Gamma radiation is used to sterilize packed disposable plastic syringes because it:

Penetrates the packing without heating it

Acts only on the surface of the items

Works best on wet, unpacked items

Leaves a protective chemical residue

A disinfectant has a Rideal-Walker coefficient of 4. This means it:

Is four times as active as phenol

Is four times as toxic as phenol

Works only at a 1 in 4 dilution

General microbiology: sterilization, disinfection and biomedical waste

54 Kerala PSC Lab Technician Gr II practice questions on General microbiology: sterilization, disinfection and biomedical waste, 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 · Easy
All of the following can be sterilized in a hot air oven EXCEPT:
1. AGlass Petri dishes
2. BRubber tubing
3. CLiquid paraffin
4. DDusting powder
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Answer: B · Rubber tubing
Dry heat in a hot air oven suits materials that steam cannot penetrate or that moisture would spoil, such as glassware, oils and powders. Holding periods such as 160 °C for 1 to 2 hours are used, longer than for moist heat. Rubber perishes and plastics melt at these temperatures, so they are autoclaved or sterilized by other means. The oven should not be overloaded, so that hot air can circulate.
- A: Glass withstands dry heat and comes out dry and ready to use.
- C: Oils repel steam, so dry heat is the method of choice for liquid paraffin.
- D: Steam cannot penetrate powders, so they are sterilized by dry heat.
Question 2 · Easy
Saturated steam at 15 lb/in² above atmospheric pressure has a temperature of about:
1. A134 °C
2. B100 °C
3. C115 °C
4. D121 °C
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Answer: D · 121 °C
Raising the pressure raises the boiling point of water, so steam at 15 lb/in² (about 103 kPa) above atmospheric pressure reaches about 121 °C. Held for 15 minutes, this kills vegetative organisms and bacterial spores. Moist heat at this temperature coagulates microbial proteins far faster than dry heat at the same temperature.
- A: 134 °C needs about 30 lb/in², as in high-speed pre-vacuum autoclaves.
- B: 100 °C is the boiling point of water at normal atmospheric pressure.
- C: 115 °C needs only about 10 lb/in² and is used for some sugar-containing media.
Question 3 · Medium
Gamma radiation is used to sterilize packed disposable plastic syringes because it:
1. AActs only on the surface of the items
2. BPenetrates the packing without heating it
3. CWorks best on wet, unpacked items
4. DLeaves a protective chemical residue
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Answer: B · Penetrates the packing without heating it
Ionizing radiation from a cobalt-60 source passes through packaging and plastic and damages microbial DNA. The temperature hardly rises, so it is called cold sterilization and suits heat-sensitive items such as syringes, catheters and gloves. Items are irradiated already sealed in their final packs. Ultraviolet light, by contrast, penetrates poorly and acts only on exposed surfaces.
- A: Surface-only action describes ultraviolet light, not gamma rays.
- C: Items are irradiated dry and sealed in their final packs.
- D: Gamma rays leave no residue; residue is a concern with ethylene oxide.
Question 4 · Medium
Which of these may still be present in serum after filtration through a 0.22 µm membrane filter?
1. AViruses
2. BStaphylococci
3. CBacterial spores
4. DEscherichia coli
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Answer: A · Viruses
A 0.22 µm membrane retains bacteria, their spores and fungi, so it is used to sterilize heat-labile fluids such as serum, antibiotic solutions and sugar solutions. Most viruses are 20-300 nm across and pass through the pores. Mycoplasmas can also pass occasionally because they lack a rigid cell wall. The filter's integrity can be checked with a bubble-point test before use.
- B: Staphylococci are about 1 µm across and are retained.
- C: Bacterial spores are about 1 µm across and are retained.
- D: E. coli rods are far larger than 0.22 µm pores and are retained.
Question 5 · Medium
A disinfectant has a Rideal-Walker coefficient of 4. This means it:
1. AIs four times as active as phenol
2. BKills Salmonella in 4 minutes
3. CIs four times as toxic as phenol
4. DWorks only at a 1 in 4 dilution
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Answer: A · Is four times as active as phenol
In the Rideal-Walker test, dilutions of the disinfectant and of phenol act on Salmonella Typhi, and samples are subcultured at fixed intervals. The coefficient is the dilution of the disinfectant that kills in a given time divided by the dilution of phenol that does the same. A coefficient of 4 means the disinfectant can be diluted four times more than phenol and still kill. The test has no organic matter, which the Chick-Martin test adds to mimic real use.
- B: The coefficient is a ratio of dilutions, not a killing time.
- C: The test measures killing power, not toxicity to tissues.
- D: The coefficient compares activity with phenol; it is not a working dilution.
Question 6 · Hard
After an autoclave run of discarded cultures, the indicator tape on the bags has changed colour, but the Geobacillus stearothermophilus spore strip from the centre of the load shows growth. The most likely cause is:
1. ATape stuck on the outside of the bags
2. BHolding time set longer than 15 minutes
3. CStrip incubated at 37 °C, not 56 °C
4. DAir trapped in a tightly packed load
Answer and explanation in LearnFree account
Question 7 · Hard
To prepare 1 litre of 1% available chlorine for cleaning blood spills, how much bleaching powder containing 25% available chlorine is needed?
