Brucella

Taxonomy and biological properties

The genus Brucella includes 6 species, first discovered in 1886 by D. Bruce. Three of them are of greatest epidemiological and clinical significance for humans: B. melitensis (the most dangerous), B. abortus, and B. suis.

Morphologically, they are small, non-motile, Gram-negative coccobacilli lacking spores. An interesting detail: the bacterium can form a capsule, but this occurs exclusively when grown on nutrient media supplemented with serum.

Brucella species are extremely fastidious regarding cultivation conditions. They require complex media enriched with blood, liver infusion, or tryptone soy broth. They grow very slowly (up to a month). According to their type of respiration, they are strict aerobes, although B. abortus is distinguished by requiring an elevated carbon dioxide concentration (5%) for growth. In liquid media, the bacteria produce uniform turbidity. Virulent strains form smooth S-colonies, while avirulent strains form rough R-colonies.

Biochemically, the causative agents are active: they produce urease, catalase, and oxidase, reduce nitrates, and oxidize carbohydrates without forming gas and acid. Hemolytic and proteolytic enzymes are absent. Species are distinguished by their ability to produce hydrogen sulfide (characteristic of B. abortus), sensitivity to dyes (thionin, fuchsin), and the ratio of somatic A and M antigens.

Epidemiology and stability

Brucellosis is a classic zoonotic infection. A sick person is not a source of infection for others. Animals serve as the reservoir, with each pathogenic species having a primary host:

• B. melitensis infects small ruminants (sheep and goats);
• B. abortus is dangerous for cattle;
• B. suis infects pigs.

Animals shed the microbes into the external environment via milk, urine, excrement, and amniotic fluid. Humans can become infected alimentary (via unpasteurized milk or cheese), through direct contact with animals, and via airborne dust transmission by inhaling particles of infected wool or manure.

The pathogen demonstrates high resistance to low temperatures. In soil and water, it survives for up to 4 months, in frozen meat for up to 5 months, and in unpasteurized milk cheeses for up to 8 weeks. However, Brucella species are extremely sensitive to high temperatures (killed at 60 °C in 30 minutes, instantly upon boiling) and are rapidly destroyed by standard disinfectant solutions.

Pathogenesis and clinical presentation

Brucella species are facultative intracellular parasites, and their entire arsenal of pathogenicity factors is aimed at survival inside host cells. Adhesion to target cells occurs by binding to sialic acid residues. Invasion is mediated by outer membrane proteins, lipopolysaccharide (LPS), and a type 4 secretion system that injects effector proteins into the cell. In the early stages, the enzyme urease plays a crucial role by protecting the microbe from phagocytosis.

After internalization by phagocytes, the microbes do not die, but multiply inside them. Subsequently, the pathogen is disseminated via lymph and blood flow, settling in the organs of the reticuloendothelial system. Because of intracellular parasitism, periodic releases of bacteria into the blood occur—forming foci of bacteremia.

The disease has an infectious-allergic nature and tends to become chronic with alternating periods of remission and exacerbation. Clinically, it manifests with prolonged fever and the formation of specific granulomas in the nervous, urogenital, and musculoskeletal systems. In response to the infection, non-sterile immunity and delayed-type hypersensitivity (DTH) develop. Severe complications are possible, up to brain and heart damage and death.

Microbiological diagnostics and prevention

Diagnosis of brucellosis is based on three methods:

1. Bacteriological ("Gold Standard"): culture of blood, CSF, or lymph node aspirates is performed strictly in specialized containment laboratories. Two bottles are used (one is incubated in capnophilic conditions at 37 °C for up to 30 days, with subcultures every 5 days).
2. Biological: infection of laboratory animals is used if the material contains few pathogens or is contaminated with other microflora.
3. Serological: the main clinical method. In the acute phase, the Wright agglutination test, indirect hemagglutination assay (IHA), and ELISA are used. In the subacute or latent phase, standard reactions are often negative due to incomplete blocking antibodies—in this case, the Coombs test is used. For screening, the Huddleson rapid test and the Burnet test to detect DTH are used.

Treatment comes down to targeted antibiotic therapy. Specific prophylaxis is carried out according to strict epidemiological indications using a live vaccine from the B. abortus 19-BA strain (developed by P.A. Vershilova), which provides cross-immunity to all Brucella species.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of action of the type IV secretion system in intracellular parasitism
• Protocol for performing and interpreting the Coombs test
• Antigenic structure: practical application of the A and M antigen ratio
• Differences between smooth (S) and rough (R) Brucella colonies
• Burnet intradermal skin test: mechanism of delayed-type hypersensitivity

Other topics in the section «Gram-negative aerobic rods (Pertussis, Brucella, Tularemia, Legionella, Coxiella/Q fever, Bartonella, Pseudomonas, Burkholderia, Acinetobacter, etc.)»

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