mycobacterium

Ashish Kelwa

B.tech Bioinformatics

III Sem

INTRODUCTIONMycobacterium, genus of rod-shaped bacteria of the

family Mycobacteriaceae (order Actinomycetales), the

most important species of which, M. tuberculosis and

M. leprae, cause tuberculosis and leprosy, respectively,

in humans. M. bovis causes tuberculosis in cattle and in

humans. Some mycobacteria are saprophytes (i.e., they

live on decaying organic matter), and others are

obligate parasites. Most are found in soil and water in a

free-living form or in diseased tissue of animals.

Streptomycin, rifampin, and species-specific

antimicrobial agents have had some success in treating

Mycobacterium infections. Mycobacterium leprae

Classification

Mycobacteria are slender rods that sometimes show branching, filamentous forms resembling fungal mycelium.

The genus Mycobacterium contains three groups:

o Obligate parasites

o Opportinistic pathogens

o Saprophytes

Obligate Parasites

• Mycobacterium tuberculosis complex

Contains M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti, M. capraeand M. pinnipedii

• Mycobacterium leprae

Opportunistic Pathogens

Non-tuberculosis mycobacteria (NTM)

This group contains mixed group of isolates from diverse sources: birds, cold-blooded and warm-blooded animals, from skin ulcers, and from soil, water and other environmental sources.

They are opportunistic pathogens and can cause many types of disease.

Pathogenesis

Source of infection:

Open case of pulmonary tuberculosis

Mode of infection

Direct inhalation of aerosolized bacilli contained in the droplet nuclei of expectorated sputum.

Infection also occurs infrequently by ingestion for example, through infected milk, and rarely by inoculation.

Millions of tubercle bacilli in lungs (mainly in cavities).

Coughing projects droplet nuclei into the air that contain tubercle bacilli.

One cough can release 3,000 droplet nuclei.

One sneeze can release tens of thousands of droplet nuclei

Transmission of M.tuberculosis

Infection

The initial infection with M. tuberculosis is referred to as a

Primary infection

Subsequent disease in a previously sensitized person, either from an exogenous source or by reactivation of a primary infection is known as

Postprimary tuberculosis

Both forms exhibit quite different pathological features.

Characteristics Primary Postprimary

Site Any part of lung

Apical region

Local lesion Small Large

Cavity formation Rare Frequent

Lymphatic involvement

Yes Minimal

Infectivity* Uncommon Usual

Local spread Uncommon Frequent

*Pulmonary cases

Difference b/w Primary and Postprimarytuberculosis

ImmunologyTubercle bacilli do not contain or secrete a toxin.

The exact basis of their virulence is not understood, butseems to be related to their ability to survive and multiply in macrophages.

Humoral immunity appears to be irrelevant.

The only specific immune mechanism effective is the CMI.

The key cell is the activated CD4+ helper T cell which can develop along two different paths: The Th1 and Th2 cells

Th1 dependent cytokines activate macrophages, resulting in protective immunity and containment of the infection

Th2 cytokines induce delayed type hypersensitivity (DTH), tissue destruction and progressive disease

Laboratory Diagnosis

Early morning sputum samples should be collected for 3 consecutive days in a sterile container

In case of renal tuberculosis, 3-6 morning urine samplesshould be collected

Type of lesion Specimen

Pulmonary tuberculosis SputumLaryngeal swabs or bronchial washings

Gastric lavage

Renal tuberculosis Urine

Tuberculosis meningitis CSF

Specimen Collection

Direct Microscopy

Ziehl-Neelsen staining (hot staining method)

Kinyoun’s method (cold staining method)

Acid fast bacilli resist decolourisation with acid and alcohol once they have been stained with carbolfuchsin.

AFB appear as pink, long, slender bacilli with beaded appearance.

Ziehl-Neelsen Staining

Fluorescent staining by Auramine O or auramine rhodamine

Mycobacterium spp. will fluoresce yellow against dark background under fluorescent microscope

Culture

Concentrated specimen is inoculated on Lowenstein – Jensen’s medium andincubated at 370C for 2 – 8 weeks

Colonies appear as buff coloured, dry, irregular colonies with wrinkled surface and not easily emulsifiable(Buff, rough and tough colonies)

Colonies are creamy white to yellow colourwith smooth surface and easily emulsifiable

Detection of antibodies

Various methods such as enzyme linked immune sorbent assay (ELISA), radio immunoassay (RIA), latex agglutination assay have been employed for detection of antibodies in patient serum.

However, diagnostic utility of these methods is doubtful.

WHO has recommended that these tests should not be use for diagnosis of active tuberculosis.