Growth of Listeria monocytogenes in salmon roe – A kinetic analysis

Listeria monocytogenes in aquatic food products: Spotlight on epidemiological information, bio-based mitigation strategies and predictive approaches

Listeria monocytogenes is the foodborne pathogen responsible for listeriosis in humans. Its ability to grow at refrigeration temperatures, particularly in products that support its growth and have a long-refrigerated shelf-life, poses a significant health risk, especially for vulnerable consumer groups such as pregnant woman and immunocompromised individuals. A comprehensive analysis of L. monocytogenes in aquatic food products (AFPs) was conducted, examining the prevalence of the bacterium, the associated outbreaks, and the resulting deaths. Data from 66 studies, comprising a total of 19,373 samples, were analysed from the scientific literature to determine prevalence of the pathogen. The mean pooled prevalence of L. monocytogenes was 11% (95% CI: 8-14%) among different AFPs categories. Additionally, an overview of worldwide listeriosis outbreaks associated with contaminated AFPs between 1980 and 2023 was provided, totalling 1,824 cases, including 41 deaths. Furthermore, a compilation of bio-based mitigation strategies was presented, including the use of lactic acid bacteria (LAB) and bacteriophages as bio-protective cultures to inhibit L. monocytogenes in AFPs. A variety of predictive microbiology models, based on growth prediction and interaction for Listeria, were reviewed to assess the effectiveness of control strategies in different types of AFPs, offering insights into pathogen behaviour throughout the production chain. The reported growth models described the impact of product storage temperature on pathogen growth parameters, while interaction models, based on the inhibitory effect of LAB presence against Listeria, were described using the Jameson-effect approach and Lotka-Volterra model (i.e., predator-prey model). Both models can be used to describe the simultaneous growth of two bacterial populations and their interactions. Several Quantitative Risk assessment studies have been conducted for AFP, identifying the food category as a relevant contributor in Listeriosis risk, and providing predictive insight into how storage temperature, food microbiota, product shelf-life and population aging influence the risk by L. monocytogenes. More importantly, this quantitative approach can serve as a key tool to assess the effectiveness of specific mitigation and intervention strategies to control the pathogen, such as sampling schemes or bio-preservation techniques.

Aspergillus fumigatus AR04 obeys Arrhenius' rule in cultivation temperature shifts from 30 to 40°C

To set a benchmark in fungal growth rate, a differential analysis of prototrophic Aspergillus fumigatus AR04 with three ascomycetes applied in > 103 t year-1 scale was performed, i.e. Ashbya gosspyii (riboflavin), Aspergillus niger (citric acid) and Aspergillus oryzae (food-processing). While radial colony growth decreased 0.5-fold when A. gossypii was cultivated at 40°C instead of 28°C, A. fumigatus AR04 responded with 1.7-fold faster hyphal growth. A. niger and A. oryzae formed colonies at 40°C, but not at 43°C. Moreover, all A. fumigatus strains tested grew even at 49°C. In chemostat experiments, A. fumigatus AR04 reached steady state at a dilution rate of 0.7 h-1 at 40°C, 120% more than reported for A. gossypii at 28°C. To study mycelial growth rates under unlimited conditions, carbon dioxide increase rates were calculated from concentrations detected online in the exhaust of batch fermentations for 3 h only. All rates calculated suggest that A. fumigatus AR04 approximates Arrhenius' rule when comparing short cultivations at 30°C with those at 40°C. Linearization of the exponential phase and comparison of the slopes revealed an increase to 192% by the 10°C up-shift.

Estimating Listeria monocytogenes Growth in Ready-to-Eat Chicken Salad Using a Challenge Test for Quantitative Microbial Risk Assessment

Currently, there is a growing preference for convenience food products, such as ready-to-eat (RTE) foods, associated with long refrigerated shelf-lives, not requiring a heat treatment prior to consumption. Because Listeria monocytogenes is able to grow at refrigeration temperatures, inconsistent temperatures during production, distribution, and at consumer's household may allow for the pathogen to thrive, reaching unsafe limits. L. monocytogenes is the causative agent of listeriosis, a rare but severe human illness, with high fatality rates, transmitted almost exclusively by food consumption. With the aim of assessing the quantitative microbial risk of L. monocytogenes in RTE chicken salads, a challenge test was performed. Salads were inoculated with a three-strain mixture of cold-adapted L. monocytogenes and stored at 4, 12, and 16 °C for eight days. Results revealed that the salad was able to support L. monocytogenes' growth, even at refrigeration temperatures. The Baranyi primary model was fitted to microbiological data to estimate the pathogen's growth kinetic parameters. Temperature effect on the maximum specific growth rate (μ_max ) was modeled using a square-root-type model. Storage temperature significantly influenced μ_max of L. monocytogenes (p < 0.05). These predicted growth models for L. monocytogenes were subsequently used to develop a quantitative microbial risk assessment, estimating a median number of 0.00008726 listeriosis cases per year linked to the consumption of these RTE salads. Sensitivity analysis considering different time-temperature scenarios indicated a very low median risk per portion (<-7 log), even if the assessed RTE chicken salad was kept in abuse storage conditions.