The weekly autoclave check gives these results: chamber record 121 °C for 30 minutes; tape on the bags changed color; a Geobacillus spore vial placed inside a tightly tied, packed waste bag shows growth; the unexposed control vial from the same lot also shows growth. The most likely cause is:

Steam could not penetrate the tightly closed bag

The chamber gauge reads lower than the true temperature

The spore vial was dead before it was autoclaved

The exposed vial was incubated at too low a temperature

When working in a Class II biological safety cabinet, materials should be placed:

At least 10 cm (4 in) inside the front grille

Anywhere, as long as the sash is fully raised

Over the rear grille, as far back as possible

On the front air grille, close to the sash

A technologist sprays 1:100 household bleach straight onto a large pool of spilled blood. Disinfection is likely to fail because:

Protein in the blood inactivates the available chlorine

Bleach works only when the surface is dry

Bleach must be mixed with alcohol to activate it

Hypochlorite does not act on blood-borne viruses

Sterilization, disinfection and microbiology biosafety

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Sterilization, disinfection and microbiology biosafety, part of Microbiology. The first 9 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 · Easy
Which organism's spores are used as the biological indicator for steam autoclaves?
1. ABacillus cereus
2. BGeobacillus stearothermophilus
3. CBacillus atrophaeus
4. DClostridium sporogenes
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Answer: B · Geobacillus stearothermophilus
Geobacillus stearothermophilus forms spores that are highly resistant to moist heat, so killing them shows that the cycle was lethal. After the run, the indicator is incubated at about 55-60 °C, the organism's growth temperature, and a color change means spores survived. An unexposed indicator from the same lot is incubated as a positive control. Bacillus atrophaeus is used instead for dry heat and ethylene oxide.
- A: Bacillus cereus is a food-poisoning organism, not a sterilization indicator.
- C: Bacillus atrophaeus spores monitor dry heat and ethylene oxide, not steam.
- D: Clostridium sporogenes is not the standard indicator for laboratory autoclaves.
Question 2 · Hard
The weekly autoclave check gives these results: chamber record 121 °C for 30 minutes; tape on the bags changed color; a Geobacillus spore vial placed inside a tightly tied, packed waste bag shows growth; the unexposed control vial from the same lot also shows growth. The most likely cause is:
1. AThe chamber gauge reads lower than the true temperature
2. BThe spore vial was dead before it was autoclaved
3. CThe exposed vial was incubated at too low a temperature
4. DSteam could not penetrate the tightly closed bag
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Answer: D · Steam could not penetrate the tightly closed bag
Steam kills spores only where it touches them, and air trapped in a sealed, overfilled bag insulates the center, which can stay far below 121 °C. The chamber record and the tape reflect conditions outside the bag, not in the middle of the load. Growth in the control vial confirms that the spores were viable and the incubation worked, so the failure is real. Bags should be loosely closed, loads should not be overpacked, and the run must be repeated with a new indicator before the waste is released.
- A: A gauge reading low would mean the chamber was hotter, which would kill more spores.
- B: Spores dead beforehand would show no growth, and the control vial grew.
- C: Too low an incubation temperature would hide surviving spores, not cause growth.
Question 3 · Easy
When working in a Class II biological safety cabinet, materials should be placed:
1. AAnywhere, as long as the sash is fully raised
2. BOver the rear grille, as far back as possible
3. COn the front air grille, close to the sash
4. DAt least 10 cm (4 in) inside the front grille
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Answer: D · At least 10 cm (4 in) inside the front grille
Room air is drawn in through the front grille to protect the worker, and HEPA-filtered air flows down over the work surface to protect the culture. Work done at least 10 cm (4 in) inside the front grille stays in the clean downflow and keeps the air curtain intact. Blocking the front or rear grilles disrupts the airflow and can let aerosols escape. Load only what the task needs and move arms slowly in and out of the cabinet.
- A: Raising the sash above its set height lowers the inflow speed and protection.
- B: Covering the rear grille disturbs the downflow and exhaust.
