Virulence of Aeromonas hydrophila to channel catfish Ictaluras punctatus fingerlings in the presence and absence of bacterial extracellular products

摘要

ABSTRACT: We investigated the virulence of three 2009 west Alabama isolates of Aeromonas hydrophila (AL09-71, AL09-72 and AL09-73) to channel catfish Ictalurus punctatus fingerlings (4.6 ± 1.3 g) in the presence and absence of extracellular products (ECPs) from overnight bacterial culture using both bath immersion and intraperitoneal injection routes. At a concentration of 1.65 × 108 colony-forming units (CFU) ml–1, AL09-73 without its ECPs killed 100% of the catfish fingerlings within 2 h by bath immersion. However, at a similar concentration, AL09-73 in the presence of its ECPs killed only 23 ± 6% catfish fingerlings. The absence of ECPs in the bath immersion experiment also significantly (p 0.05) increased the virulence of AL09-71, AL09-72, and AL98-C1B, a 1998 Alabama strain of A. hydrophila, suggesting that the virulence of the 4 A. hydrophila isolates was mainly due to bacterial cells, not to their overnight ECPs. Filter-sterilized ECPs failed to kill any catfish by bath immersion or injection. The virulence order of the 4 A. hydrophila isolates, by both bath immersion and intraperitoneal injection, was: AL09-73 ≥ AL09-71 AL09-72 ≥ AL98-C1B. At 2 h post bath immersion, all 4 isolates of A. hydrophila were found in all tissues studied (skin, intestine, liver, spleen, kidney, gill and brain), with the highest bacteria count being in the gill and kidney.