Emergence and genomic characterization of Listeria monocytogenes causing human listeriosis in Pakistan, 2017–2023

BACKGROUND: Human listeriosis, caused by Listeria monocytogenes, is a serious food borne illness. Information from Pakistan on human listeriosis is sparse. This study reports the emergence and genomic characteristics of human L. monocytogenes cases from Pakistan between 2017 and 2023. L. monocytogenes cases were identified from our laboratory database in specimens submitted for culture and a commercially available multiplex PCR assay for cerebrospinal fluid from 2017 to 2023. Whole genome sequencing was performed on randomly selected isolates collected during 2022-2023. Frequency and geographic location of cases were assessed. Bioinformatics analyses included determination of traditional 7-gene MLST (multilocus sequence type), in-silico serogroup determination, phylogenetic relationships and core genome MLST (cgMLST). RESULTS: L. monocytogenes isolation from all clinical specimens increased from zero (before 2017) to 142 between 2017 and 2023. Majority of isolates were from blood stream infections (n = 90) followed by central nervous system infections (n = 47). Phylogenetic analysis divided sequenced isolates (n = 24) into three clades. The largest clade included 22 isolates (clonal complex-1, Serogroup 4b and Lineage-I) and contained sequence types ST1 (59%) and ST308 (41%). Core genome multilocus sequence typing indicated small genetic distances between some isolates within respective STs (ST1 and ST308). CONCLUSION: This study highlights human listeriosis as an emerging infection in Pakistan which should be considered when choosing empiric therapy for compatible syndromes. cgMLST data suggests possible common source(s) and emphasizes the need for identification of source(s) and robust public health measures to prevent and control transmission.