Brucella is a genus of Gram-negative bacteria. They are small (0.5 to 0.7 by 0.6 to 1.5 µm), non-motile, encapsulated coccobacilli.

Brucella is the cause of brucellosis, a true zoonotic disease (i.e. human-to-human transmission has not been identified). It is transmitted by ingesting infected food, direct contact with an infected animal, or inhalation of aerosols. Minimum infectious exposure is between 10 - 100 organisms. Brucellosis primarily occurs through occupational exposure (e.g. exposure to cattle, sheep, pigs), but also by consumption of unpasteurised milk products.

There are a few different species of Brucella, each with slightly different host specificity. B. melitensis which infects goats and sheep, B. abortus which infects cows, B. suis infects pigs, B. ovis infects sheeps and B. neotomae. Recently a new species was discovered in marine mammals: B. pinnipediae.


Brucella is isolated from a blood culture on Castenada medium. Prolonged incubation (up to 6 weeks) may be required as they're 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.

It's 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; as it is positive for the former and negative for the latter.

Brucella could also be seen in Bone marrow.

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 preventative treatment is to stop people who have been exposed to Brucella from becoming unwell 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.

Human Brucellosis

Sir David Bruce isolated B. melitensis from British soldiers who died from Malta fever in Malta. The disease is characterized by acute undulating fever, headache, night sweats, fatigue and anorexia. Human Brucellosis is not considered a contagious disease and people become infected by contact with fluids from infected animals (sheep, cows or pigs) or derived food products like unpasteurized milk and cheese. Brucellosis is also considered an occupational disease because has higher incidence in people working with animals (slaughterhouse cases). The real worldwide incidence of Brucellosis is unknown because there is a low level of surveillance and report in brucella-endemic areas.

Blue light study

In a study published in Science magazine in August 2007, it was revealed that Brucella reacts strongly to the presence of the blue spectrum in natural light, reproducing at a great rate and becoming infectious. Conversely, depriving Brucella of the blue wavelengths dropped its reproductive rate by 90%, a result one of the co-authors called "spectacular."


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