Microbiology Tutorial

Mycobacterial infections

Granulomatous disease - formation of granulomas with epithelioid cells, giant cells, lymphocytes, fibroblasts as a result of cell-mediated immune response

M. tuberculosis (MTB)

Pulmonary disease

primary - often occurs in children - pathologic hallmark is the Ghon complex (peripheral mid-lung granuloma with large hilar nodes); only a few persons develop severe disease, the majority have subclinical disease and lesions heal and calcifiy, but a few organisms may remain
secondary - reactivation or reinfection tuberculosis - typical for adults - pathologic hallmark is pulmonary granulomas with cavitation

Disseminated tuberculosis - miliary tuberculosis, either in lung or in distant organs
Debilitation and malnutrition favor development of disease; clinically active tuberculosis characterized by fever, night sweats, and weight loss

"Atypical" mycobacteria

M. kansasii
M. avium-complex (MAC)
M. fortuitum
M. bovis
M. marinum

M. leprae: Leprosy (Hansen disease) characterized by poor transmissibility (takes years of contact) and slow course over many years. Diagnosed by skin biopsy. There are two priniciple forms:

Lepromatous leprosy - poor immune response, dissemination, marked skin and nerve involvement
Tuberculoid leprosy - better immune response with localized skin and nerve involvement

Medically Important Mycobacteria

Culture and Morphology

Species	Acid Fast Staining Characteristics	Growth on Lowenstein-Jensen Agar	Disease Characteristics
Mycobacterium tuberculosis	When grown in culture rods are arranged in long serpentine cords from elaboration of the virulence factor known as "cord factor"	Buff color with rough colony surface; non-photochromogenic	Best known as the cause for pulmonary tuberculosis
Mycobacterium kansasii	When grown in culture appears as long rods with banded "candy cane" pattern	Buff color with slightly rough colony surface; photochromogenic colonies become pigmented when grown in the presence of light	Causes pulmonary disease similar to M. tuberculosis
Mycobacterium avium-complex	When grown in culture the thin rods tend to form clusters	Buff color with smooth colonies; non-photochromogenic colonies	Cause for disseminated infections involving organs of the mononuclear phagocyte system in immunocompromised patients
Mycobacterium leprae	Short rods are weakly acid fast	Does not grow on artificial media Can be cultured in foot pads of armadillos	Cause for Hansen disease, a poorly transmissible infection involving skin and peripheral nerves

Other Mycobacteria
Mycobacterium bovis	Cause for gastrointestinal tuberculosis, now rare because of pasteurization of milk
Mycobacterium marinum	Cause for superficial skin lesions, uncommon but related to superficial trauma in environments with water (e.g., cleaning the fish tank)
Mycobacterium fortuitum	Has rapid growth on media; seen uncommonly following trauma, surgery, or invasive procedures with local tissue trauma

Identification of Mycobacteria
Mycobacteria with mycolic acids in the outer cell wall form stable complexes with dyes having aromatic methane rings such as fuchsin and auramine, and these complexes are resistant to decolorization with acid alcohols and are termed acid fast. The identification of "acid fast bacilli" is an initial screening test in pulmonary specimens or in tissues.
The specific identification of M. tuberculosis and M. avium-intracellulare (MAI) complex is performed with DNA probes. The labeled DNA probe combines with the target organism's ribosomal RNA to form a stable DNA:RNA hybrid. For non-tuberculous Mycobacteria sequencing of ribosomal RNAs is performed.

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