Virulent rough filaments of Listeria monocytogenes from clinical and food samples secreting wild-type levels of cell-free p60 protein

Atypical rough cell filaments of Listeria monocytogenes (designated FR vari ants), isolated from clinical and food samples, form long filaments up to 9 6 mu m in length and demonstrated wild-type levels of adherence, invasion, and cytotoxicity to human epithelial HEp-2, Caco-2, and HeLa cells. Unlike previously described avirulent rough mutants of L. monocytogenes that secre te diminished levels of the major extracellular protein p60 and that form l ong chains that consist of multiple cells of similar size (designated MCR v ariants), FR variants secreted wild-type or greater levels of p60. This stu dy shows that virulent filamentous forms oft. monocytogenes occur in clinic al and food environments and have atypical morphological characteristics co mpared to those of the wild-type form.