TAXONOMY: cellular organisms -> Bacteria -> Proteobacteria -> Alphaproteobacteria -> Rhizobiales -> Brucellaceae -> Brucella
Brucella is a genus of Gram-negative bacteria, named after David Bruce (1855-1931). They are small (0.5 to 0.7 by 0.6 to 1.5 µm), nonencapsulated, flagellated, facultatively intracellular coccobacilli. Brucella is the cause of brucellosis, which is a zoonosis transmitted by ingesting contaminated food (such as unpasteurized milk products), direct contact with an infected animal, or inhalation of aerosols. Transmission from human to human, for example through sexual intercourse or from mother to child, is exceedingly rare, but possible. Minimum infectious exposure is between 10 and 100 organisms. The different species of Brucella are genetically very similar, although each has a slightly different host specificity. Hence, the NCBI taxonomy includes most Brucella species under Brucella melitensis. Brucella species are small, Gram-negative, facultative coccobacilli, most lacking a capsule, endospores, or native plasmids. They are intracellular within the host organism, and show environmental persistence outside the host. They survive extremes in temperature, pH, and humidity, and in frozen and aborted materials. They infect many species, but with some specificity.