A defined allelic-replacement mutant of the sly gene, encoding a thiol-acti
vated cytolysin, from a European isolate of Streptococcus suis serotype 2 w
as generated and characterized. Unlike the parental strain, it is nonhemoly
tic, noncytotoxic for cultured macrophage-like cells, avirulent in a mouse
infection model, yet only slightly attenuated in a porcine model of systemi
c infection.