Improved detection of Listeria monocytogenes outbreaks using whole genome sequencing, Gipuzkoa, Northern Spain, 2010...

Listeria monocytogenes is a major foodborne pathogen responsible for severe infections, mainly affecting pregnant women, the elderly, and immunocompromised individuals. In Spain, listeriosis has been insufficiently studied using modern molecular techniques, and genomic data from clinical isolates remain scarce. Long‑term genomic surveillance based on whole genome sequencing (WGS) has not yet been systematically applied. WGS has proven to be a powerful method for outbreak detection and for revealing transmission dynamics that remain hidden to conventional epidemiology. This study aimed to characterise clinical L. monocytogenes isolates from Gipuzkoa, a region in northern Spain, collected over more than a decade, and to investigate their genetic diversity, phylogenetic relationships, and virulence traits.

Between 2010 and 2022, a total of 131 laboratory‑confirmed human listeriosis cases were diagnosed in Gipuzkoa. Among them, 127 genomes were successfully sequenced and analysed by core genome multilocus sequence typing (cgMLST) and single nucleotide polymorphism (SNP) approaches. Genomic analysis revealed that 47 cases (36%) were part of nine distinct clusters, some extending across periods of up to nine years, while 84 cases appeared sporadic. Importantly, several outbreaks previously considered independent were shown to originate from genetically related strains, highlighting the persistence of specific clones. By comparing our dataset with publicly available international L. monocytogenes genomes, we identified putative foodborne sources for human infections and demonstrated cross‑border connections with lineages circulating in other countries. Additionally, antibiotic resistance determinants were detected infrequently, in line with the low resistance rates described for L. monocytogenes.

This study represents one of the most comprehensive WGS‑based studies of listeriosis to date in Spain. Our findings demonstrate that more than one third of apparently sporadic cases were in fact part of persistent or recurrent genomic clusters, underscoring the hidden transmission potential of this pathogen. WGS analysis revealed links between cases that would not have been detected through routine epidemiology, providing critical insights into outbreak dynamics and long‑term circulation of L. monocytogenes. This regional study highlights the importance of implementing genomic surveillance for listeriosis and integrating all parts of the infection chain - from clinical diagnosis to food production and public health - to better guide responses and strengthen prevention and control strategies.