Mycobacteria: Pathogens, Characteristics, and Identification

Mycobacterial pathogens

M. tuberculosis, M. bovis, M. ulcerans, M. leprae.

Characteristics of Mycobacteria

Non-motile, slender, slightly curved, rod-shaped bacilli; non-spore forming.

Mycobacterial cell wall component

High lipid content (mycolic acid).

Media requirements for Mycobacteria

Require complex media; slow-growers, with those associated with disease taking 2 to 6 weeks to produce growth.

Mycobacterial species enhanced by CO2

M. tuberculosis.

Mycobacterial species that fails to grow in vitro

M. leprae.

Common acid-fast staining methods

Ziehl-Neelsen and Kinyoun modification.

Acid-fast staining method using heat

Ziehl-Neelsen.

Fluorescent acid-fast staining method

Auramine rhodamine.

Acceptable respiratory specimens for Mycobacteria

Expectorated sputum, normal-saline-nebulized induced sputum, gastric aspirate, bronchoalveolar lavage, laryngeal swab.

Digesting agent for respiratory specimens

N-acetyl-L-cysteine (NALC) with NaOH.

Purpose of Sodium hydroxide (NaOH) in specimen processing

Serves as a decontaminating agent and digestant.

Use of N-acetyl-L-cysteine (NALC)

Serves as a liquefying or mucolytic agent.

Decontaminating agent for Pseudomonas aeruginosa

Oxalic acid 5%.

Egg-based culture media for Mycobacteria

Lowenstein-Jensen (LJ), Petragnani.

Ingredient to inhibit contaminants in egg-based media

Malachite green.

Advantage of Middlebrook 7H10 or 7H11 media

Clear medium for easier visualization.

Photochromogens in Mycobacteria

Organisms that produce pigment when exposed to light.

Scotochromogens in Mycobacteria

Organisms that produce pigment in light and dark.

Non-chromogens in Mycobacteria

Nonpigmented organisms, producing tan or buff colored colonies.

Non-chromogenic and non-photoreactive species

M. tuberculosis.

Positive biochemical test for M. tuberculosis isolate

Niacin test.

Nitrate Reduction test

Detects nitroreductase, which reduces nitrate to nitrite; positive result is red.

Tests for lipase production

Hydrolysis of Tween 80; distinguishes photochromogens, scotochromogens, and non-chromogens.

Arylsulfatase Test

A 3-day test that tests ability to split phenolphthalein.

Hansen's Disease (Leprosy)

Two major forms: Tuberculoid Leprosy and Lepromatous Leprosy.

Characteristics of Tuberculoid Leprosy

Usually self-limiting, single skin lesions, nerve involvement causing patches of anesthesia (loss of feeling).

Characteristics of Lepromatous Leprosy

Progressive and malignant, extensive skin involvement, organisms disseminate to all organs.

Habitats for M. leprae

Humans are the major source; armadillos are also a habitat.

Diagnosis of M. leprae

Diagnosis based on clinical symptoms and positive acid-fast smears of skin scrapings.

Pott's Disease

The skeletal TB disease.

Virulence factors of M. tuberculosis

Cord factor (mycolic acid in cell wall, inhibits migration of PMNs) and Sulfatides (glycolipids in cell wall, inhibit phagosome-lysosome fusion and enhance survival within macrophages).

Primary mode of transmission for M. tuberculosis

Droplet aerosols.

BCG Vaccine

Vaccine used for Tuberculosis (M. bovis Bacille de Calmette et Guerin).

Microscopic morphology of Mycobacteria

Rod shaped, filamentous, considered a gram-positive organism.

Culture conditions for M. tuberculosis

Raised colonies with a dry, rough appearance; nonpigmented (buff colored); optimum growth at 35°-37°C; grows best in 5% - 10% CO2.

MDR-TB

Multi-drug resistant tuberculosis.

Risk factors for acquiring MDR-TB

Previous treatment for TB, living in endemic area, close contact with MDR-TB infected patient.

Animals affected by M. bovis

Cattle, dogs, cats, swine, parrots.

MOTTs (Mycobacterium other than tuberculosis)

Generally considered opportunistic pathogens.

Can you get infected with MOTTs from another person

No, they are not transmissible from person to person.

Mycobacterium avium Complex (MAC)

Two main species: M. avium and M. intracellulare.

Common systemic bacterial infection in AIDS patients

Disseminated disease caused by Mycobacterium avium Complex (MAC).

Causative agent of Johne disease

Mycobacterium avium subsp. paratuberculosis.

Disease caused by M. marinum

Cutaneous granulomas (e.g., swimming pool granuloma) and ulcers.

M. ulcerans

Known for causing cutaneous ulcers, such as Bairnsdale ulcer (Australia) and Buruli ulcer (Africa).

Disease caused by M. scrofulaceum

Cervical lymphadenitis (one or more enlarged lymph nodes, little or no pain).

Rapid-growers of Mycobacteria

M. chelonae, M. abscessus, M. fortuitum, M. phlei, M. smegmatis.