1. A10 g
2. B2.5 g
3. C40 g
4. D4 g
Answer and explanation in LearnFree account
Question 8 · Medium
What does the daily Bowie-Dick test on a pre-vacuum autoclave check?
1. AAccuracy of the pressure gauge
2. BKilling of Geobacillus spores
3. CDryness of the load at the end
4. DAir removal and steam penetration
Answer and explanation in LearnFree account
Question 9 · Easy
Tyndallisation steams a medium at 100 °C for 20 minutes:
1. AOnce, followed by quick cooling
2. BOn three successive days
3. CTwice on the same day
4. DDaily for one full week
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Question 10 · Easy
Which method is used to sterilize Lowenstein-Jensen medium?
1. AAutoclaving
2. BTyndallisation
3. CInspissation
4. DMembrane filtration
Answer and explanation in LearnFree account
Question 11 · Easy
Under the Bio-Medical Waste Management Rules 2016, used needles and scalpel blades are discarded in a:
1. ACardboard box with blue marking
2. BRed non-chlorinated plastic bag
3. CYellow non-chlorinated plastic bag
4. DWhite translucent puncture-proof container
Answer and explanation in LearnFree account
Question 12 · Hard
A laboratory is sorting its waste under the Bio-Medical Waste Management Rules 2016. Which item is matched with the correct container?
1. ABroken contaminated slides – blue box
2. BNeedles from a needle cutter – red bag
3. CUsed examination gloves – blue box
4. DPlastic culture plates – white container
Answer and explanation in LearnFree account
Question 13 · Medium
Ultraviolet light can disinfect the work surface of a safety cabinet but cannot sterilise wrapped items, because it:
1. AHeats the items
2. BKills only vegetative cells
3. CPenetrates poorly
4. DLeaves toxic residues
Answer and explanation in LearnFree account
Question 14 · Easy
In the holder method of pasteurising milk, the milk is heated at:
1. A100 °C for 20 minutes
2. B121 °C for 15 minutes
3. C63 °C for 30 minutes
4. D72 °C for 15-20 seconds
Answer and explanation in LearnFree account
Question 15 · Easy
What material are Chamberland candle filters made of?
1. ADiatomaceous earth
2. BUnglazed porcelain
3. CSintered glass
4. DAsbestos pads
Answer and explanation in LearnFree account
Question 16 · Medium
Moist heat sterilises at a lower temperature than dry heat mainly because steam:
1. ARemoves all water from the bacterial cell
2. BReaches a higher temperature than hot air
3. CKills bacteria by oxidising their contents
4. DGives up latent heat and coagulates proteins
Answer and explanation in LearnFree account
Question 17 · Easy
Which gas is used to sterilise heat-sensitive items such as heart-lung machines and plastic catheters?
1. AFormaldehyde
2. BNitrous oxide
3. CCarbon dioxide
4. DEthylene oxide
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Question 18 · Hard
A technician autoclaves 500 mL flasks of broth at 121 °C for 15 minutes. The caps are loosened, the load is not crowded, and the air was let out before the valve was closed. To save time, she opens the release valve as soon as the timer ends, while the gauge still reads 15 lb/in². Several flasks boil over. The most likely cause is:
1. ASudden pressure drop over superheated liquid
2. BAir left in the chamber during the cycle
3. CCaps too loose on the flasks
4. DHolding time too long for the volume
Answer and explanation in LearnFree account
Question 19 · Medium
Under the BMW Rules 2016, discarded culture plates and stock cultures from a microbiology laboratory are:
1. APre-treated on site, then sent in yellow bags
2. BSent untreated in yellow bags
3. CPre-treated on site, then sent in red bags
4. DSent untreated in red bags for recycling
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Question 20 · Easy
Contaminated recyclable plastics such as gloves, IV sets and urine bags are collected, under the BMW Rules 2016, in a:
1. AWhite puncture-proof container
2. BRed bag
3. CBlue cardboard box
4. DYellow bag
Answer and explanation in LearnFree account
Question 21 · Medium
Spore strips of which organism are used to monitor ethylene oxide sterilisation?
1. AClostridium tetani
2. BGeobacillus stearothermophilus
3. CBacillus pumilus
4. DBacillus atrophaeus
Answer and explanation in LearnFree account
Question 22 · Medium
Which test checks whether a disinfectant already in use on a ward is still killing bacteria, by culturing a sample of it?
1. AKelsey-Sykes test
2. BIn-use test
3. CChick-Martin test
4. DRideal-Walker test
Answer and explanation in LearnFree account
Question 23 · Hard
A phlebotomy area produces blood-soaked cotton swabs, used plastic syringes whose needles were cut off with a needle cutter, and the cut needles. Under the BMW Rules 2016, which segregation is correct?
1. ACotton yellow; syringes yellow; needles white
2. BCotton red; syringes red; needles blue
3. CCotton yellow; syringes red; needles blue
4. DCotton yellow; syringes red; needles white
Answer and explanation in LearnFree account
Question 24 · Medium
Cetrimide, a quaternary ammonium disinfectant, loses much of its activity when mixed with:
1. ASoap
2. BEthanol
3. CDistilled water
4. DChlorhexidine
Answer and explanation in LearnFree account
Question 25 · Medium
A large blood spill is flooded with 1% hypochlorite without first soaking up the blood, and later cultures from the floor still grow bacteria. The likely reason is that hypochlorite:
1. AIs inactivated by organic matter
2. BNeeds heating before it acts
3. CDoes not act on Gram-negative bacteria
4. DIs neutralised by floor soap residues
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29 more General microbiology: sterilization, disinfection and biomedical waste questions

