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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, nonmotile, facultatively intracellular coccobacilli.

Brucella spp. are 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 National Center for Biotechnology Information taxonomy includes most Brucella species under B. 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. The intracellular trafficking includes two or three main steps, starting with endosomal vacuoles, then endoplasmic reticulum-derived compartments and finally vacuoles having several markers of atypical autophagy. They survive extremes in temperature, pH, and humidity, and in frozen and aborted materials. They infect many species, but with some specificity.

The Brucella species belongs to the Rhizobiales group, in the Alphaproteobacteria class. They are growing by unipolar growth, like Agrobacterium tumefaciens, Sinorhizobium meliloti, and Ochrobactrum anthropi. They usually have two chromosomes and their replication and segregation are temporally organized.

What Causes Brucellosis?

Brucellosis in humans occurs when a person comes into contact with an animal or animal product infected with the Brucella bacteria.

Very rarely, the bacteria may spread from person to person. Breastfeeding moms with brucellosis may pass the bacteria to their baby. Brucella may also be spread through sexual contact.

The bacteria can enter your body:

Through a cut or scratch in the skin.

When you breathe in contaminated air (rare).

When you eat or drink something contaminated with the bacteria, such as unpasteurized milk or undercooked meat.

What Are the Symptoms of Brucellosis?

General symptoms of brucellosis are often vague and similar to the flu. They may include:

Fever (the most common symptom, with high “spikes” that usually occur in the afternoon)

Back pain

Body-wide aches and pains

Poor appetite and weight loss


Night sweats


Abdominal pain


Symptoms usually appear within five to 30 days after you come in contact with the bacteria. How bad your symptoms are depends on what type of Brucella is making you sick:

  1. abortus usually causes mild or moderate symptoms, but they are more likely to become chronic (long-lasting).
  2. canis symptoms may come and go. They are similar to B. abortus infection, although people with B. canis often have vomiting and diarrhea.
  3. suis may cause areas of infection (called abscesses) in different organs.
  4. melitensis may cause sudden and severe symptoms, which may lead to disability.


Brucella is isolated from a blood culture on Castaneda medium or from bone marrow. Prolonged incubation (up to six weeks) may be required, as they are slow-growing, but on modern automated machines, the cultures often show positive results within 7 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.

Differentiating Brucella from Salmonella is crucial, as the latter could also be isolated from blood cultures and is 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. Polymerase chain reaction (PCR) shows promise for rapid diagnosis of Brucella species in human blood specimens. Positive PCR at the completion of treatment is not predictive of subsequent relapse. PCR testing for fluid and tissue samples other than blood has also been described. A history of animal contact is pivotal; in endemic area, it should be in the diagnosis of any nonspecific febrile illness.

In the laboratory, biochemical tests can be diagnostic. Oxidase and catalase tests are positive for most members of the genus Brucella.


Test B. melitensis B. abortus B. suis B. neotomae B. ovis B. canis
Need to CO2 + +
production of H2S + + +
Growth on basic fushin 0.002% + + +
Growth on thionin 0.004% + + +
Growth on thionin 0.002% + + + + +
Destroy withTb phage +


How Is Brucellosis Treated?

Brucellosis can be difficult to treat. If you have brucellosis, your doctor will prescribe antibiotics. Antibiotics commonly used to treat brucellosis include:

doxycycline (Acticlate, Monodox, Vibra-Tabs, Vibramycin)


ciprofloxacin (Cipro) or ofloxacin (Floxin)

rifampin (Rifadin, Rimactane)

sulfamethoxazole/trimethoprim (Bactrim)

tetracycline (Sumycin)

You will generally be given doxycycline and rifampin a in combination for 6-8 weeks.

You must take the antibiotics for many weeks to prevent the disease from returning.  The rate of relapse following treatment is about 5-15% and usually occurs within the first six months after treatment.

Recovery can take weeks, even months. Patients who receive treatment within one month of the start of symptomscan be cured of the disease.

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