DHA Microbiology Laboratory exam questions

6 worked Microbiology Laboratory practice questions

Original exam-style questions from the bank, spread across the blueprint domains. They are not real or recalled exam questions. Try each one, then open the answer for the rationale and reference. None of them appear in the free mock exam, so reading them will not spoil it.

1. Question 1 · Antimicrobial Susceptibility Testing and Resistance

   An E. coli isolate is screened as a possible ESBL producer. In the combination disc test, which result confirms ESBL production?

   1. AZone increase of 5 mm or more with clavulanate
   2. BZone decrease of 5 mm or more with clavulanate
   3. CNo zone around a cefoxitin disc
   4. DFlattening of zone near an erythromycin disc
   Show answer and explanation

   Correct answer: A. Zone increase of 5 mm or more with clavulanate

   An increase of 5 mm or more in the zone around ceftazidime or cefotaxime when combined with clavulanate confirms ESBL production, because clavulanate inhibits the enzyme. Cefoxitin resistance suggests AmpC rather than ESBL. Flattening near erythromycin is the D-test for inducible clindamycin resistance.

   Reference: CLSI M100: Performance Standards for Antimicrobial Susceptibility Testing

2. Question 2 · Bacteriology and Identification

   Alpha-haemolytic, draughtsman-shaped colonies of Gram-positive diplococci grow from a blood culture. The isolate is optochin sensitive. What is the most likely organism?

   1. AStreptococcus pneumoniae
   2. BStreptococcus mitis
   3. CEnterococcus faecium
   4. DStreptococcus agalactiae
   Show answer and explanation

   Correct answer: A. Streptococcus pneumoniae

   Streptococcus pneumoniae is alpha-haemolytic, forms draughtsman colonies, is optochin sensitive and bile soluble. Viridans streptococci such as Streptococcus mitis are usually optochin resistant. Streptococcus agalactiae is beta-haemolytic.

   Reference: Murray's Manual of Clinical Microbiology, 12th ed.

3. Question 3 · Bacteriology and Identification

   A wound culture grows non-lactose-fermenting Gram-negative rods that are oxidase positive, produce a green pigment and have a grape-like smell. What is the most likely organism?

   1. AKlebsiella pneumoniae
   2. BAcinetobacter baumannii
   3. CProteus mirabilis
   4. DPseudomonas aeruginosa
   Show answer and explanation

   Correct answer: D. Pseudomonas aeruginosa

   An oxidase-positive non-fermenter with green pigment (pyocyanin) and a grape-like odour is typical of Pseudomonas aeruginosa. Acinetobacter is oxidase negative. Klebsiella and Proteus are oxidase-negative Enterobacterales.

   Reference: Murray's Manual of Clinical Microbiology, 12th ed.

4. Question 4 · Bacteriology and Identification

   A hospital patient has diarrhoea. The laboratory's two-step algorithm shows a positive GDH screen but a negative toxin immunoassay. How should this result be interpreted?

   1. AActive C. difficile infection is confirmed
   2. BC. difficile is present but toxin was not detected
   3. CThe GDH result is a false positive
   4. DThe stool should be cultured anaerobically for 7 days
   Show answer and explanation

   Correct answer: B. C. difficile is present but toxin was not detected

   A positive GDH or NAAT with a negative toxin test shows that C. difficile is present but toxin was not detected, which may represent colonisation or low toxin levels. The report should say so, and clinicians decide on treatment with the clinical picture. A positive toxin test is needed to confirm likely active infection.

   Reference: Crobach et al., ESCMID update of the diagnostic guidance document for Clostridium difficile infection, Clin Microbiol Infect 2016

5. Question 5 · Biosafety, Infection Prevention and Quality

   A blood culture grows tiny Gram-negative coccobacilli after several days, and the patient drinks raw camel milk. How should the laboratory handle further work on this isolate?

   1. AUse a safety cabinet with level 3 practices
   2. BContinue identification on the open bench
   3. CSniff the plates to check for odour
   4. DPerform rapid identification strips on the open bench
   Show answer and explanation

   Correct answer: A. Use a safety cabinet with level 3 practices

   Brucella is a leading cause of laboratory-acquired infection, transmitted by aerosols. Suspected isolates must be handled in a biological safety cabinet using containment level 3 practices, and plates must never be sniffed. Open-bench work exposes staff to infection.

   Reference: WHO Laboratory Biosafety Manual, 4th ed.

6. Question 6 · Mycobacteriology

   A sputum is cultured for mycobacteria in an automated liquid culture system. For how long should the tube be incubated before it is reported as negative?

   1. A7 days
   2. B14 days
   3. C42 days
   4. D90 days
   Show answer and explanation

   Correct answer: C. 42 days

   Automated liquid mycobacterial culture tubes are usually incubated for 42 days (6 weeks) before being reported negative, as Mycobacterium tuberculosis grows slowly. Solid media are often incubated for up to 8 weeks. Shorter incubation would miss slow-growing isolates.

   Reference: Siddiqi and Rusch-Gerdes, MGIT Procedure Manual, FIND 2006