Glutamate Decarboxylase-Mediated Nisin Resistance in Listeria monocytogenes

Analysis of a complete set of glutamate decarboxylase (gad) mutants of Listeria monocytogenes strain LO28 (Delta gadD1, Delta gadDT1, Delta gadD2, Delta gadT2, and Delta gadD3 mutants) revealed that the Delta gadD1 mutant is impaired in its ability to tolerate exposure to both sublethal and lethal levels of the lantibiotic nisin. gadD1 is strain variable and is found only in approximately 50% of L. monocytogenes strains. Growth and survival experiments revealed that possession of gadD1 correlates with a higher degree of tolerance to nisin. Significantly, a similar finding using a gadB mutant of L. lactis IL1403 implies that this may be a general phenomenon in Gram-positive bacteria. Our findings thus suggest that the specific inhibition of GAD activity or a reduction in the levels of free glutamate may prevent the growth of otherwise resistant GAD(+) bacteria in foods where low pH and/or nisin is used as a preservative.