Brucella spp: B. canis, abortus, melitensis, suis - MegaMicro

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Brucella spp: B. canis, abortus, melitensis, suis

Microbiology > Bacteriology > Gram-negative bacteria
Brucella abortus
I.   Structure
A.    Gram-negative coccobacilli
1. Small rod-shaped bacteria
a.       Lack flagella or pili, therefore, they are immotile
b.      They are not encapsulated
2. Produce endospores that enable survival under long-term starvation and desiccation
3. Intracellular bacteria, therefore, they cannot replicate outside of the host

II.  Pathobiology:
A.    Physiology
1. Facultative bacteria
a.       Can grow with or without oxygen present
B.     Fastidious: requires most essential nutrients to be imported into the host cell
1. Breakdown of erythritol is the most desirable metabolic pathway to acquire energy in the cattle host
2. Some strains require carbon dioxide to grow
C.     Enters phagocytes (macrophages) in the digestive or respiratory tract of the host and then attaches to the ER. They live in vacuolar spaces along the ER, undergo replication, and downregulate apoptosis genes. This prevents the host cells from undergoing self-death. B. abortus also suppresses the transcription of pro-inflammatory mediators, therefore, preventing inflammation and enabling them to become resistant to the host’s immune system.
D.    Persistence of bacteria in tissues of mononuclear phagocyte systems, including spleen, liver, lymph nodes, and bone marrow
E.     Can remain alive in the excrement of cattle and aborted fetuses (without replicating)
F.      Type IV secretion system are expressed extracellularly
1. Transcription of the virB operon is induced within macrophages
2. Required for microgranuloma formation
3. Delivers effector proteins into the host cell cytosol that interfere with cellular response to favor the infectious process

III.  Epidemiology
A.    Acquired from:
1. Direct contact with infected animals or animal products (zoonosis)
a.       Transferred from infected animals to human hosts while remaining pathogenic
b.      Commonly acquired by slaughterhouse workers, meatpackers, and laboratory workers
2. Ingestion of unpasteurized milk or cheese products
3. Can be inhaled as an aerosol = form of bioterrorism
B.     Minimum infectious exposure is 100 organisms

IV.   Laboratory diagnosis
A.    Gram-negative stain (dense clumps)
1. May require prolonged incubation (up to 6 weeks) because they are slow growing
B.     Serology: diagnosis can be made by culturing bone marrow or blood on Castaneda medium
C.     Catalase positive
1. Can convert hydrogen peroxide into water  
D.    Oxidase positive
1. Produce cytochrome oxidase enzyme for the ETC
E.     Can reduce nitrate
V.    Disease manifestations
A.    Diagnosis
1. Brucellosis
a.       There’s usually a 2 to 4 week latency period
b.      Acute phase: fever (increases during the day and decreases at night), chills, sweating, headaches, backaches, weakness, and weight loss
c.       Chronic phase: recurring joint pain/arthritis, fatigue, headaches, liver abscesses, endocarditis, or pneumonia
B.     Differential diagnosis
1. To distinguish B. abortus from Salmonella, test for urease
a.       Brucella is positive, Salmonella is negative
C.     Therapy
1. Treated with doxycycline + gentamicin; doxycycline + rifampin (most common treatment); or doxycycline + streptomycin
D.    Prevention and control
1. Prevented by pasteurizing milk, eradicating infection from herds and flocks, and observing safety precautions in laboratory and meatpacking settings
2. Vaccinate cattle herds in the U.S. to decrease the prevalence of the disease in animals
a.       Use brucellin skin tests to identify cattle that have been vaccinated
Related concepts
1. Coccobacilli
2. Gram-negative
3. Brucellosis
4. Urease positive
5. Microgranuloma formation
6. Fever
7. Type IV Secretion System
8. Zoonosis
9. Mononuclear phagocyte systems

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