Author Archives: Raja Mahendran

Microbiology Quiz–Mycobacteria questions

1. The drug which acts at A in the above image is

(A) Bedaquiniline

(B) Delamanid

(C) PAS

(D) Pyrazinamide

Answer: Option (B) is the correct answer

Reference: Guidelines for use of delamanid in the treatment of drug resistant TB 2018; Pg 2

Option (B): Delamanid is a nitroimidazooxazole compound which inhibits mycolic acid synthesis

Option (A): Bedaquiniline is a diarylquinoline which inhibits ATP synthase

Option (C): PAS – inhibits synthesis of DNA precursor.

Option (D): Pyrazinamide – disrupts plasma membrane

2. A 49yr old aquarium worker presented to the OPD with multiple nodules and few non healing ulcers in right forearm. He gives a history of cleaning aquarium tanks regularly. Biopsy from the nodule showed acid fast bacilli. The probable etiological agent is

(A) Mycobacterium leprae

(B) Nocardia

(C) Mycobacterium marinum

(D) Mycobacterium avium intercellulare complex.

Answer: Option (C) is the correct answer

Reference: Apurba Sastry Essentials of Medical Microbiology 2^nd edition pg no: 294

Option (C): The occupational exposure is suggestive of M.marinum which causes fish tank granuloma or swimming pool granuloma. It presents as tender nodules, papules and ulcers

Option (A): Mycobacterium leprae causes leprosy with hypopigmented anaesthetic patches in early lesions

Option (B): Nocardia causes Pulmonary nocardiosis, Disseminated nocardiosis and actinomycetoma.

Option (D): MAC complex is usually associated with pulmonary lesion and lymphadenitis.

3.  Match the above

(A) a- ii, b-v, c-i, d-iii

(B) a-v, b-iv, c-iii, d-i

(C) a- ii, b- iv, c-i, d-v

(D) a- iii, b-v, c-ii, d-i

Answer: Option (C) is the correct answer

Reference: Apurba Sastry Essentials of Medical Microbiology 2^ndedition pg no: 295

Option (C): As per Runyons classification a) M.marinum – photochromogens; b) M.gordonae –  scotochromogens; c) M.abscessus – rapid grower; d) M.ulcerans – Non photochromogens

4. Which of the following statement is true regarding GeneXpert used in TB diagnosis

(A) Can detect rifampicin resistance only

(B) Can detect isoniazid resistance only

(C) Can detect both rifampicin and isoniazid resistance

(D) does not detect drug resistance

Answer: Option (A) is the correct answer

Reference: Apurba Sastry Essentials of Medical Microbiology 2^nd edition pg no: 288

Option (A): GeneXpert simultaneously detects: (i) MTB complex DNA and (ii)

rifampicin resistance (mutations of the rpoBgene). It uses five probes targeting various sequences of rpoBgene

5. This instrument is based on

1. RT PCR
2. Automated liquid culture system
3. CBNAAT
4. Line probe assay

Answer: Option (2) is the correct answer

Reference: Apurba Sastry Essentials of Medical Microbiology 2^nd edition pg no: 288

Option (2): This is  BACTEC MGIT(Mycobacteria growth indicator tube) which is an automated liquid culture system to diagnose TB

Option (1): Nested PCR targeting IS6110 gene was the most common molecular test used earlier. Other gene targets in PCR are MPT64 gene, 65 KDa and 38 KDa genes.

Option (3): Cartridge based PCR used in TB diagnosis is CBNAAT – Genexpert.

Option (4): Line probe assay involves probe-based detection of amplified DNA in the specimen. The commercial kits available to perform LPA are GenoType and INNO-LiPA