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

Abstract

Summary During in vitro cultivation of bacteria, the pathogenetic factors may change in amount and quality. Knowledge of this variability is important in vaccine production. The pathogenic variability of Pasteurella piscicida broth culture (BC), extracellular products (ECPs) and whole cells (WC) was investigated in sea bass (Dicentrarchus labrax) 2–3 g wet weight, inoculated intra-peritoneally. Ten Pasteurella piscicida field strains isolated from fish affected by different outbreaks which occurred in Italian fish farms were used. When BC and ECPs were intraperitoneally injected, no variability in virulence 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 quantitative differences inside the panel of strains and between the same strain collected at different in vitro cultivation times. ECPs produced from a BC 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 broth culture. Results seems to indicate that the toxic activity of Pasteurella piscicida is mostly due to ECP and not bounded to the cell.