Significance and context
Brucellosis, a disease that infects domestic animals and wildlife worldwide, is caused by species of the bacterium Brucella, including B. abortis and B. suis. These are facultative intracellular pathogens that survive in macrophages by inhibiting phagosome-lysosome fusion. Hoofed animals become infected by ingesting Brucella and symptoms of infection include abortion, retained placenta, reduced milk production and difficulty in becoming pregnant. Humans can become infected, resulting in Malta fever, after direct contact with infected animals or their reproductive tissues, or after drinking unpasteurized milk from an infected herd. Symptoms of brucellosis in humans include weakness, anorexia, aches, sweating and inattentiveness. Treatment is difficult, no vaccines are currently available, and B. suis is considered a candidate microorganism for bioterrorism. Paulsen et al. have now determined the entire genome sequence of B. suis.
The genome of B. suis 1330 consists of chromosomes (chr) I and II. Chr I contains 2,107,792 base-pairs (bp), has a G+C content of 57.2%, and contains 2,185 protein-coding genes with an average length of 842 bp. Chr II contains 1,207,381 bp, has a G+C content of 57.3%, and contains 1,203 protein-coding genes with an average length of 897 bp. In contrast to Chr I, Chr II contains a set of replication genes that are similar to those found on, for instance, the Agrobacterium Ti plasmids. Chr I carries mainly genes that are involved in transcription, translation and protein synthesis, whereas Chr II mainly carries genes required for energy metabolism, membrane transport, regulatory functions and plasmid functions. A comparison of the B. suis genome with that of B. melitensis revealed that more than 90% of the genes were 98-100% identical between these two organisms. The less similar genes mainly encode hypothetical proteins that are supposed to be responsible for the difference in pathogenicity and host preference between these species. Other proteins that are unique to B. suis or B. melitensis include, for example, transposases, an outer membrane protein, ATP-binding cassette (ABC) transporters, cell-surface protein, and propionyl-CoA carboxylase beta chain. The genome sequence of B. suis has a total of 1,902 open reading frames (ORFs) with significant similarity to ORFs in the plant symbiotic bacteria Mesorhizobium loti and Sinorhizobium meliloti, and the plant pathogen Agrobacterium tumefaciens, which are members of the same bacterial order (the Rhizobiales) as Brucella. In addition, analysis of the genome sequence suggests that B. suis has the capacity to use plant-derived compounds, and that 29 insertion sequences are present. Finally, the genome sequence reveals the presence of genes encoding proteins involved in adhesion (for example, adhesin with autotransporter domain, putative surface-exposed proteins, putative outer membrane proteins), in invasion and trafficking (for example, invasin, hemolysins), and in detoxification (for example, ureases, Fe-Mn superoxide dismutase, alkyl hydroperoxide reductase).
The entire genome sequence of B. suis 1330 can be accessed at The Institute for Genomic Research: Brucella suis 1330.
The analysis of the genome sequence of the animal pathogenic bacterium B. suis by Paulsen et al. reveals a striking similarity between an animal pathogen and plant symbiotic/pathogenic bacteria. It would be tempting to determine the molecular requirements to change a plant symbiotic bacterium into a plant pathogenic bacterium and subsequently into an animal pathogen. When supported by expression and mutant analysis, and biochemical investigation, this study will enhance our understanding of which molecules are required for pathogenesis.