Characterization of Novel Brucella Strains Originating from Wild Native Rodent Species in North Queensland,Australia

We report on the characterization of a group of seven novel Brucella strains isolated in 1964 from three native rodent species in North Queensland, Australia, during a survey of wild animals. The strains were initially reported to be Brucella suis biovar 3 on the basis of microbiological test results. Our results indicated that the rodent strains had microbiological traits distinct from those of B. suis biovar 3 and all other Brucella spp. To reinvestigate these rodent strains, we sequenced the 16S rRNA, recA, and rpoB genes and nine housekeeping genes and also performed multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA). The rodent strains have a unique 16S rRNA gene sequence compared to the sequences of the classical Brucella spp. Sequence analysis of the recA, rpoB, and nine housekeeping genes reveals that the rodent strains are genetically identical to each other at these loci and divergent from any of the currently described Brucella sequence types. However, all seven of the rodent strains do exhibit distinctive allelic MLVA profiles, although none demonstrated an amplicon for VNTR 07, whereas the other Brucella spp. did. Phylogenetic analysis of the MLVA data reveals that the rodent strains form a distinct clade separate from the classical Brucella spp. Furthermore, whole-genome sequence comparison using the maximal unique exact matches index (MUMi) demonstrated a high degree of relatedness of one of the seven rodent Brucella strains (strain NF 2653) to another Australian rodent Brucella strain (strain 83-13). Our findings strongly suggest that this group of Brucella strains isolated from wild Australian rodents defines a new species in the Brucella genus.Brucella species are facultative intracellular Gram-negative members of the Alphaproteobacteria class capable of causing brucellosis in a range of animal hosts, including domesticated livestock, wildlife, marine mammals, and humans (1, 5, 7, 29, 32, 33, 36, 47). Brucellosis is the most prevalent zoonotic disease worldwide, causing spontaneous abortion and fetal death in animals and severe flu-like symptoms, focal complications, and often, chronic disease in humans (7, 11, 22, 27, 40, 41, 49, 50). Brucella species are typically transmitted to humans through consumption of unpasteurized dairy products or exposure to fluids or tissues from infected animals (45, 49). Animals are primary hosts of all Brucella spp., which include Brucella abortus (cattle), B. canis (dogs), B. melitensis (goats, cows, and sheep), B. suis (swine), B. ovis (rams), and B. neotomae (desert rats) (3, 7, 8). Recently, three additional Brucella species have been recognized: B. pinnipedialis (seals), B. ceti (dolphins) (5), and B. microti. B. microti was initially isolated from the common vole in the Czech Republic (33, 35). In the mid-1980s, DNA-DNA hybridization studies demonstrated a very high level of genetic similarity (98.5%) among the Brucella spp., which led to the adoption of a monospecies concept for the Brucella genus, with all the species at that time renamed as biovars of B. melitensis (46). However, 20 years later, the use of a phylogenetic-evolutionary approach to Brucella taxonomy was accepted. By that approach, host preferences, virulence, and pathogenicity were considered important criteria in the delineation of Brucella species, and consequently, the multispecies taxonomy was restored to the Brucella genus (28). With the development of more advanced molecular typing methodologies such as multilocus sequence analysis (MLSA) (48), multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA) (23), and techniques interrogating single nucleotide polymorphisms (SNPs) (17, 47), Brucella spp. can be quickly genotyped and strains can be readily examined for their phylogenetic and evolutionary relationships (15).Recently, we reported on two unusual human brucellosis cases, one of which led to the identification of a novel species, Brucella inopinata, whose type strain is strain BO1 and which was associated with a breast implant infection in a patient in Oregon (12, 36). The second brucellosis case involved an atypical Brucella strain (strain BO2) isolated from the lung biopsy fluid of a patient with chronic destructive pneumonia in Australia (44). Because both patients denied common risk factors associated with human brucellosis, the primary hosts of these strains remain unknown. However, nucleotide sequence analysis of the outer membrane proteins (omp2a and omp2b) of both strain BO2 and strain BO1^T demonstrated close clustering to an atypical B. suis strain (strain 83-210) isolated from a rodent in Australia (30, 44). Further genetic analysis of the 16S rRNA genes from strains BO1^T and BO2 and the 16S rRNA gene from atypical Brucella strain 83-13 (available at http://www.broadinstitute.org), which was isolated from a rodent in Australia and briefly described by Corbel and Brinley-Morgan in 1984 (9), showed that strain 83-13 yielded notable genetic similarity to the novel human isolates, which led us to speculate that atypical human Brucella strains BO1^T and BO2 may have an animal reservoir in rodents from Australia.Rodent brucellosis is self-limiting and is mostly associated with wild rodents that cohabitate among domestic livestock presumably infected with classical Brucella spp. (27). Over 22 different wild rodent species worldwide have been reported to be susceptible to Brucella infection, as demonstrated by serology and/or culture (41). Earlier field studies in Argentina, Venezuela, and Denmark reported on the prevalence of B. suis in hares, opossums, and rats and B. abortus in ferrets and capybaras (10, 11, 27). However, two rodent-specific Brucella spp. have been identified, including B. neotomae, isolated from the desert wood rat (Neotomae lepida) in Utah (39), and most recently, B. microti, isolated from the common vole (Microtus arvalis) in the Czech Republic (35). In the early 1960s, Cook et al. reported on the isolation and biochemical identification of seven Brucella suis biovar 3 strains from three known species of wild native rats from Australia (6). We reinvestigated the microbiological characteristics of these seven rodent B. suis biovar 3 strains and performed genetic analyses with respect to the microbiological characteristics and genetics of the classical and atypical Brucella species. In this report, we describe this group of Brucella strains isolated from wild rodents in Australia, which confer unique microbiological and molecular characteristics distinct from those of any of the currently described species.