Microbiology · MU Sofia

64. Genus Brucella. Genus Francisella.

Covers the genus Brucella, its gram-negative coccobacilli, zoonotic transmission from several livestock and dog reservoirs, intracellular survival in macrophages, and brucellosis with its clinical forms, diagnosis, treatment and prevention. It then turns to Francisella tularensis, covering its high infectivity, reservoirs and vectors, the four clinical forms of tularaemia presented in a table, and its diagnosis, treatment and prevention. Several comparison tables summarise virulence factors and disease forms.

Inside: 3 sections · 8 labelled pictures · 3 tables · 15 flashcards · 3 min

Topic 64 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 3 sections, about 378 words (3 min of reading), with 15 flashcards.

In this note

  1. Genus Brucella. Genus Francisella
  2. Brucella spp.
  3. Francisella tularensis

Brucella spp.

General Characteristics

  • Gram-negative small coccobacilli, non-motile
  • Non-encapsulated
  • Strict aerobes, oxidase (+), catalase (+)
  • Facultative intracellular bacteria — live in macrophages
  • Grow slowly on enriched media (3 days to 3 weeks)
  • No exotoxins, only endotoxins

Epidemiology & Transmission

  • Zoonosis: transmitted from animals to humans
  • Reservoirs:
  • B. melitensis – goats/sheep (most pathogenic)
  • B. abortus – cattle
  • B. suis – pigs
  • B. canis – dogs
  • Infection via:
  • Unpasteurised dairy
  • Occupational contact (e.g. veterinarians, slaughterhouse workers)
  • Direct contact with animal secretions
  • No human-to-human transmission

Pathogenesis

  • Enters via mucous membranes, skin, or ingestion → local replication
  • Phagocytosed by macrophages but survive intracellullarly
  • Spread via lymphatics to reticuloendothelial organs: liver, spleen, BM
  • Causes granulomatous inflammation, focal abscesses, and chronic infection

Virulence Factors

Diagnosis

  • Specimens: blood, bone marrow, tissue biopsy
  • Culture: enriched media + 10% CO₂, slow growth
  • Serology:
  • Slide agglutination test with Brucella antiserum
  • ELISA or standard agglutination test (SAT)
  • PCR available in reference labs

Clinical Manifestations – Brucellosis

  • Incubation: 1–3 weeks
  • Nonspecific flu-like symptoms:
  • Undulant fever (intermittent), chills, fatigue
  • Muscle aches, night sweats, weight loss
  • Hepatosplenomegaly, lymphadenopathy
  • Chronic form: depression, arthritis, endocarditis, orchitis

Treatment

  • DOC: Doxycycline + Rifampin (min. 6 weeks)
  • Alternative (children, pregnancy): TMP-SMX
  • Avoid monotherapy to prevent relapse

Prevention

  • Avoid raw milk/cheese
  • Control infection in livestock
  • Protective clothing for high-risk workers
  • No human vaccine available

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