Bacteriology: specimen collection and transport

11 Kerala PSC Lab Technician Gr II practice questions on Bacteriology: specimen collection and transport, part of Microbiology. The first 11 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Easy

   For which suspected infection is stool sent in Venkatraman-Ramakrishnan (VR) medium?

   1. ACholera
   2. BAmoebic dysentery
   3. CBacillary dysentery
   4. DTyphoid fever

   Show answerHide answer

   Answer: A · Cholera

   VR medium is a holding medium for Vibrio cholerae developed in India: an alkaline sea salt and peptone solution in which vibrios survive for weeks at room temperature. A small amount of rice-water stool is added and sent to the laboratory, where it is enriched in alkaline peptone water and plated on TCBS agar. Cary-Blair medium can also be used. Buffered glycerol saline, used for Shigella, is unsuitable because glycerol harms vibrios.

   • B: Amoebic dysentery needs fresh stool examined at once for motile trophozoites.
   • C: Stool for Shigella is sent in buffered glycerol saline or Cary-Blair medium.
   • D: Typhoid is diagnosed mainly by blood culture; stool for Salmonella is sent in Cary-Blair or buffered glycerol saline.

2. Question 2 · Medium

   Blood for culture is added to broth at about 1 part in 5 to 10. What is the main purpose of this dilution?

   1. ATo stop the blood clotting in the bottle
   2. BTo lyse red cells so growth is easy to see
   3. CTo keep the broth free of dissolved oxygen
   4. DTo dilute antibacterial substances in blood

   Show answerHide answer

   Answer: D · To dilute antibacterial substances in blood

   Fresh blood contains antibodies, complement, phagocytes and sometimes antibiotics that can kill bacteria in the bottle. Diluting it 1 in 5 to 1 in 10 in broth weakens these effects so that a few organisms can multiply. Sodium polyanethol sulfonate (SPS) in the broth prevents clotting and also inactivates complement and some aminoglycosides. Adults need 8-10 mL of blood per bottle, because organisms in the blood are often few.

   • A: Clotting is prevented by SPS in the broth, not by the dilution.
   • B: Dilution does not lyse red cells; lysis-centrifugation needs a special tube.
   • C: Oxygen content depends on the bottle type, not on the blood-to-broth ratio.

3. Question 3 · Medium

   Why does Stuart's transport medium contain no nutrients?

   1. ATo keep organisms alive without multiplying
   2. BTo stop the swab from drying out in transit
   3. CTo kill the normal flora before plating
   4. DTo let the pathogen outgrow the commensals

   Show answerHide answer

   Answer: A · To keep organisms alive without multiplying

   Stuart's medium is a semisolid, non-nutritive buffered medium with a reducing agent (sodium thioglycollate). Without nutrients the organisms neither multiply nor die quickly, so pathogens and commensals stay in the proportions present at collection. If commensals could grow, they would overgrow delicate pathogens such as gonococci. Amies medium is a modification that adds charcoal to absorb toxic substances.

   • B: Moisture is kept by the semisolid agar, not by leaving out nutrients.
   • C: Transport media preserve all organisms; they do not kill the normal flora.
   • D: Letting organisms grow would let fast commensals overgrow the pathogen.

4. Question 4 · Hard

   CSF from a 5-year-old with suspected meningitis, taken before any antibiotic, reaches the laboratory at midnight. Gram smear shows intracellular gram-negative diplococci, but culture on chocolate agar in 5-10% CO2 next morning is sterile. Which step most likely caused this?

   1. ARefrigeration overnight
   2. BUse of chocolate agar
   3. CIncubation in CO2
   4. DDelay in Gram staining

   Show answerHide answer

   Answer: A · Refrigeration overnight

   The smear proves that organisms were present, and no antibiotic had been given, so they died after collection. Neisseria meningitidis, like Haemophilus influenzae and Streptococcus pneumoniae, is fragile and dies quickly in the cold. CSF for bacterial culture is processed at once or, if a delay cannot be avoided, kept at room temperature or 37 °C, never in the refrigerator. Chocolate agar in 5-10% CO2 is the correct way to culture meningococci, and a delay in staining does not affect the culture.

   • B: Chocolate agar is the right medium for Neisseria and Haemophilus.
   • C: Meningococci grow better in 5-10% CO2; it does not inhibit them.
   • D: A delay in staining does not affect whether the culture grows.

5. Question 5 · Easy

   Sachs' buffered glycerol saline is used to transport stool from patients with suspected:

   1. ABacillary dysentery
   2. BAmoebic dysentery
   3. CCampylobacter enteritis
   4. DCholera

   Show answerHide answer

   Answer: A · Bacillary dysentery

   Buffered glycerol saline at pH 7.4 keeps Shigella and Salmonella alive during transport while it holds back the commensal flora. It contains phenol red, and the medium is discarded if it turns yellow, because acid kills shigellae. It is not suitable for vibrios, so VR medium or Cary-Blair medium is used for cholera. Cary-Blair medium is also used for Campylobacter.

   • B: Amoebic trophozoites need fresh stool examined at once, not a bacterial transport medium.
   • C: Campylobacter is sent in Cary-Blair medium.
   • D: Vibrio cholerae does not survive well in glycerol saline; VR or Cary-Blair medium is used.

6. Question 6 · Easy

   Which preservative is added to urine for culture when it cannot be plated within 2 hours?

   1. AConcentrated hydrochloric acid
   2. B10% formalin
   3. CThymol
   4. DBoric acid

   Answer and explanation in LearnFree account

7. Question 7 · Hard

   A midstream urine from an outpatient was collected at 8 a.m. and left on the bench until it was plated at 4 p.m. Culture grows three different organisms, each at more than 10^5 CFU/mL. The patient has no catheter. What is the best action?

   1. ARequest a fresh, promptly plated sample
   2. BDo sensitivity tests on all three
   3. CReport the predominant organism with its sensitivity
   4. DReport significant growth of all three

   Answer and explanation in LearnFree account

8. Question 8 · Easy

   Sputum for acid-fast bacilli should be collected in a:

   1. ANarrow-necked glass bottle with a cork stopper
   2. BTube containing EDTA anticoagulant
   3. CPlastic bag tied with a rubber band
   4. DWide-mouthed, screw-capped, leak-proof container

   Answer and explanation in LearnFree account

9. Question 9 · Easy

   Charcoal is added to Amies or Stuart transport medium for gonococcal swabs in order to:

   1. ANeutralise substances toxic to the organism
   2. BSupply carbon as a nutrient for the gonococci
   3. CKeep the pH acidic during transport
   4. DInhibit the normal flora of the swab

   Answer and explanation in LearnFree account

10. Question 10 · Medium

    A deep abscess is to be cultured for anaerobes. Which specimen is best?

    1. ADry cotton swab rubbed over the abscess surface
    2. BPus in a container left open
    3. CSwab kept in a refrigerator
    4. DNeedle aspirate sent without air

    Answer and explanation in LearnFree account

11. Question 11 · Medium

    In a clean-catch midstream urine sample, the first part of the stream is discarded because it:

    1. AHas the most dilute urine
    2. BCarries vaginal epithelial cells only
    3. CWashes out urethral bacteria
    4. DContains too little urine for culture

    Answer and explanation in LearnFree account