Development of a set of Listeria monocytogenes strains for conducting challenge tests

One of the recommendations of the EURL Listeria monocytogenes (Lm) Technical Guidance Document on shelf-life studies for L. monocytogenes in ready-to-eat foods (Afssa, 2008) dealt with the choice of strains to conduct microbial challenge tests assessing growth potential. It was noticed that they should be performed with a mixture of at least 3 strains to account for variations in growth or survival among strains. It was recommended that one of the strains must be chosen from reference strains; the others must be chosen between strains isolated from the same or similar food products. A major outcome of the enquiry launched in April 2010 by EURL Lm on the need to revise the EURL Lm Technical Guidance Document was related to the choice of the strains. EURL Lm had thus settled a working group of volunteering National Reference Laboratories (NRLs) to share knowledge and technical approaches to deal with strain variability, as it may be difficult to have available in any lab well characterised strains (e.g. origin or growth). Strain variability of the behaviour of foodborne bacterial pathogens is found, whatever the species considered (Lianou and Koutsoumanis, 2013). Yet, the question of its consequence on the results of challenge tests could be raised. A short review was proposed on the most interesting works during the last years on intraspecific variability. This review would serve as a basis for discussion in the working group. Growth rates and growth limits of L. monocytogenes in various media, foods and conditions had been illustrated in numerous papers. We focused the review on studies which included a large number of strains in their experiments. Growth rate of a strain at a determined temperature only depends on environmental conditions (medium or food). Lag-time is dependent on the last two factors but also on the conditions preceding contamination of that medium/food or on the initial number of bacteria (Guillier and Augustin 2006). Thus, apparent lag-time differences between strains may reflect other factor(s) than strain variability. For this reason, strain variability was only assessed on growth rate.Work conducted in the frame of the EURL for Listeria monocytogenes co-funded by the European Commissio