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

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 x 10(8) 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 > or = AL09-71 > AL09-72 > or = 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.