Inefficient Replication of Listeria innocua in the Cytosol of Mammalian Cells

Abstract

The efficiency of adherence to, internalization by, and replication in the cytosol of J774 macrophages and HEp-2 epithelial cells was compared between a nonspreading Listeria monocytogenes actA mutant and L. innocua. The studied L. innocua strains were equipped either with listeriolysin alone or with listeriolysin O (LLO) and the recently identified hexose-phosphate transporter of L. monocytogenes. All listerial strains expressed green fluorescent protein (GFP) under the control of the PrfA-dependent actA promoter. GFP expression was observed exclusively in the cytosol of host cells. Escape from the phagosome of LLO-expressing L. innocua strains was as efficient as that from L. monocytogenes. hpt-positive L. innocua showed significantly enhanced adherence to HEp-2 cells, but internalization was only slightly increased, compared with hpt-negative L. innocua. Subsequent replication of L. monocytogenes in the cytosol of the host cells proceeded within the next 6 h in most infected host cells, with a generation time <40 min. L. innocua prfA hly replicated more slowly (with a generation time of 60-90 min), and, in most host cells, bacterial replication stopped after 2-3 rounds of replication. In some cells, bacterial replication did not occur. Twenty-four hours after infection, the majority of J774 cells (>90%) infected with L. monocytogenes actA were dead, whereas most host cells infected with L. innocua were still alive. L. innocua equipped with the prfA, hly, and hpt genes of L. monocytogenes did not show significantly increased cytosolic replication, which indicates that expression of this sugar phosphate uptake system is not sufficient for extensive listerial replication in the cytosol of host cells.