Poor Invasion of Trophoblastic Cells but Normal Plaque Formation in Fibroblastic Cells despite actA Deletion in a Group of Listeria monocytogenes Strains Persisting in Some Food Processing Environments

We determined mammalian cell invasion and virulence gene (inlA, inlB, and actA) sequences of Listeria monocytogenes strains belonging to a molecular subtype (RAPD 9) that often persists in Danish fish-processing plants. These strains invaded human placental trophoblasts less efficiently than other L. monocytogenes strains, including clinical strains, and they carry a premature stop codon in inlA. Eight of 15 strains, including the RAPD 9 and maternofetal strains, had a 105-nucleotide deletion in actA that did not affect cell-to-cell spread in mouse fibroblasts. The RAPD 9 strains may still be regarded as of low virulence with respect to human listeriosis.