Heterogeneity, Characteristics, and Public Health Implications of Listeria monocytogenes in Ready-to-Eat Foods and Pasteurized Milk in China

Abstract

<i>Listeria monocytogenes</i> is a foodborne pathogen with a high mortality rate in humans. This study aimed to identify the pathogenic potential of <i>L. monocytogenes</i> isolated from ready-to-eat (RTE) foods and pasteurized milk in China on the basis of its phenotypic and genotypic characteristics. Approximately 7.7% (44/570) samples tested positive for <i>L. monocytogenes</i> among 10.8% (39/360) RTE and 2.4% (5/210) pasteurized milk samples, of which 77.3% (34/44) had < 10 MPN/g, 18.2% (8/44) had 10-110 MPN/g, and 4.5% (2/44) had > 110 MPN/g. A total of 48 strains (43 from RTE foods and five from milk samples) of <i>L. monocytogenes</i> were isolated from 44 positive samples. PCR-serogroup analysis revealed that the most prevalent serogroup was II.2 (1/2b-3b-7), accounting for 52.1% (25/48) of the total, followed by serogroup I.1 (1/2a-3a) accounting for 33.3% (16/48), serogroup I.2 (1/2c-3c) accounting for 12.5% (6/48), and serogroup II.1 (4b-4d-4e) accounting for 2.1%. All isolates were grouped into 11 sequence types (STs) belonging to 10 clonal complexes (CCs) and one singleton (ST619) via multi-locus sequence typing. The most prevalent ST was ST87 (29.2%), followed by ST8 (22.9%), and ST9 (12.5%). Virulence genes determination showed that all isolates harbored eight virulence genes belonging to <i>Listeria</i> pathogenicity islands 1 (LIPI-1) (<i>prfA</i>, <i>actA</i>, <i>hly</i>, <i>mpl</i>, <i>plcA</i>, <i>plcB</i>, and <i>iap</i>) and <i>inlB</i>. Approximately 85.4% isolates carried full-length <i>inlA</i>, whereas seven isolates had premature stop codons in <i>inlA</i>, six of which belonged to ST9 and one to ST5. Furthermore, LLS (encoded by <i>llsX</i> gene, representing LIPI-3) displays bactericidal activity and modifies the host microbiota during infection. LIPI-4 enhances neural and placental tropisms of <i>L. monocytogenes</i>. Results showed that six (12.5%) isolates harbored the <i>llsX</i> gene, and they belonged to ST1/CC1, ST3/CC3, and ST619. Approximately 31.3% (15/48) isolates (belonging to ST87/CC87 and ST619) harbored <i>ptsA</i> (representing LIPI-4), indicating the potential risk of this pathogen. Antimicrobial susceptibility tests revealed that > 95% isolates were susceptible to 16 antimicrobials; however, 60.4 and 22.9% isolates were intermediately resistant to streptomycin and ciprofloxacin, respectively. The results show that several isolates harbor LIPI-3 and LIPI-4 genes, which may be a possible transmission route for <i>Listeria</i> infections in consumers.