A major catalase (KatB) that is required for resistance to H2O2 and phagocyte-mediated killing in Edwardsiella tarda

Abstract

Edwardsiella tardacauses haemorrhagic septicaemia in fish and gastro- and extra-intestinal infections in animals including humans. Resistance to phagocyte-mediated killing is one of the virulence factors ofEd. tarda. The authors' previous studies using TnphoAtransposon mutagenesis indicated thatkatBmutants derived from the strain PPD130/91 are at least 1·6 log higher in LD50values than the wild-type strain. These findings suggest the involvement of catalase (KatB) inEd. tardapathogenesis. In this study, experiments were conducted to characterize the contribution of KatB toEd. tardainfection. Zymographic analyses indicated that the 22Ed. tardastrains examined expressed three different types of catalase-peroxidases (Kat1–3) based on their mobility in non-denaturing polyacrylamide gels. KatB (Kat1), the major catalase enzyme, was expressed in eight out of 22Ed. tardastrains, and was commonly found in virulent strains except AL9379. AL9379 has a mutatedkatB, which has a base substitution and a deletion that translate into stop codons in the catalase gene. KatB produced by PPD130/91 was located in both periplasmic and cytoplasmic fractions and was constitutively expressed in various growth phases. Kinetics studies indicated that the catalase provided resistance to H2O2- and phagocyte-mediated killing. Infection kinetics studies ofkatBmutant 34 in gourami fish demonstrated its inability to survive and replicate in phagocyte-rich organs and this prevented the dissemination of infections when compared to the wild-type. Complementation of catalase mutants restored the production of catalase, and led to an increase in the resistance to H2O2- and phagocyte-mediated killing, and a decrease in LD50values. This study has identified and characterized a major catalase gene (katB) that is required for resistance to H2O2- and phagocyte-mediated killing inEd. tarda. The results also suggest that catalase may play a role as a virulence factor inEd. tardapathogenesis.