Transmission electron microscopy
(TEM) studies revealed that rough cell-forms of L. monocytogenes (designated FR variants),
isolated from clinical and food samples (and under conditions of sublethal heat stress),
consist of either single or paired long-®laments. These FR variants markedly contrast in cell
morphology from other previously described avirulent rough-mutants of L. monocytogenes
that form long chains consisting of multiple cells of similar size (designated MCR variants).
The identity of these Listeria isolates was determined using a commercially available, anti-
Listeria polyclonal KPL antibody and by the API Listeria biochemical gallery. This study
shows that ®lamentous rough-forms of L. monocytogenes may occur in clinical and food samples that are of undetermined pathogenicity.