The Brucellae: Morphology and Identification, Antigenic Structure, Pathogenesis and Pathology, Clinical Findings, Diagnostic Laboratory Tests, Immunity,

Question 1

Brucellae: morphology and identification

Answer 1

Brucellae are small, nonmotile, non–spore-forming, Gram-negative coccobacilli. They are strict aerobes and require enriched media like blood or liver infusion agar for growth. Colonies are small, smooth, and convex. Brucella melitensis, B. abortus, B. suis, and B. canis are the four main pathogenic species in humans.

Question 2

Brucellae: antigenic structure

Answer 2

Brucellae have two antigenic phases: smooth (S) and rough (R), depending on their lipopolysaccharide (LPS) structure. The smooth phase has complete LPS and is more virulent. The O-polysaccharide chain of LPS is important in pathogenesis and used in serologic testing. No capsule is present.

Question 3

Brucellae: pathogenesis and pathology

Answer 3

Brucellae are facultative intracellular pathogens that survive and replicate inside macrophages. After penetration through mucous membranes or skin, they are phagocytosed and carried to lymph nodes, liver, spleen, and bone marrow. They inhibit phagosome-lysosome fusion, allowing intracellular replication. Granulomatous inflammation can occur in infected organs.

Question 4

Brucellae: clinical findings

Answer 4

Brucellosis (undulant fever) causes nonspecific symptoms: fever (intermittent or continuous), chills, sweats, fatigue, arthralgia, and weight loss. Hepatosplenomegaly and lymphadenopathy may occur. Chronic brucellosis can cause spondylitis, arthritis, endocarditis, and neurobrucellosis. Relapses are common without adequate treatment.

Question 5

Brucellae: diagnostic laboratory tests

Answer 5

Blood or bone marrow culture is the gold standard; Brucellae grow slowly (may take up to 2 weeks). Automated blood culture systems increase yield. Serology (standard agglutination test, SAT) detects antibodies; a titer ≥1:160 is diagnostic. ELISA and PCR are also useful, especially in chronic cases.

Question 6

Brucellae: immunity

Answer 6

Cell-mediated immunity is essential for controlling infection. Th1 response activates macrophages to kill intracellular Brucellae. Antibodies develop but are not protective alone. Recovery provides partial immunity, but relapses may occur without proper therapy.

Question 7

Brucellae: treatment

Answer 7

Treatment requires prolonged dual antibiotic therapy. First-line regimen: doxycycline (6 weeks) plus rifampin (6 weeks). Alternative combinations include doxycycline plus streptomycin or gentamicin (shorter aminoglycoside course). Monotherapy or short courses have high relapse rates. Endocarditis requires extended therapy and sometimes surgery.

Question 8

Brucellae: epidemiology

Answer 8

Brucellosis is a zoonotic infection acquired from contact with infected animals or consumption of unpasteurized dairy products. B. melitensis is found in goats/sheep (most virulent in humans), B. abortus in cattle, B. suis in pigs, B. canis in dogs. It is an occupational hazard for veterinarians, farmers, and slaughterhouse workers. Endemic in Mediterranean, Middle East, Latin America, and Asia.

Question 9

Brucellae: prevention and control

Answer 9

Preventive measures include pasteurization of milk, control of infection in livestock (animal vaccination, test and slaughter), and protective equipment for at-risk workers. No human vaccine is available. Avoiding unpasteurized dairy and proper handling of animals reduces risk. Infected animals should be reported and culled.