Measurements of bacteriolysis in vitro were performed as previously described (28) with slight modifications. Cultures of L. monocytogenes carrying the lacZ gene encoding a transposon (Tn917-LTV3) were grown to mid-logarithmic or early stationary phase in BHI medium. Half of the culture was subjected to centrifugation to separate bacteria from the supernatant. The other half of the culture was subjected to complete bacteriolysis by addition of SDS to reach a concentration of 0.1% and to bead beating performed with 0.1-mm-diameter silicon beads for 10 min at 2,000 rpm. One hundred microliters of culture supernatant or 100 µl of serially diluted lysate (standard curve) was monitored for β-galactosidase activity by combining the supernatant or the lysate with 100 µl of 200 µM MUG (4-methylumbelliferyl β-d-galactopyranoside; Invitrogen) (excitation/emission [ex/em] wavelengths = 360 nm/449 nm) resuspended in Z buffer (0.1 M phosphate, 0.01 M KCl, 1 mM MgSO4, 50 mM β-mercaptoethanol, pH 7.0). The rate of β-galactosidase activity in the culture supernatant was compared to the standard curve to calculate percent bacteriolysis in the broth culture.