- C: Items on the front grille block the inflow that protects the worker.
Question 4 · Medium
A technologist sprays 1:100 household bleach straight onto a large pool of spilled blood. Disinfection is likely to fail because:
1. AProtein in the blood inactivates the available chlorine
2. BBleach works only when the surface is dry
3. CBleach must be mixed with alcohol to activate it
4. DHypochlorite does not act on blood-borne viruses
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Answer: A · Protein in the blood inactivates the available chlorine
Hypochlorite reacts with organic matter, so a large amount of blood uses up the free chlorine before microorganisms are killed. For spills with a heavy organic load, the bulk is first absorbed with paper towels, or a stronger 1:10 dilution of household bleach (about 5,000 ppm available chlorine) is used. The disinfectant is then left for the stated contact time before final cleaning. Dilute bleach should be made fresh, because its available chlorine falls during storage.
- B: Bleach acts on wet surfaces; it needs contact time, not drying.
- C: Mixing bleach with other chemicals is unsafe and is not needed.
- D: Hypochlorite kills hepatitis B virus and HIV; the problem is the organic load.
Question 5 · Easy
Which of these microbial forms is most resistant to chemical disinfectants?
1. ABacterial endospores
2. BEnveloped viruses
3. CMycobacteria
4. DNon-enveloped viruses
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Answer: A · Bacterial endospores
Resistance to germicides runs from prions (most resistant) through bacterial spores, mycobacteria, non-enveloped viruses, fungi and vegetative bacteria to enveloped viruses (least resistant). Spores have a thick coat, a dehydrated core and almost no metabolic activity. Only sterilizing methods, such as steam or long contact with glutaraldehyde, reliably kill them. This ranking decides which level of disinfectant a task needs.
- B: Enveloped viruses, such as HIV, are the easiest to kill.
- C: Mycobacteria are resistant, but less so than spores.
- D: Non-enveloped viruses are less resistant than mycobacteria and spores.
Question 6 · Medium
Autoclave tape on a bag of discarded cultures has changed color after a cycle. This shows that:
1. AThe outside of the bag was exposed to heat
2. BThe cycle held 121 °C for at least 15 minutes
3. CSpores at the center of the load were killed
4. DThe contents of the bag are now sterile
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Question 7 · Medium
Why should a Bunsen burner or other open flame not be used inside a Class II biological safety cabinet?
1. AThey disrupt the airflow and can damage the HEPA filter
2. BThey contaminate the work surface with soot particles
3. CThey interfere with the reading of the airflow gauge
4. DThey raise the cabinet temperature enough to kill cultures
Answer and explanation in LearnFree account
Question 8 · Easy
What is the usual minimum cycle for a small load of culture media in a gravity-displacement autoclave?
1. A121 °C at 103 kPa (15 psi) for 15 minutes
2. B160 °C of dry heat for 2 hours
3. C63 °C in a water bath for 30 minutes
4. D100 °C at normal pressure for 30 minutes
Answer and explanation in LearnFree account
Question 9 · Easy
Why is ethanol for bench disinfection used at about 70% rather than 95%?
1. AOnly 70% ethanol kills bacterial spores
2. BEthanol at 95% corrodes stainless steel surfaces
3. CWater is needed for it to denature microbial proteins
4. DEthanol at 70% is not flammable, unlike 95%
Answer and explanation in LearnFree account

Sterilization, disinfection and microbiology biosafety

Which organism's spores are used as the biological indicator for steam autoclaves?

The weekly autoclave check gives these results: chamber record 121 °C for 30 minutes; tape on the bags changed color; a Geobacillus spore vial placed inside a tightly tied, packed waste bag shows growth; the unexposed control vial from the same lot also shows growth. The most likely cause is:

When working in a Class II biological safety cabinet, materials should be placed:

A technologist sprays 1:100 household bleach straight onto a large pool of spilled blood. Disinfection is likely to fail because:

Which of these microbial forms is most resistant to chemical disinfectants?

Autoclave tape on a bag of discarded cultures has changed color after a cycle. This shows that:

Why should a Bunsen burner or other open flame not be used inside a Class II biological safety cabinet?

What is the usual minimum cycle for a small load of culture media in a gravity-displacement autoclave?

Why is ethanol for bench disinfection used at about 70% rather than 95%?