DAO 53:223-229 (2003)  -  doi:10.3354/dao053223

ABSTRACT: Addition of benzalkonium chloride (BKC) at 0, 0.3, 0.6 and 1.0 mg l^-1 to tryptic soy broth (TSB) had no effect on growth of Lactococcus garvieae, a bacterial pathogen of the giant freshwater prawn Macrobrachium rosenbergii. However, injection of the cultured cells into prawns at a dose of 4 x 10⁶ colony-forming units (cfu) prawn^-1 resulted in significantly higher mortality at 120 h (p <0.05) in prawns injected with cells grown in the absence of BKC than in prawns injected with cells grown in the presence of BKC. In other experiments, prawns were injected with TSB-grown L. garvieae (4 x 10⁶ and 3 x 10⁵ cfu prawn^-1) and then held in water containing BKC at 0, 0.3, 0.6 and 1.0 mg l^-1. After 120 h, mortality was significantly higher in all the BKC treatments than in the control without BKC. Prawns showed no significant differences in total hemocyte count (THC) or differential hemocyte count (DHC) amongst treatment and control groups. However, 96 h exposure to 0.3 mg l^-1 BKC or more resulted in a decrease in phenoloxidase activity and an increase in respiratory burst to levels considered to be cytoxic. In summary, exposure of L. garvieae to BKC at 0.3 mg l^-1 or more decreased its virulence to M. rosenbergii, while exposure of M. rosenbergii to BKC at 0.3 mg l^-1 or more increased its susceptibility to L. garvieae infection.

KEY WORDS: Macrobrachium rosenbergii · Lactococcus garvieae · Benzalkonium chloride · Challenge · Virulence · Hemocyte count · Phenoloxidase activity · Superoxide anion