Phenotypic and genetic characterization of a non-hemolytic variant of Listeria monocytogenes from cold-smoked salmon

The identity of 11 strains of Listeria monocytogenes and L. innocua isolated from various processed seafood products was confirmed by the polymerase chain reaction (PCR). All species' identities determined by a combination of selective media, beta-hemolysis, and biochemical fermentation profiles were confirmed by PCR, with one exception. One strain (4-4), previously identified as L. innocua because of the lack of hemolysis on blood agar, was shown to be L. monocytogenes. In order to determine whether this isolate was a weakly hemolytic variant and thus potentially virulent, or whether it was truly unable to express listeriolysin (LLO), a number of phenotypic and genetic characterizations were performed. The strain was competent for expression of other L. monocytogenes Virulence factors and characteristics such as the phosphatidylinositol-specific phospholipase C and the ability to adhere to and invade Caco-2 and J774 cells, but could not grow intracellularly and was avirulent in mice. DNA sequencing of the LLO gene (hly) from strain 4-4 showed that the genetic lesion was a single base-pair deletion, resulting in a frame-shift and subsequent stop codon in the translated open reading frame. Such a mutation is unique from those seen in previously described naturally occurring non-hemolytic L. monocytogenes strains. (C) U.S. Government.