Lm is a facultative intracellular microorganism and many of the bacterial determinants necessary for pathogenesis, including intracellular growth and spread of Lm, have been identified and are clustered on a 10-kb region of the chromosome termed the virulence gene cluster (vgc) which encodes the prfA, plcA, hly, mpl, actA and plcB genes organized in three transcriptional units [6] .