mycobacteria - prac. microbiology

MYCOBACTERIA

Characters

• Acid-alcohol fastness

• Slow rate of growth

Members of the Genus Mycobacterium

1. Mycobacterium tuberculosis Complex: M. tuberculosis, M. bovis, M. africanum & M. microti.

2. Non-tuberculous mycobacteria (NTM): e.g. M. avium complex & M. fortuitum.

3. Mycobacterium leprae: causes leprosy.

4.The Bacillus of Calmette-Guerin (BCG)

Mycobacterium Tuberculosis

Morphology:- Acid-alcohol-fast bacillus- Slender - Straight or curved - Arranged singly or in small groups

- Ziehl-Neelsen (Z-N) --- pink in a blue background.

Lipid-Rich Cell Wall

Gram Stain

Ziehl-Neelsen (Z-N) Stain

• Carbol-fuchsin dye

• Heat

• Acid-alcohol (decolourization)

• Methylene blue

• Acid-fast = Mycobacteria

Cultural Characters

Media include:

- Lowenstein-Jensen (L-J) medium

- Middlebrook's medium

Cultural Characters

• Strict aerobe (5 - 10% CO2 stimulates its

growth).

• Colonies appear after 2-4 weeks of incubation at 37°C and may be delayed to 8 weeks.

Human TuberculosisMode of Infection

Inhalation of droplet nuclei from a patient with open pulmonary tuberculosis.

Ingestion of milk contaminated with M. bovis.

TB may affect other systems (e.g. TB meningitis, lymphadenitis, renal, genital & intestinal TB).

Oral mainfestations of tuberculosis

1. Primary complex

2. Oral ulceration secondary to pulmonary tuberculosis

3. Lupus vulgaris

4. Tuberculous periapical infection and osteomyelitis.

Laboratory Diagnosis of Open Pulmonary TB

Specimens: include sputum, BAL

Early morning sputum specimens collected on 3 consecutive days, from a

deep productive cough, give the best results

Direct Detection:

1. Smears:

Ziehl-Neelsen (Z-N).

Under the ordinary light microscope, AFB appear pink in a blue background.

2. Molecular tests:

• For rapid detection

• Expensive

Cultivation:

Conventional culture media: (incubated in 5 – 10% CO2 at 37oC):

L-J tube

- Results may be relatively delayed (2-8 weeks).

Fluid Medium Systems(4-14 days identification)

Middle brook broth and Bactec AFB media

Laboratory Diagnosis of Extra-Pulmonary TB

Same as mentioned for pulmonary TB, but by collecting the appropriate specimens.

However, specimens collected from normally sterile sites, e.g. CSF, do not require decontamination.

Tuberculin Skin Test (TST)

Principle:

detects a delayed type hypersensitivity (DTH)

response to previous exposure of the host to

tubercle bacilli.

Interpretation of the Tuberculin Skin Test

An induration of 10 millimeters or more is considered positive

Prevention is Always Better than Treatment

1. General Measures e.g., Pasteurization of milk.

2. Treatment of latent tuberculosis.

3. BCG Vaccine.

MCQ Questions

1.M. tuberculosis are characterized by being:

a.Strict anaerobes

b.Acid fast bacilli

c.Capsulated

d.Motile

e.Able to grow on ordinary media

2.One of the following media is used for growing M. tuberculosis:

a.MacConkey’s agar

b.Blood agar

c.Nutrient agar

d.Lowenstein Jensen medium

e.Loeffler’s serum

3.The following methods are used in the diagnosis of pulmonary tuberculosis except:

a.Examination of ZN stained direct sputum smear

b.PCR of sputum

c.Culture on selective medium

d.BACTEC AFB system

e.Detection of specific antibodies

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