Streptococcus suis mutants were generated by allelic replacement with a spectinomycin (spc) or chloramphenicol (cat) resistance gene cassette, using the method we described previously (Li et al., 2008) . Genetic complementation was performed by transforming the mutant strain with an E. coli-S. suis shuttle plasmid (pSET1 or pVA838) construct containing the functional copy of the mutated gene fused in-frame to a cloned S. suis suilysin gene promoter (Lun and Willson, 2004) .