Brucellosis – new paradigms for a classical pathogen

Introduction Globally, brucellosis remains one of, if not the major bacterial zoonosis, still devastating productivity of livestock and affecting mankind, either directly through infection, or indirectly through reduction of productivity among foodproducing livestock. It is estimated by the WHO that more than 500,000 new cases of human brucellosis occur each year. Many nations have deployed eradication campaigns, with some countries such as the UK and several other Northern European countries successfully gaining brucellosis-free status. Even in these countries, comprehensive surveillance is an essential prerequisite to maintain this status, exemplified by recent introductions of Brucella-infected cattle to the UK, despite preand post-export screening. The aetiological agent of brucellosis is a small, pleomorphic Gram-negative rod, which was first isolated from human clinical cases of undulant fever by David Bruce (1887). Later, in 1904, the Mediterranean Fever Commission under the leadership of David Bruce, identified the reservoir of infection in goats and subsequent transmission to man through consumption of unpasteurised milk. In subsequent years, similar microbes were identified in cattle (B. abortus) and swine (B. suis). This microbe belongs within the α-2 proteobacteria clustering with Bartonella, Ochrobactrum, Agrobacteria and Rhizobia as phylogenetic neighbours. The genus was originally divided into species based on various biochemical capabilities, dye and phage susceptibilities and host preferences (see Table). These species in some cases were further divided into biovars (see Table). Following whole genomic hybridization studies, the high degree of homogeneity among the brucellae was noted, prompting the re-classification into a single species, B. melitensis1. Although justifiable according to phylogenetic criteria, this classification has proved unpopular, largely through distinct host susceptibilities and differences in host-pathogen interactions. Brucellae generally possess two chromosomes with a large replicon of 2.1 Mbp (chromosome 1) and a smaller replicon of 1.2 Mbp (chromosome 2); however, B. suis biovar 3 possesses a single replicon of 3.1 Mbp. Plasmids have not been reported within the brucellae.