PhD study trip

Mechanism of non-linear inactivation

Listeria monocytogenes is a food-borne pathogen causing listeriosis. Its robustness and ability to grow under harsh conditions make it a very hard pathogen to eliminate. L. monocytogenes may be exposed to acidic conditions in foods and during gastric passage, indicating the importance of acid shock survival capacity.

An additional feature that can have implications for food safety is the occurrence of subpopulations of stress resistant variants of L. monocytogenes. The presence of acid resistant subpopulations can lead to tailing of the inactivation curves. This is a major concern for the food industry since tailing can lead to a higher number of surviving organisms than expected which, when ingested, are more likely to survive stomach passage and potentially cause disease.


The aim of this project is to extend our knowledge of the generation and occurrence of stress resistant variants in populations of L. monocytogenes and the mechanisms leading to increased resistance, and to investigate the predictability of the occurrence of these mutations and corresponding phenotypes in other strains and species.


Previous research showed that exposure to high hydrostatic pressure (HHP) [1] and heat [2] results in inactivation curves exhibiting considerable tailing. A set of colonies isolated from the tail were tested on stress resistance and were confirmed to have a stable phenotype being more resistant than the wild type. In an attempt to extend the previously described set of variants, and to investigate the effect of different types of stress on the type of variants being found, acid stress was applied to L. monocytogenes. After optimization of the experimental set-up, L. monocytogenes was exposed to different pH conditions. This resulted in a set of inactivation curves showing considerable tailing, as well as the isolation of a  set of acid resistant variants with a stable phenotype.

Future research

The set of stable resistant variants, retrieved after HHP, heat and acid stress, is being genotypically and phenotypically characterized in order to get more insight in the mechanisms underlying stress resistance and heterogeneity therein. Furthermore we will  investigate if the mutations, leading to a more stress resistant phenotype in L. monocytogenes, also occur in other strains and species.


Acid stress is an important hurdle to eliminate pathogens in both the stomach and food products. However, acid stress can lead to tailing of inactivation curves of L. monocytogenes and the survival of acid resistant subpopulations. Understanding the mechanisms underlying these stress resistant variants is important for food industry to control the presence of these subpopulations in its food products.


1.    Van Boeijen, I.K.H., R. Moezelaar, T. Abee, and M.H. Zwietering. 2008. Inactivation kinetics of three Listeria monocytogenes strains under high hydrostatic pressure. Journal of Food Protection 71, 2007 – 2013.
2.    Van Boeijen, I.K.H., C. Francke, R. Moezelaar, T. Abee, M.H. Zwietering. 2011. Isolation of highly heat-resistant Listeria monocytogenes variants by use of a kinetic modelling-based sampling scheme. Applied and Environmental Microbiology 77, 2617 – 2624.