Listeria monocytogenes serotype 4b strains demonstrate a fitness advantage over strains of serotype 1/2a and 4b-v1 on cantaloupe rind and flesh at different temperatures

Authors: Gorski, Lisa; Aviles Noriega, Ashley; Carter, Michelle

Submitted to: Microbiology Spectrum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2025
Publication Date: 8/7/2025
Citation: Gorski, L.A., Aviles Noriega, A., Carter, M.Q. 2025. Listeria monocytogenes serotype 4b strains demonstrate a fitness advantage over strains of serotype 1/2a and 4b-v1 on cantaloupe rind and flesh at different temperatures. Microbiology Spectrum. 13(9). Article e01252-25. https://doi.org/10.1128/spectrum.01252-25.
DOI: https://doi.org/10.1128/spectrum.01252-25

Interpretive Summary: Certain bacterial strains cause a disproportionate number of the illnesses acquired by eating cantaloupe contaminated with Listeria monocytogenes. USDA researchers therefore sought to understand whether an emerging variant responsible for three recent nationwide outbreaks (serotype 4b-v-1, sequence type 382) outcompetes other strains at growth and storage temperatures. On rinds, serotype 4b outcompeted another serotype (1/2a) as well as the new variant. On flesh, serotypes 4b again outcompeted 1/2a, but did not outcompete the new variant. Although all strains reproduce on cantaloupe surfaces, the new variant does not enjoy a competitive advantage observable under these experimental conditions. Instead, disproportionate persistence in soil, harvesting equipment, or other transmission factors may conceivably explain this variant’s disproportionate harm to food safety, grower liability, and consumer confidence. Microbiologists, food safety professionals, and consumers will benefit by ultimately identifying what drives pathogen transmission, offering needed means to reduce their harms.

Technical Abstract: Listeria monocytogenes is a foodborne pathogen implicated in several produce-related outbreaks. In nature, it is saprophytic and is isolated from agricultural areas and surface waters. The ability of the pathogen to grow at cold temperatures results in population growth when contaminated foods are stored in the refrigerator. Most L. monocytogenes outbreaks are caused by serotypes 1/2a and 4b. In the last decade, several produce-related outbreaks have been caused by an atypical serotype 4b subtype termed 4b-v1, specifically strains of Sequence Type (ST) 382. ST382 strains are overrepresented in environmental isolates. This study asked if strains of ST382 strains displayed better fitness on produce. Strains of serotypes 1/2a, 4b, and 4b-v1 were co-inoculated onto cantaloupe rinds and flesh and incubated for 24h at 25oC or for 7d at 4oC. After rinsing, 30 resulting colonies were screened for serotype. Serotype 4b strains demonstrated significantly better fitness on cantaloupe rind than either serotype 1/2a or 4b-v1 strains. On cantaloupe flesh, serotype 4b and 4b-v1 strains were statistically similar, and serotype 4b was more fit than serotype 1/2a. Serotype 4b colonies represented approximately 50% of the colonies recovered from rind and 40% of the colonies recovered from flesh. Incubation temperature was not a factor. While all the strains could grow on both cantaloupe surfaces, the data indicate that strains of ST382 did not have a fitness advantage on cantaloupe. Importance Most Listeria monocytogenes outbreaks are caused by serotypes 4b and 1/2a. An atypical variant of serotype 4b, 4b-v-1, specifically sequence type (ST) 382, caused three nationwide produce-related outbreaks in the last decade. To address fitness of ST382 on produce, these strains were inoculated in mixtures including serotypes 1/2a and 4b onto cantaloupe rinds and flesh and incubated at 25oC and 4oC. Fitness was determined by measuring the serotypes of the population recovered from the cantaloupe surfaces. Serotype 4b strains were recovered consistently at higher levels than both serotype 1/2a and 4b-v1 strains from cantaloupe rind. On cantaloupe flesh, serotypes 4b and 4b-v1 were statistically equivalent, but serotype 4b had the advantage over serotype 1/2a strains. While all the strains can reproduce on the cantaloupe surfaces, ST382 was not more fit on cantaloupe. Furthermore, serotype 4b strains had a fitness advantage in mixed populations on cantaloupe.