Use of Double-Gradient Plates To Study Combined Effects of Salt, pH, Monolaurin, and Temperature on Listeria monocytogenes†

Abstract

This study evaluated the combined effects of pH, NaCl, incubation temperature, and sublethal concentrations of monolaurin on the survival of Listeria monocytogenes using the double-gradient diffusion technique. L. monocytogenes tolerance to NaCl was greatest (>78 g/liter) at neutral pH (6.8 to 7.2) and increased in the pH range 7.0 to 5.4 as the incubation temperature was lowered. Monolaurin at 2 μg/ml lowered the salt tolerance of L. monocytogenes to 60 g/liter independently of pH. At 4 μg of monolaurin per ml, salt tolerance was reduced to approximately 40 g/liter with no growth occurring at pH 6.0 to 5.4 and 25 g of NaCl per liter. At 8 μg of monolaurin per ml, only a subpopulation of the initial inoculum tolerated 25 g of NaCl per liter at neutral pH (6.5 to 7.5). Monolaurin reduced the tolerance of L. monocytogenes to NaCl and low pH.