Listeriolysin O secretion by Listeria monocytogenes in the presence of cysteine and sorbate

The influence of cysteine or cysteine-HCl and their combination with potassium sorbate on growth of Listeria monocytogenes and listeriolysin O (LLO) secretion, and on activation of LLO in the haemolysin assay system was studied. Both cysteine and cysteine-HCl (10 and 20 mmol l-1) enhanced LLO secretion in tryptic soy broth supplemented with yeast extract during 24 h incubation at 35 degrees C. While sorbate did not affect growth, it suppressed both LLO secretion and LLO activation by cysteine in the haemolysin activity assay. These findings provide further evidence that sulphydryl-containing enzymes are implicated in the mechanism of microbial inhibition of sorbate. Addition of sorbate to foods has the potential of inactivating listeriolysin and other thiol-dependent toxins.