Bacterial Strains

A total of 48 L. monocytogenes isolates classified by PCR-serogrouping (as detailed below) into PCR-serogroup IIb (n = 17) and PCR-serogroup IVb (n = 31) were selected for the whole genome sequencing and genomic analyses. They were isolated in official laboratories between 2014 and 2017 from different kinds of ready-to-eat (RTE) food of animal origin (e.g., ham, sausages or meat) (n = 33) and from food processing environment (n = 15), using the standard ISO 11290-1:1996+A1:2004 (ISO, 1996). The isolates were originated from 9 voivodeships (administrative regions) of Poland and sent to the laboratory of the Department of Hygiene of Food of Animal Origin, National Veterinary Research Institute in Pulawy. Subsequently, the isolates were cultured on TSYEA (tryptone soya yeast extract agar; Oxoid, United Kingdom) at 37 ± 1°C for 18–4 h and the isolates were identified biochemically at species level using API Listeria (Biomerieux, France) according to the manufacturer’s instructions. The confirmed L. monocytogenes were stored at −70°C for further analysis.