed significantly (P<0.05) and the 72 h LD₅₀ values ranged between 273 and 3,040 cells/ml. The chloramphenicol-resistant strains were more virulent to shrimp larvae compared to chloramphenicol-sensitive strain. Prevalence of highly virulent V. harveyi strains harbouring a transferable chloramphenicol-resistance determinant together with other extracellular virulence factors may hamper the hatchery production of penaeid shrimp larvae.