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Microbiology · Other Gram-negative rods

Brucella !

  • Bacterium
  • Gram-negative
  • For completeness

Livestock bacteria that reach people through unpasteurized milk or cheese or contact with animals: a fever that comes and goes.

Updated 2026-11-01

Also called: Brucella melitensis, Brucella abortus, Brucella suis, Brucella canis

At a glance

Gram
Gram-negative
Shape
Small coccobacillus
Oxygen
Aerobic
Family
Brucellaceae
Stain
Gram: small red coccobacilli

Morphology

  • Small, non-motile, non-encapsulated Gram-negative coccobacillus
  • Facultative intracellular: lives in macrophages
  • Slow-growing; a hazard for laboratory staff

Transmission

  • Unpasteurized milk and fresh cheese, undercooked meat
  • Contact with blood, placentas, aborted fetuses and animal secretions (farmers, slaughterhouse workers, vets)
  • Inhalation in the laboratory
  • Person-to-person spread is extremely rare

Reservoir: Goats and sheep (B. melitensis, the most common in humans), cattle (B. abortus), pigs (B. suis) and dogs (B. canis)

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Survives and multiplies inside macrophages
  • Spreads through the reticuloendothelial system: liver, spleen, bone marrow and lymph nodes
  • Low-toxicity lipopolysaccharide that favors chronic infection
  • In animals it causes abortion (affinity for the placenta)

Diseases it causes

Clinical picture

  • Fever that rises and falls, heavy sweating, fatigue, headache and joint pain
  • Hepatosplenomegaly and enlarged nodes
  • Complications: sacroiliitis and spondylitis, orchitis, endocarditis (the main cause of death)
  • Untreated it can become chronic

Diagnosis

  • Prolonged blood or bone marrow culture; warn the lab because of the infection risk
  • Serology: agglutination (Rose Bengal, tube agglutination)
  • PCR

Treatment

Combination antibiotics to prevent relapse: doxycycline with rifampicin, or doxycycline with an aminoglycoside (streptomycin or gentamicin). Young children and pregnant women receive other combinations, such as trimethoprim-sulfamethoxazole with rifampicin.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Pasteurization of milk and dairy products
  • Vaccination and brucellosis control in livestock
  • Gloves, goggles and protective clothing for people handling animals or birth products
  • Strict laboratory precautions

Test yourself

Think of the answer, then tap the question to see it.

What are the most common ways to catch brucellosis?

Drinking unpasteurized milk or eating fresh cheese, and contact with infected animals or their birth products.

Which Brucella species causes most human disease worldwide, and which animals carry it?

Brucella melitensis, from goats and sheep.

Which bone and joint complication is typical of brucellosis?

Sacroiliitis and spondylitis.

Why is brucellosis treated with two antibiotics?

It is intracellular and single-drug treatment is linked to more relapses.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Brucellosis (WHO, 2020); About Brucellosis (CDC, 2026).

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Representative model, for educational purposes only.