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), non-motile, non-encapsulated coccobacilli, which function as facultative intracellular parasites.
Brucella is the cause of brucellosis, which is a zoonosis. It is 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 - 100 organisms.
The different species of Brucella are genetically very similar although each has a slightly different host specificity (see below). Hence the NCBI taxonomy includes most Brucella species under Brucella melitensis.
Human brucellosis 
Sir David Bruce isolated B. melitensis from British soldiers who died from Malta fever in Malta. After exposure to Brucella, humans generally have a two- to four-week latency period before exhibiting symptoms. Symptoms include acute undulating fever (>90% of all cases), headache, arthralgia (>50%), night sweats, fatigue and anorexia. Later complications may include arthritis or epididymoorchitis, spondylitis, neurobrucellosis, liver abscess formation, and endocarditis, the latter potentially fatal.
Human brucellosis is usually not transmitted from human to human; people become infected by contact with fluids from infected animals (sheep, cattle or pigs) or derived food products like unpasteurized milk and cheese. Brucellosis is also considered an occupational disease because of a higher incidence in people working with animals (slaughterhouse cases). People may also be infected by inhalation of contaminated dust or aerosols. Human and animal brucellosis share the persistence of the bacteria in tissues of the mononuclear phagocyte system, including the spleen, liver, lymph nodes, and bone marrow. Brucella can also target the male reproductive tract.
Globally, there are an estimated 500,000 cases of brucellosis each year.
Brucella is isolated from a blood culture on Castaneda medium. Prolonged incubation (up to 6 weeks) may be required as they are slow-growing, but on modern automated machines the cultures often show positive results within seven days. On Gram stain they appear as dense clumps of Gram-negative coccobacilli and are exceedingly difficult to see. In recent years molecular diagnostic techniques based on the genetic component of the pathogen have become more popular.
It is crucial to be able to differentiate Brucella from Salmonella which could also be isolated from blood cultures and are Gram-negative. Testing for urease would successfully accomplish the task; it is positive for Brucella and negative for Salmonella.
Brucella can also be seen in bone marrow biopsies.
Laboratory acquired brucellosis is common. This most often happens when the disease is not thought of until cultures become positive, by which time the specimens have already been handled by a number of laboratory staff. The idea of preventive treatment is to stop people who have been exposed to Brucella from becoming ill with the disease.
There are no clinical trials to be relied on as a guide for optimal treatment, but a three week course of rifampicin and doxycycline twice daily is the combination most often used, and appears to be efficacious; the advantage of this regimen is that it is oral medication and there are no injections; however, a high rate of side effects (nausea, vomiting, loss of appetite) has also been reported.
Host specificity and animal brucellosis 
Brucella species have been found primarily in mammals:
|B. melitensis||goats and sheep|
|B. neotomae||desert woodrat (Neotoma lepida)|
|B. ceti||dolphin, porpoise, whale|
|B. microti||common vole (Microtus arvalis)|
|Brucella sp. NVSL 07-0026||baboon|
Pathogenic Brucella can cause abortion in female animals by colonization of placental trophoblasts, and sterility in male animal.
Possible pathogens responsible for the plague in Thebes 
According to a January 2012 article in Emerging Infectious Diseases, pathogens that fit the characteristics of the plague described in Oedipus Rex include Leishmania spp., Leptospira spp., Brucella abortus, Orthopoxviridae, and Francisella tularensis. While the authors note that the disease progression of brucellosis in modern times may make it seem unlikely, they posit that it may have been an agent impactingthe livestock, while humans might have been infected with Salmonella enterica serovar Typhi or another pathogen or that the ancestral versions of Brucella may have been more lethal.
- Brucella abortus A13334, 3,401 ORFs
- Brucella canis ATCC 23365, 3,408 ORFs
- Brucella melitensis 16M, 3,279 ORFs
- Brucella microti CCM 4915, 3,346 ORFs
- Brucella ovis ATCC 25840, 3,193 ORFs
- Brucella pinnipedialis B2/94, 3,505 ORFs
- Brucella suis 1330, 3,408 ORFs
Effect of blue light 
Infection of macrophages by B. abortus is stimulated by blue light in the wild type but is limited in photochemically inactive and null mutants, indicating that a flavin-containing histidine kinase functions as a photoreceptor regulating B. abortus virulence. Conversely, depriving Brucella of the blue wavelengths dropped its reproductive rate by 90%.
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