Listeria monocytogenes F2365 carries several authentic mutations potentially leading to truncated gene products, including inlB, and demonstrates atypical phenotypic characteristics.

Searches of the genome annotation of Listeria monocytogenes F2365, an isolate from the 1985 listeriosis epidemic in California, showed that this strain carries 20 authentic mutations resulting in premature stop codons, including a nonsense mutation in inlB. Here we showed that L. monocytogenes F2365 demonstrates atypical virulence-associated characteristics, including significantly (P < 0.05) reduced invasion efficiency in Caco-2 cells as compared with a closely related lineage I serotype 4b strain as well as significantly (P < 0.05) greater variation in invasiveness when grown under different conditions compared with standard laboratory control and other lineage I serotype 4b strains. In addition, L. monocytogenes F2365 demonstrated distinct growth characteristics, including a significantly (P < 0.05) reduced exponential growth rate when compared with laboratory control and other lineage I serotype 4b outbreak-associated strains as well as a significantly (P < 0.05) longer lag phase duration time compared with another lineage I serotype 4b strain. Our results support that L. monocytogenes F2365 is characterized by genotypic and phenotypic properties that are atypical of other L. monocytogenes strains.