Construction of core genome multi-locus sequence typing schemes for population structure analyses of Nocardia species

Nocardia, a member of the Actinobacteria phylum, populates diverse habitats globally, with certain species being the cause of various clinical infections in humans. There is paucity of data regarding the population structure of this genus and of established genomic-based phylogenetic methods. We examined the whole genome sequences of 193 isolates spanning five major pathogenic Nocardia species sourced from public databases, encompassing diverse geographic regions. Using the chewBBACA pipeline, a species-specific core genome multilocus sequence typing (cgMLST) schema was created for N. cyriacigeorgica, N. farcinica, N. brasiliensis, N. wallacei, and N. abscessus. Additional genomic features that were examined included virulence factor (VF) profile, total length and open-reading frame count, the core genome length and core gene count, and GC content. Our findings indicated that: (i) N. brasiliensis diverges significantly from the other four species, underscoring its distinct evolutionary trajectory; (ii) the population structures of all species were polyclonal, with phylogenetic clustering occurring in the minority of isolates; (iii) clonal complexes were largely restricted to specific geographical locations, rather than demonstrating a global distribution, and (iv) initial evidence suggests no direct common-source transmission amongst the studied strains. Our study establishes a comprehensive genome-based phylogenetic methodology for population structure of Nocardia species.