Brucella species are significant zoonotic pathogens responsible for brucellosis, a disease that can lead to severe and chronic health issues in humans. Understanding their characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Brucella. Ongoing surveillance, vaccination, and preventive measures are essential to control and reduce the impact of this public health concern.
General Character
- Genus: Brucella
- Key Species:
- Brucella abortus (primarily affects cattle)
- Brucella melitensis (primarily affects goats and sheep)
- Brucella suis (primarily affects pigs)
- Brucella canis (affects dogs)
- Family: Brucellaceae
- Gram Staining: Brucella species are Gram-negative bacteria, appearing pink due to their thin peptidoglycan layer and outer membrane.
- Shape and Arrangement:
- Shape: Small coccobacilli (short rod-shaped).
- Arrangement: Typically found as single cells or in small clusters.
- Oxygen Requirements:
- Brucella species are facultative intracellular anaerobes.
Morphology
- Cell Wall Structure:
- Composed of a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS), contributing to virulence.
- Non-motile: Brucella species do not have flagella and are non-motile.
Cultural Characteristics
- Growth Media:
- Blood Agar: Supports growth, but Brucella is slow-growing and may require enriched media.
- Brucella Agar: A selective medium for isolating Brucella, often supplemented with antibiotics to inhibit competing flora.
- Colony Appearance:
- Colonies appear small, smooth, and transparent, requiring several days to become visible.
- Temperature and pH Range:
- The optimal growth temperature is around 37°C, with a pH range of 6.5 to 7.5.
Biochemical Reactions
- Catalase Test: Brucella species are catalase-positive.
- Oxidase Test: Brucella species are oxidase-positive.
- Lactose Fermentation: Brucella does not ferment lactose.
- Urease Test: Brucella species are urease-positive, particularly B. suis and B. melitensis.
Pathogenicity
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Virulence Factors:
- Intracellular Survival: Brucella can survive and replicate within macrophages, evading the host’s immune response.
- Endotoxin: The lipopolysaccharide component contributes to inflammatory responses.
- Modulation of Host Cell Signaling: Brucella can manipulate host cell signalling pathways to promote its survival.
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Clinical Infections:
- Brucellosis: Characterized by fever, sweats, malaise, weight loss, and arthralgia. It can present in acute or chronic forms and may affect multiple organs (e.g., liver, spleen, joints).
- Undulant Fever: A classic symptom marked by fluctuating fever patterns.
- Complications: This can lead to osteoarticular infections, endocarditis, and neurological involvement.
Laboratory Diagnosis
- Specimen Collection: Clinical specimens may include blood, bone marrow, or other tissue samples.
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Microscopic Examination:
- Gram staining reveals small, Gram-negative coccobacilli, often difficult to visualize.
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Culture Techniques:
- Inoculation on blood or selective Brucella agar, with prolonged incubation (up to several weeks).
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Serological Testing:
- Agglutination tests (e.g., Wright’s test) to detect antibodies against Brucella.
- Enzyme-linked immunosorbent assays (ELISAs) can also be used for diagnosis.
- Molecular Methods:
- PCR techniques are available for rapid identification.
Antibiotic Resistance
- Emergence of Resistance: Brucella is generally sensitive to antibiotics, but there may be rare resistance.
- Treatment Options: The standard treatment for brucellosis typically involves a combination of doxycycline and rifampin or streptomycin for several weeks.
Prevention
- Vaccination: Vaccination of livestock (e.g., the RB51 vaccine for cattle) can help reduce the incidence of brucellosis in animal populations.
- Hygiene Practices: Good hygiene and safety measures in handling animals and animal products are essential to prevent transmission.
- Food Safety: Avoiding consumption of unpasteurized dairy products and undercooked meat reduces the risk of infection.