Cellular morphology of rough forms of Listeria monocytogenes isolated from clinical and food samples

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‐filaments. 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 filamentous rough‐forms of L. monocytogenes may occur in clinical and food samples that are of undetermined pathogenicity.