Influence of the capsular layer on the virulence of Pasteurella piscicida for fish

The influence of the capsule of Pasteurella piscicida on the cell surface properties of this microorganism as well as on the virulence and the capacity of the strains to grow in fish sera was examined. Although all the P. piscicida strains synthetized an additional exostructure in glucose-enriched-medium, only virulent strains constitutively synthetized capsule. The cell surface of all the P. piscicida isolates showed a low hydrophobic nature. No strains pelleted in broth culture (SP-) and all of them were stable after boiling (PAB). All isolates attached to the fish cell line CHSE-214 with values of adherence ranging from 2 to 5% of the initial bacterial inoculum. The presence of induced capsular material caused changes in some cell surface characteristics such as hydrophobicity and stability after boiling. A decrease in the adherence capacity of all the P. piscicida strains was also observed. However, the capsule increased the degree of virulence for fish of the nonpathogenic strains (LD50 was reduced in about 4 log) and conferred to all the isolates resistance to serum killing. Therefore, these results indicate that the presence of capsule can play an important role in the pathogenesis of P. piscicida.