Veterinary Bacteriology and Mycology - Mycobacterium

Genus Mycobacterium

Overview

• Strictly aerobic, non-sporeforming, curved or straight Gram-positive rods.

• Forms filaments that can break into rods or cocci upon disturbance.

• Notable for being acid-fast.

• Lipid content of cell wall constitutes 60% of dry weight.

Key Features

• Hydrophobicity of bacteria: Due to high lipid content.

• Slow growth: Characteristic of the genus.

• Resistance:

  • Acid resistance

  • Resistance to disinfectants, antibiotics, and desiccation.

• Cell wall lipids include mycolic acid and trehalose 6,6-dimycolate (cord factor).

Runyon’s Classification

• Based on growth rate and pigment production.

Group I: Photochromogenic Strains

• Grow slowly and produce yellow pigment when exposed to light.

• Associated with skin ulcers and pulmonary diseases in humans.

Group II: Scotochromogenic Strains

• Grow slowly and form orange-yellow pigment in both light and dark.

• Generally considered saprophytes.

Group III: Non-chromogenic Strains

• Grow slowly and do not produce pigments.

• Form smooth colonies and are resistant to isoniazid.

• Highly pleomorphic with filament production resembling Nocardia; associated with pulmonary diseases in humans.

Group IV: Rapid Growers

• Mature in less than 1 week at 25 to 37 °C.

Alternative Grouping of Mycobacteria

• Obligate Parasites

• Facultative Intracellular Parasites

• Saprophytes

Disease-Causing Mycobacterium in Domestic Animals

• M. bovis: Pathogenic for humans; causes tuberculosis in cattle, pigs, cats, horses, primates, dogs, and sheep.

• M. avium: Pathogenic; significant in various animals.

• M. paratuberculosis: Causes Johne’s disease; characterized by chronic or intermittent diarrhea in cattle.

• M. farcinogenes: Restricted to tropical countries; causes bovine farcy.

• M. tuberculosis: Primarily affects humans and other primates but also found in dogs and cattle.

Morphology

• In Tissues: Short, plump rods.

• In Culture Media: Large, slender, and beaded.

• Acid-fast characteristics.

Cultural and Biochemical Features

• Commonly used media includes egg yolk-based media, e.g., Dorset’s and Stonebrink’s media.

• Glycerol is inhibitory.

• Primary culture requires 6 to 8 weeks at 37 °C for colonies to develop.

• Colonies initially appear as minute dull flakes that mature into dry, irregular masses.

• Color changes: Light yellow progressing to deep yellow, eventually turning brick red when exposed to light.

• Growth characteristics vary with media: In fluid medium, growth is limited to surface unless wetting agents (e.g., Tween 80) are added.

• No growth at 25 °C.

Antigens

• Mycosides (glycolipids), waxes D (mycolic acid), cord factor (trehalose 6,6-dimycolate).

• Tuberculoprotein or tuberculin: A protein or polypeptide released into the medium by the organism.

Pathogenesis

• Transmission: Primarily aerogenous; can also be milk-borne, congenital, or sexually transmitted.

• Cattle are the natural primary host.

• Lacks traditional toxins or extracellular virulence factors.

• The importance of cord factor in virulent strains: Stimulates granulomatous responses.

Infection Process

1. Inhalation of bacteria.

2. Bacteria deposited in alveolar sacs.

3. Ingested by alveolar macrophages.

4. Bacteria multiply and kill phagocytes.

5. Bacteria are reingested by other macrophages.

6. Organisms filtered from the regional lymphatic system.

7. Intracellular multiplication occurs.

8. Bacteria enter bloodstream and disseminate to other body sites.

9. Some may return to lungs.

• The bacteria can prevent the fusion of lysosome and phagosome, leading to the antifusion effect and hindering intracellular digestion.

Lesion Development

• Tubercles are formed by macrophages, lymphocytes, and other leukocytes at primary sites, regional lymph nodes, and secondary metastasis sites.

