VIRULENCE MECHANISMS OF PHOTORHABDUS SP STRAIN K122 TOWARD WAX MOTH LARVAE

Virulence of Photorhabdus sp. strain K122 was assessed by injecting whole cells, spent culture medium, and lysate fractions into the hemocoel of larvae of wax moth, Galleria mellonella. Virulence correlated with the growth rate of the cultures and all larvae died after the cultures entered the stationary phase. Maximal production of protease and lipase exoenzymes occurred at the stationary phase and the extracellular fraction (spent culture medium from stationary phase cells) was found to be toxic to the insects. Part, but not all, of that toxicity was attributed to secreted lipase. Total lysates of K122 were toxic to G. mellonella, though they took. three times longer to kill the larvae than did the same number of viable cells. The toxicity of the lysates was located in the membrane fraction, but this was found to be a nonspecific effect because the membrane fractions of Escherichia coli and Bacillus subtilis were toxic to the same extent, whereas viable whole cells of these species were nonpathogenic, It is proposed that the nonspecific toxicity of bacterial lysates to G. mellonella derives from the nonlocalized activation of its humoral immune response by components of the membrane fraction followed by damage to its tissues by a product of its own immune response. (C) 1995 Academic Press, Inc.