Prevalence, antibiotic resistance, and molecular epidemiology of Listeria monocytogenes isolated from imported foods in China during 2018 to 2020

A total of 1797 imported food samples collected during 2018 to 2020 were investigated for Listeria mono-cytogenes. Antibiotic susceptibility tests and whole genome sequencing analysis were performed for the obtained isolates. The overall prevalence of L. monocytogenes was 5.62 %; the highest prevalence was observed for pork (13.65 %), followed by fish (6.25 %), sheep casing (6.06 %), chicken (3.61 %), and beef (2.06 %). Geographical differences in prevalence were also observed for pork. Resistance to oxacillin (39.33 %) and clindamycin (16.85 %) was common, whereas resistance rates for other antibiotics were relatively low, ranging from 0 % to 6.74 %. Pork and fish isolates showed resistance to more antibiotics than beef isolates. Tetracycline and chloramphenicol resistance phenotypes strongly correlated with genotypes. The predominant serogroup was 1/2a, 3a, at 44.44 %, while the percentages of three other serogroups were similar and relatively lower, from 17.28 % to 19.75 %. Significant genetic differences were observed among lineage I and II isolates. LIPI-3 was carried by 19.75 % (16/ 81) of isolates and LIPI-4 by 6.17 % (5/81); all were lineage I. The stress survival island was present in 31.03 % (9/29) of lineage I and 83.02 % (44/53) of lineage II. Benzalkonium chloride tolerance genes were carried by 10.34 % (3/29) of lineage I and 23.08 % (12/52) of lineage II isolates. A total of 25 sequence types (STs) were identified, among which one was novel; ST9 and ST121 were the most prevalent. Disparate distribution of STs among food types was observed, and geographical and food related characteristics were also found for some STs. Hypervirulent STs, such as ST1, ST4 and ST6, belonged to 4b,4e,4e; carried LIPI-3 and/or LIPI-4; and some even were ECI or ECII; while only one carried SSI or BC tolerance genes. In contrast, hypo-virulent STs such as ST9 and ST121 carried SSI and BC tolerance genes, while none had LIPI-3/LIPI-4. Certain STs were detected frequently from a particular food of a particular country for a long time, indicating more attention should be given to these special persistent isolates. These findings are valuable for source tracking, prevention and control of L. mono-cytogenes in the global food chain.