Benzalkonium chloride adaptation induces filamentation in Listeria monocytogenes HL06 and enhances its growth in pasteurized milk and pork sausage at 4 °C.

Benzalkonium chloride (BC) is a commonly used disinfectant in the food industry. However, inappropriate use of BC leads to BC adaptation in Listeria monocytogenes. BC adaptation can induce L. monocytogenes filamentation; however, the underlying mechanism remains unclear. In this study, we investigated the effects of BC stress, including BC shock, BC growth, and BC adaptation, on the cell length of L. monocytogenes, identified the mutation sites in BC-adapted strain through whole-genome sequencing, and screened the gene(s) involved in BC adaptation-induced filamentation in L. monocytogenes via transcriptomic analysis. Additionally, we compared the growth of BC-adapted strains and their wild-type strains in food systems. Our results showed that neither BC shock nor BC growth affects the cell length of L. monocytogenes. BC adaptation resulted in filamentation of L. monocytogenes. Alignment to the wild-type reference genome showed that the BC-adapted strain harbors 11 single nucleotide polymorphisms (SNPs) and 10 insertion-deletion mutations (InDels). Of these, 4 SNPs and 1 InDel were located within coding regions. Results from the transcriptomic analysis and quantitative real-time PCR showed overexpression of the cell division gene ftsL in BC-adapted strains. We then constructed an ftsL-overexpressing strain and found that this mutant had increased resistance to BC and increased cell length, indicating that the ftsL gene is involved in the BC adaptation-induced filamentation in L. monocytogenes. We also found that BC-adapted L. monocytogenes exhibited a better growth rate than the wild-type strain when grown in pasteurized milk and pork sausage at 4 °C. These findings demonstrate that BC adaptation in L. monocytogenes poses significant risks to food safety and human health. It is recommended to further standardize the use of the disinfectant BC to avoid the emergence of BC-adapted strains of L. monocytogenes, reducing the risk of food contamination by BC adapted strains.