Expression of hlyII in recombinant B. subtilis BD170 renders bacteria hemolytic. Recombinant B. subtilis::hlyII (BD170-EH2) can lead to the death of Daphnia magna macroorganisms. Expression of HlyII in the crustacean gut leads to the destruction of intestine cells followed by a gradual disruption of other tissues. Fluorescent microscopy reveals post-infection changes in mitochondrial potential of intestine tissues in the first 24 h of infection, suggesting that formation of ionic pores leads to cell lysis and, finally, host death [15] and [16]. Expression of hlyII in B. subtilis::hlyII-hlyIIR (BD170-EH2R) is strongly suppressed by HlyIIR regulator, and this recombinant is not pathogenic to the crustacean. These data show that hemolysin II acting alone is a sufficient virulence factor, and the regulation of hlyII expression could be an important step in the adaptation of bacteria to various environmental niches.