Cultures from patients' bronchial washings grow the same Pseudomonas aeruginosa. The bronchoscope is soaked in 2% glutaraldehyde for 20 minutes after each use, but the channels are not brushed first. What is the most likely cause?

Organic debris in the channels shielded the bacteria

Glutaraldehyde does not kill Pseudomonas at all

The disinfectant should have been 70% ethanol

Twenty minutes is too long and corrodes the scope

Beta-propiolactone is used in vaccine production mainly to:

Inactivate viruses such as rabies virus

Sterilise the glass vials before filling

Preserve opened multi-dose vials

Adsorb antigen onto alum adjuvant

A suprapubic bladder aspirate from an infant grows 2,000 CFU/mL of Escherichia coli. How should this be interpreted?

Significant, as bladder urine should be sterile

Repeat as a clean-catch sample before reporting

Not significant, as it is below 100,000 CFU/mL

Contamination by perineal flora is likely

Sterilisation, disinfection and specimen processing

40 Lab technician recruitment practice questions on Sterilisation, disinfection and specimen processing, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Low-temperature plasma sterilisers for heat-sensitive instruments generate the plasma from:
1. AHydrogen peroxide vapour
2. BEthylene oxide gas
3. CFormaldehyde vapour
4. DCarbon dioxide gas
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Answer: A · Hydrogen peroxide vapour
In a gas plasma steriliser, hydrogen peroxide vapour is injected into an evacuated chamber and excited by radio-frequency energy into a plasma of free radicals. The radicals kill microbes, including spores, at about 45-55 °C. The by-products are water and oxygen, so no aeration is needed, unlike ethylene oxide. Cellulose materials such as linen and paper absorb the peroxide and cannot be processed.
- B: Ethylene oxide is used in separate gas sterilisers that need long aeration.
- C: Formaldehyde is used in low-temperature steam-formaldehyde sterilisers.
- D: Carbon dioxide is only a diluent for ethylene oxide, not a sterilant.
Question 2 · Hard
Cultures from patients' bronchial washings grow the same Pseudomonas aeruginosa. The bronchoscope is soaked in 2% glutaraldehyde for 20 minutes after each use, but the channels are not brushed first. What is the most likely cause?
1. AGlutaraldehyde does not kill Pseudomonas at all
2. BThe disinfectant should have been 70% ethanol
3. CTwenty minutes is too long and corrodes the scope
4. DOrganic debris in the channels shielded the bacteria
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Answer: D · Organic debris in the channels shielded the bacteria
Cleaning must come before disinfection: mucus, blood and biofilm left in the channels block the disinfectant from reaching the organisms. Glutaraldehyde at 2% for 20 minutes is a standard high-level disinfection, and it kills Pseudomonas when it can reach it. Scope-borne outbreaks typically arise from skipped brushing or poor channel flushing. Manual cleaning, brushing and enzymatic detergent come first, then disinfection, rinsing and drying.
- A: Glutaraldehyde kills Pseudomonas readily once it can reach the organisms.
- B: 70% ethanol is not a high-level disinfectant for endoscopes.
- C: Twenty minutes is the usual contact time and does not harm the scope.
Question 3 · Medium
Beta-propiolactone is used in vaccine production mainly to:
1. ASterilise the glass vials before filling
2. BInactivate viruses such as rabies virus
3. CPreserve opened multi-dose vials
4. DAdsorb antigen onto alum adjuvant
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Answer: B · Inactivate viruses such as rabies virus
Beta-propiolactone is an alkylating agent that inactivates viruses for killed vaccines, notably cell-culture rabies vaccines. It is sporicidal and acts faster than formaldehyde. It breaks down by hydrolysis into non-toxic products, so no harmful residue stays in the vaccine. It is too carcinogenic for routine surface use.
- A: Vials are sterilised by dry heat, not by beta-propiolactone.
- C: Opened vials are protected by preservatives such as thiomersal or phenol.
- D: Adsorption on alum is a separate step that uses aluminium salts.
Question 4 · Easy
Savlon, a common hospital antiseptic, is a mixture of chlorhexidine and:
1. ACetrimide
2. BPovidone-iodine
3. CChloroxylenol
4. DHexachlorophene
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Answer: A · Cetrimide
Savlon combines chlorhexidine gluconate with cetrimide, a quaternary ammonium compound. Both act on bacterial cell membranes and are active mainly against Gram-positive organisms. The mixture is used for wound cleaning and skin and surface antisepsis. Like other quaternary ammonium preparations, it is inactivated by soap and organic matter.
- B: Povidone-iodine is an iodophor sold on its own, as in Betadine.
- C: Chloroxylenol is the active ingredient of Dettol, a different product.
- D: Hexachlorophene is a phenolic used in some skin washes, not in Savlon.
Question 5 · Medium
A suprapubic bladder aspirate from an infant grows 2,000 CFU/mL of Escherichia coli. How should this be interpreted?
1. ARepeat as a clean-catch sample before reporting
2. BNot significant, as it is below 100,000 CFU/mL
3. CContamination by perineal flora is likely
4. DSignificant, as bladder urine should be sterile
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Answer: D · Significant, as bladder urine should be sterile
Suprapubic aspiration takes urine directly from the bladder, bypassing the urethra and perineum, so normal bladder urine is sterile. Any growth of a urinary pathogen in such a sample is significant. The 100,000 CFU/mL (Kass) level applies to voided midstream samples, which pick up urethral flora. Suprapubic aspiration is used mainly in infants when a clean sample is otherwise hard to obtain.
- A: A clean-catch repeat is less reliable than the aspirate already obtained.
- B: The 10^5 CFU/mL cut-off is for voided midstream urine, not bladder aspirates.
- C: A suprapubic needle bypasses the perineum, so contamination is unlikely.
Question 6 · Medium
