Genetic variations and accelerated growth at low temperatures in a Listeria monocytogenes isolate

Listeria monocytogenes contamination of fresh-cut produce poses a threat to consumers due to its high mortality rate in humans. Because it is widespread in the environment, as well as able to grow and survive in harsh conditions, such as low temperatures, it is difficult to control. A strain of L. monocytogenes, L008, with significantly faster growth than other strains, was identified in our laboratory during experiments with fresh-cut produce. The L008 strain had a consistently shorter lag phase and reached maximum spectrophotometric absorbance (optical density) faster than three other L. monocytogenes strains during growth curve studies at 4 and 8°C. The lag phase was also shorter when compared with two of the three strains at 12°C. A genetic comparison between L008 and the other three strains identified missense mutations in the fapR, mngB, pfkB, and uvrC genes of L008 that may account for the faster growth. These genes are involved in metabolism, cell membrane maintenance, and stress tolerance, which may allow L008 to adapt to lower temperatures better than the other strains. Accelerated growth at low temperatures may elevate the risk associated with this isolate and other L. monocytogenes strains with similar genetic variants. This study highlights the importance of including bacterial isolates that demonstrate a worst-case scenario for lag phase or more rapid growth in foodborne pathogen challenge studies.

Listeria monocytogenes is responsible for listeriosis in humans and has a relatively high incidence of hospitalization and mortality when compared to other foodborne pathogens. The psychrotrophic nature of L. monocytogenes allows it to grow at low temperatures, makes it difficult to control, and we identified an isolate with faster growth at 4, 8, and 12°C. A comparison of the isolate's genome with three other L. monocytogenes strains found changes in genes involved in energy usage, cell membrane maintenance, and stress resistance. These genetic differences may allow the isolate to adjust to lower temperatures better than other strains and replicate more quickly. The ability to grow faster at low temperatures could increase the risk of foodborne illness from this isolate and other L. monocytogenes with similar genetic changes.