Genomic epidemiology of the clinically dominant clonal complex 1 in the Listeria monocytogenes population in the UK

Listeria monocytogenes is a food-borne pathogen, typically affecting the elderly, immunocompromised patients and pregnant women. The aim of this study was to determine the population structure of L. monocytogenes clonal complex 1 (CC1) in the UK and describe the genomic epidemiology of this clinically significant CC. We interrogated a working dataset of 4073 sequences of L. monocytogenes isolated between January 2015 and December 2020 from human clinical specimens, food and/or food-production environments. A minimum spanning tree was reconstructed to determine the population structure of L. monocy-togenes in the UK. Subsequent analysis focused on L. monocytogenes CC1, as the cause of the highest proportion of invasive listeriosis in humans. Sequencing data was integrated with metadata on food and environmental isolates, and information from patient questionnaires, including age, sex and clinical outcomes. All isolates either belonged to lineage I (n=1299/4073, 32%) or lineage II (n=2774/4073, 68%), with clinical isolates from human cases more likely to belong to lineage I (n=546/928, 59%) and food isolates more likely to belong to lineage II (n=2352/3067, 77%). Of the four largest CCs, CC1 (n=237) had the highest proportion of isolates from human cases of disease (CC1 n=160/237, 67.5 %; CC121 n=13/843, 2 %; CC9 n=53/360, 15 %; CC2 n=69/339, 20%). Within CC1, most cases were female (n=95/160, 59%, P=0.01771) and the highest proportion of cases were in people >60 years old (39/95, 41%, P=1.314×10⁻⁶) with a high number of them aged 20–39 years old (n=35/95, 37%) most linked to pregnancy-related listeriosis (n=29/35, 83%). Most of the male cases were in men aged over 60 years old (40/65, 62%), and most of the fatal cases in both males and females were identified in this age group (42/55, 76%). Phylogenetic analysis revealed 23 5 SNP single linkage clusters comprising 80/237 (34%) isolates with cluster sizes ranging from 2 to 19. Five 5 SNP clusters comprised isolates from human cases and an implicated food item. Expanding the analysis to 25 SNP single linkage clusters resolved an additional two clusters linking human cases to a potential food vehicle. Analysis of demographic and clinical outcome data identified CC1 as a clinically significant cause of invasive listeriosis in the elderly population and in women of child-bearing age. Phylogenetic analysis revealed the population structure of CC1 in the UK comprised small, sparsely populated genomic clusters. Only clusters containing isolates from an implicated food vehicle, or food processing or farming environments, were resolved, emphasizing the need for clinical, food and animal-health agencies to share sequencing data in real time, and the importance of a One Health approach to public-health surveillance of listeriosis.