Alkaline (2%) glutaraldehyde is 'activated' before use by adding a buffer that raises its pH. This change:
1. AStops it fixing proteins on instruments
2. BMakes it safe to use as a skin antiseptic
3. CTurns it into a low-level disinfectant
4. DIncreases its activity but shortens its usable life
Answer and explanation in LearnFree account
Question 7 · Easy
Seitz filters, once widely used to sterilise serum and other heat-labile fluids, are made of:
1. AAsbestos pads
2. BUnglazed porcelain
3. CDiatomaceous earth
4. DSintered glass
Answer and explanation in LearnFree account
Question 8 · Medium
Blood culture bottles collected at night are kept in the refrigerator and loaded into the automated system next morning. The main risk is that:
1. AThe instrument will read every bottle as positive
2. BThe anticoagulant will precipitate and clot the blood
3. CGrowth may be delayed or missed, giving false negatives
4. DSkin contaminants will multiply and overgrow pathogens
Answer and explanation in LearnFree account
Question 9 · Medium
A laboratory hot-air oven is fitted with an electric fan mainly to:
1. ACool the load quickly at the end of the cycle
2. BKeep the temperature even throughout the load
3. CPush out air the way an autoclave removes it
4. DRemove moisture so glassware does not crack
Answer and explanation in LearnFree account
Question 10 · Easy
Before serum is used in complement-dependent tests, it is 'inactivated' by heating at:
1. A60 °C for 1 hour
2. B100 °C for 20 minutes
3. C56 °C for 30 minutes
4. D63 °C for 30 minutes
Answer and explanation in LearnFree account
Question 11 · Hard
A boric acid urine container is meant to be filled to the 20 mL mark, but only 3 mL is collected. The patient has dysuria and many pus cells, yet the culture grows nothing. What is the most likely cause?
1. AThe patient has no urinary infection
2. BThe sample was too dilute to grow
3. CBoric acid only works after refrigeration
4. DExcess boric acid inhibited the bacteria
Answer and explanation in LearnFree account
Question 12 · Medium
Castaneda's biphasic medium, with an agar slope and broth in the same bottle, is used for blood culture in suspected:
1. ABrucellosis
2. BTyphoid fever
3. CLeptospirosis
4. DInfective endocarditis
Answer and explanation in LearnFree account
Question 13 · Medium
Which of these is the most resistant to standard steam sterilisation?
1. AMycobacteria
2. BNon-enveloped viruses
3. CPrions
4. DBacterial spores
Answer and explanation in LearnFree account
Question 14 · Hard
Only 0.5 mL of CSF is obtained from a febrile infant with a bulging fontanelle. Requests are for culture, Gram stain, cell count, protein and glucose. Which should take the fluid first?
1. ACryptococcal antigen test
2. BTotal and differential count
3. CGram stain and culture
4. DProtein and glucose
Answer and explanation in LearnFree account
Question 15 · Easy
Which heat method works at a temperature below 100 °C?
1. APasteurisation
2. BAutoclaving
3. CTyndallisation
4. DHot-air oven
Answer and explanation in LearnFree account
Question 16 · Medium
Working dilutions of hypochlorite for discard jars are made fresh each day because the solution:
1. ABecomes contaminated with spore-formers
2. BForms a precipitate that blocks the sink
3. CTurns acidic and corrodes plastic jars
4. DLoses available chlorine on standing
Answer and explanation in LearnFree account
Question 17 · Medium
Hypochlorite and bleaching powder kill microorganisms mainly through the formation of:
1. AHypochlorous acid
2. BHydrochloric acid
3. CSodium hydroxide
4. DFree chloride ions
Answer and explanation in LearnFree account
Question 18 · Easy
Before giving a sputum sample for culture, the patient should be asked to:
1. ADrink a glass of saline solution
2. BRinse the mouth with plain water
3. CGargle with an antiseptic mouthwash
4. DBrush the teeth with toothpaste
Answer and explanation in LearnFree account
Question 19 · Hard
Discard jars need 5 litres of 2% cresol. The stock Lysol contains 50% cresol. How is it prepared?
1. A200 mL Lysol made up to 5 L
2. B100 mL Lysol made up to 5 L
3. C125 mL Lysol made up to 5 L
4. D500 mL Lysol made up to 5 L
Answer and explanation in LearnFree account
Question 20 · Medium
In the usual three-tube collection of CSF, the second tube is normally sent for:
1. ACell count
2. BChemistry and serology
3. CMicrobiology
4. DCytology
Answer and explanation in LearnFree account
Question 21 · Easy
Bacterial vaccines made from non-sporing organisms are heat-killed in a vaccine bath at:
1. A72 °C for 20 seconds
2. B121 °C for 15 minutes
3. C56 °C for 30 minutes
4. D60 °C for 1 hour
Answer and explanation in LearnFree account
Question 22 · Hard
A 30-year-old man has fever for 5 days and suspected typhoid. One adult blood culture bottle received about 2 mL of blood and shows no growth after 5 days. Which is the most likely reason?
1. AThe bottle should have been held for 10 days
2. BBlood was drawn too early in the illness
3. CThe volume of blood cultured was too small
4. DSalmonella cannot grow in blood culture broth
Answer and explanation in LearnFree account
Question 23 · Medium
A patient has fever with chills every afternoon. When are blood cultures best drawn to catch the bacteraemia?
1. AAt the height of the fever spike
2. BAfter the fever has settled each evening
3. CTwo hours after the antipyretic dose
4. DAt the start of a chill, before the fever peaks
Answer and explanation in LearnFree account
Question 24 · Medium
A urine container for culture arrives with a completed request form, but the container itself carries no label. What should the laboratory do?
1. ARefrigerate it until the ward confirms the patient
2. BReject it and request a fresh, labelled sample
3. CLabel it from the request form and process it
4. DProcess it and add a remark to the report
Answer and explanation in LearnFree account
Question 25 · Medium
A phlebotomist uses butterfly tubing to collect a pair of blood culture bottles. In what order should they be filled?
1. AAnaerobic, to keep oxygen out of it
2. BAerobic, as the air in the tubing enters it
3. CAnaerobic, since it needs more blood
4. DAerobic, since it holds more broth
Answer and explanation in LearnFree account

