Identifying suitableListeria innocuastrains as surrogates forListeria monocytogenesfor horticultural products

Abstract

AbstractWe conducted a laboratory-based study testing nineListeria innocuastrains independently and a cocktail of 11Listeria monocytogenesstrains. The aim was to identify suitableL. innocuastrain(s) to modelL. monocytogenesin inactivation experiments. Three separate inactivation procedures and a hurdle combination of the three were employed: thermal inactivation (55°C), UV-C irradiation (245 nm) and chemical sanitiser (Tsunami™ 100, a mixture of acetic acid, peroxyacetic acid and hydrogen peroxide). The responses were strain dependent in the case ofL. innocuawith different strains responding differently to different regimes.L. innocuaisolates generally responded differently to theL. monocytogenescocktail and had different responses among themselves. In the thermal inactivation treatment, inactivation of all strains including theL. monocytogenescocktail plateaued after 120 minutes. Chemical sanitiser, inactivation could be achieved at concentrations of 10 and 20 ppm with inactivation increasing with contact time up to 8 minutes, beyond which there was no significant benefit. Although most of theL. innocuastrains in the study responded similarly toL. monocytogeneswhen subjected to a single inactivation treatment, when the treatments were applied as hurdle, allL. innocuastrains except PFR16D08 were more sensitive than theL. monocytogenescocktail. PFR16D08 almost matched the resistance of theL. monocytogenescocktail but was much more resistant to the individual treaments. A cocktail of twoL. innocuastrains (PFR 05A07 and PFR 05A10) had the closest responses to the hurdle treatment to those of theL. monocytogenescocktail and is therefore recommended for hurdle experiments.ImportanceOwing to researcher safety risks it is often difficult to use actual pathogens, such asListeria monocytogenes, to explore different inactivation procedures under field conditions. Organisms that are closely related to the pathogen but without its virulence are therefore used as surrogates for the actual pathogen. However, this assumes that the surrogate will behave in a similar manner to the pathogen and it is difficult to predict the responses of the surrogate compared to the actual pathogen. This study compares the responses of individual and combined “cocktails” of strains of non-pathogenicListeria innocuato different inactivation procedures when compared to the response of a cocktail ofL. monocytogenes. Our study highlights the importance of evaluating a number of strains when choosing surrogates.