Biocide use in the food industry and the disinfectant resistance of persistent strains of Listeria monocytogenes and Escherichia coli.

Aims: The aims of the project were threefold: to survey the use of disinfectants in the UK food industry; to assess the product and environmental microflora of selected food factories for the persistence of Listeria monocytogenes and Escherichia coli; and to determine the disinfectant resistance of any persistent strains.

Methods and results: A survey of the use of disinfectants in the UK food industry was undertaken in which a total of 40 sites were visited and a further 77 postal questionnaires were returned from farms, food manufacture, food transport and food retail sites. Quaternary ammonium compounds (QACs) were predominantly used, applied in small volumes as a mist. Approximately 30,000 samples from the product and environment of five chilled food factories were examined for L. monocytogenes and E. coli over a 3 year period. A total of 181 L. monocytogenes and 176 E. coli isolates were ribotyped to yield 19 and 34 ribogroups, respectively. Some strains were isolated only from the product, a number only from the environment and others from both niches. Some strains were seen to be persistent for the duration of the sampling exercise (2-3 years). The most common L. monocytogenes and E. coli strains, together with two environmental L. monocytogenes strains, were assessed for any resistance to commercial disinfectants as compared with a laboratory L. monocytogenes disinfectant testing strain. The resistance of the L. monocytogenes and E. coli strains isolated from the factory were not significantly different from the laboratory control strain.

Conclusions: Persistent strains of L. monocytogenes and E. coli are found in the UK food industry, though this persistence is not related to their increased susceptibility to the most commonly used disinfectants.

Significance and impact of the study: The concept of a persistent microflora in food factories will have an impact on the future selection of suitable control options, including the use of biocides.