15 more Sterilisation, disinfection and specimen processing questions

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Sterilisation, disinfection and specimen processing

Low-temperature plasma sterilisers for heat-sensitive instruments generate the plasma from:

Cultures from patients' bronchial washings grow the same Pseudomonas aeruginosa. The bronchoscope is soaked in 2% glutaraldehyde for 20 minutes after each use, but the channels are not brushed first. What is the most likely cause?

Beta-propiolactone is used in vaccine production mainly to:

Savlon, a common hospital antiseptic, is a mixture of chlorhexidine and:

A suprapubic bladder aspirate from an infant grows 2,000 CFU/mL of Escherichia coli. How should this be interpreted?

Alkaline (2%) glutaraldehyde is 'activated' before use by adding a buffer that raises its pH. This change:

Seitz filters, once widely used to sterilise serum and other heat-labile fluids, are made of:

Blood culture bottles collected at night are kept in the refrigerator and loaded into the automated system next morning. The main risk is that:

A laboratory hot-air oven is fitted with an electric fan mainly to:

Before serum is used in complement-dependent tests, it is 'inactivated' by heating at:

A boric acid urine container is meant to be filled to the 20 mL mark, but only 3 mL is collected. The patient has dysuria and many pus cells, yet the culture grows nothing. What is the most likely cause?

Castaneda's biphasic medium, with an agar slope and broth in the same bottle, is used for blood culture in suspected:

Which of these is the most resistant to standard steam sterilisation?

Only 0.5 mL of CSF is obtained from a febrile infant with a bulging fontanelle. Requests are for culture, Gram stain, cell count, protein and glucose. Which should take the fluid first?

Which heat method works at a temperature below 100 °C?

Working dilutions of hypochlorite for discard jars are made fresh each day because the solution:

Hypochlorite and bleaching powder kill microorganisms mainly through the formation of:

Before giving a sputum sample for culture, the patient should be asked to:

Discard jars need 5 litres of 2% cresol. The stock Lysol contains 50% cresol. How is it prepared?

In the usual three-tube collection of CSF, the second tube is normally sent for:

Bacterial vaccines made from non-sporing organisms are heat-killed in a vaccine bath at:

A 30-year-old man has fever for 5 days and suspected typhoid. One adult blood culture bottle received about 2 mL of blood and shows no growth after 5 days. Which is the most likely reason?

A patient has fever with chills every afternoon. When are blood cultures best drawn to catch the bacteraemia?

A urine container for culture arrives with a completed request form, but the container itself carries no label. What should the laboratory do?

A phlebotomist uses butterfly tubing to collect a pair of blood culture bottles. In what order should they be filled?