An evaluation of Lux technology as an alternative methodology to determine growth rates of Listeria in laboratory media and complex food matrices

Abstract

Listeria monocytogenes is a foodborne pathogen which is a significant challenge in food production, particularly for ready-to-eat (RTE) products. Incidence of Listeria in food can be reduced by the application of multiple preservative factors or “hurdles”, which include acids, water activity and salts. Studying the growth of Listeria in complex foods is often reliant on laborious plate-counting techniques, therefore alternative methodologies are required. In this study we investigated the use of bioluminescence produced by chromosomally integrated genes encoding luciferase and its substrate to determine microbial growth rates in media and complex food matrices. Five Listeria innocua strains, used as a non-pathogenic surrogate for L. monocytogenes, were transformed with plasmid pPL2luxPhelp, resulting in a collection of Lux-tagged strains. Three test matrices (BHI broth, zucchini purée and béarnaise sauce) were adjusted, testing various combinations of pH (4.7–5.3), water activity (0.96 & 0.98) and undissociated acetic and propionic acid concentrations (0-2 mM). Adjusted matrices were inoculated with a cocktail of Lux-tagged strains and growth monitored over time by both bioluminescence and viable plate counts. Specific growth rates were calculated using both the bioluminescence and plate count data and compared. Statistical analysis showed that specific growth rates determined by bioluminescence did not significantly differ from those determined by plate counts (t-test, P > 0.05), while measurements of bias and accuracy showed good agreement between growth rates determined by both methods. Although plate counts remain the method of choice for detection of very low specific growth rates, this study demonstrates the potential of bioluminescence as a rapid alternative to plate counts for determining microbial growth rates in complex foods.