Pathogenic variability of Pasteurella piscicida during in vitro cultivation as a preliminary study for vaccine production

During in vitro cultivation of bacteria, the pathogenetic factors may chang e in amount and quality. Knowledge of this variability is important in vacc ine production. The pathogenic variability of Pasteurella piscicida broth c ulture (BC), extracellular products (ECPs) and whole cells (WC) was investi gated in sea bass (Dicentrarchus labrax) 2-3 g wet weight, inoculated intra peritoneally. Ten Pasteurella piscicida field strains isolated from fish af fected by different outbreaks which occurred in Italian fish farms were use d. When BC and ECPs were intraperitoneally injected, no variability in viru lence was observed between strains of different geographical origin and no attenuation was registered between strains of different in vitro passages. The electrophoretic (SDS-PAGE) analysis of ECP showed evidence of only quan titative differences inside the panel of strains and between the same strai n collected at different in vitro cultivation times. ECPs produced from a B C incubated for 18-60 h were highly toxic. This ability was heat labile and not active when ECPs were administered by immersion. No toxic capacity was observed with WC where infection was greater with a 18-60 h incubation bro th culture. Results seems to indicate that the toxic activity of Pasteurell a piscicida is mostly due to ECP and not bounded to the cell.