Rhodococcus Genus Study Notes

Genus Rhodococcus

  • Belongs to the order Actinomycetales

    • Contains 9 soil-associated species that produce a red pigment

    • Only Rhodococcus equi has been reported to cause lesions in animals

    • Associated conditions include:

      • Purulent pneumonia

      • Mesenteric lymphadenitis

      • Arthritis in foals

      • Lesions similar to tuberculosis in cervical lymph nodes of swine and cattle

Rhodococcus equi

  • Causative agent of purulent pneumonia in foals

Morphology and Staining

  • Large organism with pleomorphic characteristics

    • Forms bacillary and coccoid forms

    • Coccoid on solid media

    • Bacillary in fluid, sometimes appears in short chains

  • Metachromatic granules can usually be demonstrated if grown in milk

  • Gram-positive and variably acid-fast

  • Stains readily with other dyes

  • Non-sporeforming

  • Possesses a lamellar polysaccharide capsule

Cultural and Biochemical Features

  • Grows well on all ordinary media

    • Colonies characteristics:

    • Diameter: 1 cm

    • Appearance: raised, moist, translucent, regular outline

    • Initial color: white, changes to rose pink especially when grown on potato

  • Grows poorly in milk

  • Biochemical characteristics:

    • Catalase positive

    • Urease positive

    • Cytochrome c negative

    • Does not ferment carbohydrates (CHO)

    • Reduces nitrate

    • Does not form indole

    • Non-hemolytic

  • Produces phospholipase and cholesterol oxidase:

    • Reacts with phospholipase D of C. pseudotuberculosis, beta toxin of S. aureus, and hemolysin of L. monocytogenes to completely hemolyze sheep, cattle, and rabbit RBC.

    • Implication: Rapid presumptive test for identification of R. equi

Antigens

  • Polysaccharide capsule present

  • Divided into 4 groups and further into 14 serotypes

Epizootiology and Pathogenesis

  • Acquired from soil, gut contents, and feces of herbivores

  • Heat resistant

    • Killed at 60°C for 1 hour

    • Resists extreme pH, 0.01% sodium azide, 0.5% formaldehyde, sunlight, and desiccation

  • Route of infection in foals:

    • Enters through the respiratory tract

    • Pathway includes entry into alveoli, phagocytosis by macrophages, and subsequent processes leading to degeneration and necrosis:

    • Massive infiltration of macrophages and multinucleated giant cells into the alveolar space

    • Formation of foci of alveolar necrosis where bacteria-laden macrophages degenerate

    • Granulomatous response in the lung

    • Massive invasion of polymorphonuclear leukocytes (PMNs) leading to suppurative bronchopneumonia with prominent abscesses

Clinical Signs

  • Common in foals aged 2 to 5 months and older

  • Clinical manifestations include:

    • Anorexia

    • Nasal discharge

    • Arthritis

    • Diarrhea

  • Mortality rate: 64%

  • Lymphadenitis with abscessed lymph nodes

    • Contrast with strangles where lymph nodes of the head are seldom involved

  • In addition to pneumonia, internal abscesses and pleurisy can occur

  • In swine, infections affect submandibular and cervical lymph nodes, sometimes associated with tubercle bacilli

Immunity

  • Involves cell-mediated immune responses

  • Surface components inhibit bactericidal mechanisms of PMNs

  • Antibody production measurable by:

    • Precipitin reaction

    • Complement fixation

    • Agglutination

    • Indirect hemagglutination

    • ELISA

  • Antibodies present in the serum of normal mares and passively transferred to foals

  • Generally poor antibody responses noted

Diagnosis

  1. Recognized based on the source isolate

  2. Gram staining reaction

  3. Pleomorphism

  4. Appearance of pinkish, mucoid, spreading colonies

  5. Catalase and urease positivity

  6. Lack of fermentative activity on carbohydrates (CHO)

  7. Equi factor test (Prescott confirmatory)

  • Transtracheal aspirates and Gram staining cultures are of great value

  • Selective medium containing:

    • Nalidixic acid

    • Novobiocin

    • Actidione (cycloheximide)

    • Potassium tellurite (NANAT medium)

    • Resulting colonies turn black

Antimicrobial Susceptibility

  • Sensitive to:

    • Penicillin G

    • Doxycycline

    • Erythromycin

    • Lincomycin

    • Gentamycin

    • Neomycin

    • Streptomycin

  • Effective combinations include:

    • Erythromycin and rifampicin

    • Penicillin and gentamycin

Diseases in Humans

  • Particularly affects individuals with:

    • AIDS or undergoing immunosuppressive therapy for malignant neoplasms

Conclusion

  • Importance of awareness of Rhodococcus equi infection in veterinary and human health.