Purpose: This study was conducted to (i) measure the ability of persistent and transient Listeria isolates to form biomass and (ii) determine the amount of genetic change persistent isolates from the same harborage site accumulated over time.
Methods: Thirty Listeria isolates were selected based on classification as persistent or transient from previous longitudinal studies that used combined testing and molecular subtyping. Isolates were characterized by ability to form biomass at 12°C and 30°C using a crystal violet assay; each isolate was assayed in triplicate in two biological replicates. Persistent isolates from the same harborage site were also characterized by whole genome sequencing and comparative genomic analyses.
Results: Significantly more biomass production was observed (P < 0.05) at 30°C versus 12°C. Transient Listeria isolates produced more biomass at 30°C compared to persistent isolates (P < 0.05), while no difference in biomass was observed between persistent and transient isolates at 12°C. Among the different Listeria species, L. monocytogenes showed the most (P < 0.05) biomass production across both temperatures. Paired Listeria isolates from the same harborage site in three facilities accumulated a range of genetic change over time as evidenced by 16, 18, and 252 single nucleotide polymorphisms.
Significance: Our results show that Listeria forms more biomass at ambient temperature as compared to food processing temperature and transient Listeria isolates form more biomass at ambient temperature as compared to persistent isolates. Paired Listeria isolates from the same harborage site within a facility accumulated a range of genetic change over time.