Estimation of Listeria Monocytogenes Transfer Coefficients and Efficacy of Bacterial Removal Through Cleaning and Sanitation

Listeria monocytogenes is readily found in the environment of retail deli establishments and can occasionally contaminate food handled in these establishments. Here we synthesise the available scientific evidence to derive probability distributions and mathematical models of bacterial transfers between environmental surfaces and foods, including those during slicing of food, and of bacterial removal during cleaning and sanitising (models available at www.foodrisk.org).

Transfer coefficients varied considerably by surface type, and after log(10) transformation were best described by normal distributions with means ranging from -0.29 to -4.96 and standard deviations that ranged from 0.07 to 1.39. 'Transfer coefficients' during slicing were best described by a truncated logistic distribution with location 0.07 and scale 0.03. In the absence of protein residues, mean log inactivation indicated a greater than 5 log(10) reduction for sanitisation with hypochlorite (mean: 6.5 log(10); 95% confidence interval (CI): 5.0-8.1 log(10)) and quaternary ammonium compounds (mean: 5.5 log(10); 95% CI: 3.6-7.3 log(10)), but in the presence of protein residues efficacy reduced dramatically for hypochlorite (mean: 3.8 log(10); 95% CI: 2.1-5.4 log(10)) as well as quaternary ammonium compounds (mean: 4.4log(10); 95% CI: 2.5-6.4 log(10)).

Overall, transfer coefficients are therefore low, even though cross-contamination can be extremely efficient under certain conditions. Dozens of food items may consequently be contaminated from a single contaminated slicer blade, albeit at low concentrations. Correctly performed sanitizing efficiently reduces L. monocytogenes contamination in the environment and therefore limits cross-contamination, even though sanitisation is only performed a few times per day. However, under unfavourable conditions reductions in bacterial concentration may be far below 5 log(10). The probability distributions and mathematical models derived here can be used to evaluate L. monocytogenes cross-contamination dynamics in environments where foods are handled, and to assess the potential impact of different intervention strategies.