Prevalence, serotype diversity, biofilm-forming ability and eradication of Listeria monocytogenes isolated from diverse foods in Shanghai, China

Listeria monocytogenes is a major foodborne pathogen that is well known for its high mortality rate in humans upon infection. This study aimed to investigate the prevalence and serotype diversity of L. monocytogenes isolates in various foods. From July 2013 to June 2014, a total of 648 food samples, originating from raw milk, quick-frozen food, vegetables, raw meats, cooked meats, aquatic products, dairy products and bean products, were collected from open-air markets, large supermarkets, delis and dairy farms, in Shanghai, China. Forty samples (6.2%) tested positive for L. monocytogenes, confirmed by both bacteriological and molecular methods. Among the food categories surveyed, the highest prevalence was observed in raw meat samples at 16.0% (15/94), followed by 9.9% (10/101) in quick-frozen food samples and 7.3% (4/55) in cooked meat samples. Serotyping showed that majority of the isolates (67.5%) belonged to serotype 1/2a, followed by 1/2b (22.5%), 1/2c (7.5%) and 3c (2.5%). Isolates were assessed for their ability to form biofilms. Significant differences in biofilm forming abilities were observed between storage temperatures and serotypes. Anti-biofilm efficacy of industrial disinfectants, chlorine dioxide (ClO2) and hydrogen peroxide (H2O2), was examined by measuring minimal effective concentration (MEC) on L. monocytogenes in biofilm. A positive correlation was observed among the biofilm’s resistance to H2O2 and the biomass of biofilms. ClO2 was found to be more effective against biofilm and inactivated all the listerial cells in the biofilm state, whereas H2O2 was not able to kill all the cells in biofilms at the recommended concentrations according to National Food Safety Standard of China. Our study suggests that there is potential public health risk of L. monocytogenes infection in Shanghai. Efficacy against L. monocytogenes biofilm should be considered when evaluating disinfectants.