• Lesions visually resemble translucent pearly structures similar to small tapioca grains.

• As growth continues, necrosis occurs in the centers of tubercles leading to yellow-white opaque areas.

• Presence of giant cells of Langhans (fusion or continuous growth of macrophages).

• Necrotic lesions consist of dry, cheesy material with calcium deposits, surrounded by dense connective tissue.

• Affected lymph nodes include bronchial, mediastinal, submaxillary, and retropharyngeal.

Old Lesions and Complications

• Older lesions show caseocalcareous characteristics in anterior lobes.

• Tubercles may be difficult to demonstrate in older lesions.

• Tuberculous pleuritis or peritonitis: Characterized by the presence of large masses of smooth, grape-like bodies on serous surfaces.

Specific Lesion Locations in Various Animals

• Swine: Lesions commonly found in lymph nodes of the head, neck, and abdomen.

• Horses: Lesions occur in pharyngeal region, mesentery, lungs, liver, and spleen.

• Sheep and Goats: Pulmonary lesions are commonly observed, especially in young kids.

• Cats: Involvement of abdominal organs and lungs is frequent.

Immunity

• Cell-mediated immune response becomes effective approximately 4 weeks after infection.

• Macrophages are activated by lymphokines released by T-lymphocytes sensitized to antigens.

• This immune response does not lead to the complete elimination of all organisms; T-lymphocytes remain sensitized for life.

Diagnosis

• Identifying characteristic lesions at necropsy.

• Utilizing Ziehl-Neelsen’s staining technique.

• Culture and isolation for identification, typically on Lowenstein-Jensen or Kirchner media.

• Culturing duration is 6 to 8 weeks at 37 °C.

Animal Inoculation Results

• Tuberculin Test:

  • M. tuberculosis is more potent but less specific than M. bovis.

  • Injection sites include caudal fold, lip of vulva, and side of the neck (most sensitive).

  • Tests are intradermal and examined after 72 hours.

  • Comparative tests between avian and mammalian strains can be performed.

  • BCG (Bacillus of Calmette and Guerin) and additional tests like ophthalmic, thermal, and Stormont tests (requires 2 injections at the same site, examined 7 days later).

Mycobacterium avium

Characteristics

• Runyon’s group 3.

• Morphology is more variable in shape than M. bovis.

• Strongly acid and alcohol-fast.

Cultural and Biochemical Features

• Typical growth observed in less than 1 week and well developed in about 2 weeks.

• Colonies are soft, moist, and yellow, transitioning to yellowish-pink as they age and are exposed to light.

Antigens

• M. intracellulare complex consists of serotypes 1 to 21; serotypes 1 and 2 are predominant in chickens and avians, while serotype 3 is commonly found in cattle.

Pathogenesis

• Birds are the natural host for M. avium.

• Serotypes 1, 2, and 3 exhibit virulence for chickens.

• Tuberculosis in birds develops slowly and is rarely seen in birds younger than a year old.

• Symptoms include loss of weight, weakness, listlessness, and eventual death.

• Lesions consist of caseous forms found in intestines, liver, and spleen.

• Swine are easily affected, and lesions are observed in mandibular and mesenteric lymph nodes.

• Minimal lesions are noted in cattle caused by M. avium, but it sensitizes the animal to avian tuberculin and to a lesser extent, mammalian tuberculin, as well as johnin (a tuberculin-like extract from M. paratuberculosis).

• Rare lesions in sheep, goats, dogs, and cats.

Immunity

• Both cell-mediated and humoral responses are involved in immunity against M. avium.

Diagnosis

• Detected using Ziehl-Neelsen’s staining technique.

• Isolation performed on Dorset's medium.

• Notable biochemical ability: Reduces tellurite but does not hydrolyze Tween 80.

Conclusion

• Understanding the genus Mycobacterium is crucial for veterinary bacteriology and mycology, as Mycobacteria species are significant pathogens in both humans and animals.

Thank You

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