Intracellular replication of L. monocytogenes deletion mutants. (A) Caco-2 cells were infected at a multiplicity of infection (MOI) with either the wildtype strain or in-frame deletion mutants. The mutants EGDΔinlACGHE and EGDΔinlAB served as controls for the adhesion and invasion properties of the mutants. The number of viable bacterial cells recovered from the epithelial cells 8 h after infection was determined, and reduced survival of the ten mutants was calculated as a percentage in comparison with the wildtype strain. Error bars show the standard deviations from the mean. Each experiment was performed independently at least three times. The significance level was < 0.05 according to student's t test. (B) Phase contrast microscopy of a Caco-2 cell layer 8 h after infection with the wildtype strain (top) and mutant EGDΔlmo0135-0137 (buttom). The scale bars corresponds to 10 μm. The deletion of the transporter operon results in an approximately 10-fold reduced number of L. monocytogenes cells.