Listeria monocytogenes Contamination in Commercial Foods and Characterization of Slow-growth Strains

Abstract

Listeria monocytogenes (LM) is a foodborne pathogen found in both food and environment. Its ability to multiply under refrigeration and in high-salt conditions is a major concern for food poisoning in dairy and ready-to-eat foods. This risk warrants attention in Japan, given the numerous reported cases in Europe and the US. This study investigated the contamination status of LM in food products and examined the characteristics of two food-derived isolates. The LM detection rate was determined for various food products, including ground chicken (thigh), ground pork, spicy pollack roe, and natural cheese. The detection rates were 40%, 17%, 20%, and 3.3%, respectively. While most strains isolated in this study, such as strain MP01, isolated from ground pork, formed typical colonies within 24 hr on agar Listeria according to Ottaviani and Agosti medium, strain SC14, isolated from spicy pollack roe, exhibited delayed colony formation, appearing only on the second day after incubation. To investigate the cause of this delay, the whole-genome sequence of strain SC14 was determined and compared with that of other LM strains. Genome analysis suggested that pepE found in strains MP01 and JCM32892^T but absent in strain SC14, may contribute to colony formation and colony growth rate.