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General microbiology: sterilization, disinfection and biomedical waste

All of the following can be sterilized in a hot air oven EXCEPT:

Saturated steam at 15 lb/in² above atmospheric pressure has a temperature of about:

Gamma radiation is used to sterilize packed disposable plastic syringes because it:

Which of these may still be present in serum after filtration through a 0.22 µm membrane filter?

A disinfectant has a Rideal-Walker coefficient of 4. This means it:

After an autoclave run of discarded cultures, the indicator tape on the bags has changed colour, but the Geobacillus stearothermophilus spore strip from the centre of the load shows growth. The most likely cause is:

To prepare 1 litre of 1% available chlorine for cleaning blood spills, how much bleaching powder containing 25% available chlorine is needed?

What does the daily Bowie-Dick test on a pre-vacuum autoclave check?

Tyndallisation steams a medium at 100 °C for 20 minutes:

Which method is used to sterilize Lowenstein-Jensen medium?

Under the Bio-Medical Waste Management Rules 2016, used needles and scalpel blades are discarded in a:

A laboratory is sorting its waste under the Bio-Medical Waste Management Rules 2016. Which item is matched with the correct container?

Ultraviolet light can disinfect the work surface of a safety cabinet but cannot sterilise wrapped items, because it:

In the holder method of pasteurising milk, the milk is heated at:

What material are Chamberland candle filters made of?

Moist heat sterilises at a lower temperature than dry heat mainly because steam:

Which gas is used to sterilise heat-sensitive items such as heart-lung machines and plastic catheters?

Under the BMW Rules 2016, discarded culture plates and stock cultures from a microbiology laboratory are:

Contaminated recyclable plastics such as gloves, IV sets and urine bags are collected, under the BMW Rules 2016, in a:

Spore strips of which organism are used to monitor ethylene oxide sterilisation?

Which test checks whether a disinfectant already in use on a ward is still killing bacteria, by culturing a sample of it?

A phlebotomy area produces blood-soaked cotton swabs, used plastic syringes whose needles were cut off with a needle cutter, and the cut needles. Under the BMW Rules 2016, which segregation is correct?

Cetrimide, a quaternary ammonium disinfectant, loses much of its activity when mixed with:

A large blood spill is flooded with 1% hypochlorite without first soaking up the blood, and later cultures from the floor still grow bacteria. The likely reason is that hypochlorite: