Fitness differences between Listeria monocytogenes serotypes 1/2a, 4b, and 4bv-1 in competition for growth on lettuce leaf sections

Authors: Gorski, Lisa; Aviles Noriega, Ashley

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2024
Publication Date: 7/24/2024
Citation: Gorski, L.A., Aviles Noriega, A. 2024. Fitness differences between Listeria monocytogenes serotypes 1/2a, 4b, and 4bv-1 in competition for growth on lettuce leaf sections. Journal of Food Protection. 87(9). Article 100333. https://doi.org/10.1016/j.jfp.2024.100333.
DOI: https://doi.org/10.1016/j.jfp.2024.100333

Interpretive Summary: Foodborne illness due to Listeria monocytogenes contamination results in listeriosis, which has a high rate of hospitalization and death. Two of the three serotypes that cause the most listeriosis are serotypes 1/2a and 4b. Serotype 4b has a variant called 4bv-1. Serotype 4bv-1 strains, specifically those identified as Sequence Type (ST) 382, have been responsible for three produce-related outbreaks since 2014. These outbreaks were linked to contaminated bagged leafy lettuce, caramel apples, and stone fruit. One reason for the link of ST 382 to produce outbreaks may be that this strain-type is more fit on produce. High fitness results when a strain contains factors that make it grow or survive better on produce samples. Fitness is assessed through competition experiments where different subtypes of strains are co-inoculated onto the produce, allowed to grow, and the percentages of the different subtypes that result are calculated. In this project, the fitness of ST 382 strains in competition with strains of serotypes 1/2a and 4b was assessed on cut lettuce sections after growth at 4 oC and 25 oC. Thirty L. monocytogenes colonies resulting from the growth on each lettuce piece were characterized for serotype. In the individual mixtures with three strains, none of the ST 382 strains showed better fitness for growth on lettuce at either 4 oC or 25 oC. Statistical analysis of the recovery of twelve strains tested indicated that ST 382 strains were less fit for lettuce growth than the other serotypes. The data indicate that ST 382 strains do not have a competitive fitness advantage on cut lettuce sections.

Technical Abstract: Listeria monocytogenes is a foodborne pathogen that lives in nature as a saprophyte. Two of the three most common serotypes that cause foodborne listeriosis are 1/2a and 4b. Within serotype 4b, there is a variant called 4bv-1. In the last decade several produce-related outbreaks (linked to leafy salad, caramel apples, and stone fruit) were linked to 4bv-1 strains, specifically those of Sequence Type 382. This study assessed the fitness of ST 382 strains on lettuce leaf sections to determine if they are more fit on produce than strains of other serotypes. Strains of serotypes 1/2a, 4b, and ST 382 were inoculated as mixtures onto lettuce and incubated at 4 oC for 7 days or 25 oC for 24 hours. Thirty L. monocytogenes colonies resulting from the growth on each lettuce piece were characterized for serotype by multiplex PCR, and the percentages of each serotype recovered compared. In the individual mixtures with three strains, none of the ST 382 strains showed better fitness for growth on lettuce at either 4 oC or 25 oC. Overall, ST 382 strains showed better recovery from lettuce sections grown at 4 oC than at 25 oC. Statistical analysis of the recovery of twelve strains tested in competition experiments indicated that ST 382 strains were less fit for lettuce growth when competing against the other serotypes. The data indicate that ST 382 strains do not have a competitive fitness advantage on cut lettuce sections.