Development and evaluation of a core genome multilocus sequence typing (cgMLST) scheme for Brucella spp.

Brucellosis is a zoonotic disease caused by Brucella spp. Brucella spp. can be sub-typed by multilocus sequence typing (MLST) method, which targets a set of housekeeping genes. We have developed a core genome MLST (cgMLST) typing scheme to distinguish and differentiate species of Brucella up to biovar level. A total of 407 whole (complete and draft) genome sequences of different Brucella strains were used in this study. Genome sequences were filtered using the BLAST score ratio (BSR)-based allele calling algorithm, and we found that 164 cgMLST target loci are shared in all the 407 genome sequences. These 164 loci were used to develop the cgMLST scheme and further evaluated to sub-type different species of Brucella. Based on our cgMLST scheme, Brucella spp. were classified into 287 sequence types (STs). A phylogenetic tree was constructed based on the STs derived from the cgMLST analysis. The phylogenetic tree differentiated all the 11 Brucella spp. and five biovars of B. suis. B. vulpis formed the outmost clade followed by B. inopinata and B. microti. Among the four subgroups of B. abortus, group A and B were differentiated based on their geographic origins. Similarly, three subgroups of B. melitensis were separated based on their geographical origins with few exceptions. B. neotomae that infect rodents were distinguished from other Brucella spp. B. canis showed the closest relationship with B. suis bv. 4, followed by B. suis bv. 3 and bv. 1. Brucella spp. associated with the marine mammals, such as B. ceti and B. pinnipedialis were closely related. Of these, B. ceti strains isolated from dolphins and porpoise were differentiated into two groups. We incorporated our cgMLST tool in BrucellaBase (https://www.dbtbrucellosis.in/brucella_cgmlst.html), which will be helpful to predict the cgMLST allelic profile and the ST of a newly sequenced genome.