Abstract
BackgroundBrucella species include economically important zoonotic pathogens that can infect a wide range of animals. There are currently six classically recognised species of Brucella although, as yet unnamed, isolates from various marine mammal species have been reported. In order to investigate genetic relationships within the group and identify potential diagnostic markers we have sequenced multiple genetic loci from a large sample of Brucella isolates representing the known diversity of the genus.ResultsNine discrete genomic loci corresponding to 4,396 bp of sequence were examined from 160 Brucella isolates. By assigning each distinct allele at a locus an arbitrary numerical designation the population was found to represent 27 distinct sequence types (STs). Diversity at each locus ranged from 1.03–2.45% while overall genetic diversity equated to 1.5%. Most loci examined represent housekeeping gene loci and, in all but one case, the ratio of non-synonymous to synonymous change was substantially <1. Analysis of linkage equilibrium between loci indicated a strongly clonal overall population structure. Concatenated sequence data were used to construct an unrooted neighbour-joining tree representing the relationships between STs. This shows that four previously characterized classical Brucella species, B. abortus, B. melitensis, B. ovis and B. neotomae correspond to well-separated clusters. With the exception of biovar 5, B. suis isolates cluster together, although they form a more diverse group than other classical species with a number of distinct STs corresponding to the remaining four biovars. B. canis isolates are located on the same branch very closely related to, but distinguishable from, B. suis biovar 3 and 4 isolates. Marine mammal isolates represent a distinct, though rather weakly supported, cluster within which individual STs display one of three clear host preferences.ConclusionThe sequence database provides a powerful dataset for addressing ongoing controversies in Brucella taxonomy and a tool for unambiguously placing atypical, phenotypically discordant or newly emerging Brucella isolates. Furthermore, by using the phylogenetic backbone described here, robust and rationally selected markers for use in diagnostic assay development can be identified.
Highlights
Brucella species include economically important zoonotic pathogens that can infect a wide range of animals
The remaining loci used are a fragment of omp25, encoding a 25 kDa outer membrane protein, included as a potentially more variable surface marker that might facilitate discrimination between closely related classical species and a fragment, labelled int-hyp, that is largely intergenic though it does include the extreme 5' of a hypothetical protein
A strong argument could undoubtedly be made for reclassifying B. canis as an additional B. suis biovar, given that the existing biovars already have distinct host specificities
Summary
Brucella species include economically important zoonotic pathogens that can infect a wide range of animals. There are currently six classically recognised species of Brucella as yet unnamed, isolates from various marine mammal species have been reported. Members of the genus Brucella are causative agents of brucellosis, a widespread disease of various animal species, and a common zoonotic infection of man [1]. Some of the classical species are divided into biovars the distinction of some of these biovars is based on very slight differences and can be difficult and somewhat subjective. Multiple biovars of B. abortus, B. melitensis and B. suis are recognized currently [3] the status of some biovars, those of B. abortus, remains unresolved
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