Superoxide dismutase and catalase in Photobacterium damselae subsp. piscicida and their roles in resistance to reactive oxygen species

Photobacterium damselae subsp. piscicida (formerly Pasteurella piscicida) i s the causative agent of pasteurellosis or pseudotuberculosis in warm water marine fish. Enzymes which neutralize reactive oxygen species, produced du ring aerobic metabolism or during respiratory burst in fish macrophages, ar e important virulence factors in many pathogens. This study characterizes a periplasmic superoxide dismutase (SOD) and a cytoplasmic catalase in P. da mselae. Purification and partial amino-terminal sequencing confirmed the SO D to be iron-cofactored, with a high degree of homology to other bacterial FeSODs. The SOD was common to all strains analysed in terms of type, locati on and activity, whilst the catalase varied in activity between strains. Th e catalase was constitutively expressed, but the SOD appeared to be repress ed under low oxygen conditions. In spite of the presence of a periplasmic S OD, P. damselae was susceptible to killing by exogenous superoxide anion ge nerated in a cell-free system. Addition of exogenous SOD to this system did not abolish the bactericidal effect; however, addition of catalase was pro tective. These results suggest that lack of periplasmic catalase may be imp licated in susceptiblity to killing by reactive oxygen species.