History, sterilization, disinfection and biosafety

56 Kerala MSc MLT Entrance practice questions on History, sterilization, disinfection and biosafety, 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.

1. Question 1 · Medium

   Under the Bio-Medical Waste Management Rules 2016, the longest that untreated bio-medical waste may be stored before treatment or disposal is:

   1. A7 days
   2. B72 hours
   3. C24 hours
   4. D48 hours

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   Answer: D · 48 hours

   The 2016 Rules say untreated bio-medical waste must not be stored beyond 48 hours. If it cannot be treated in time, the occupier must take steps to prevent harm to health. Laboratory cultures and stocks are also pre-treated on site by autoclaving or microwaving before they leave the laboratory. Waste is segregated at source into yellow, red, white and blue categories.

   • A: 7 days is far beyond the permitted storage time.
   • B: 72 hours exceeds the 48-hour limit.
   • C: 24 hours is shorter than the limit set in the Rules.

2. Question 2 · Easy

   Both the steam autoclave and the unglazed porcelain bacterial filter were introduced by:

   1. AJohn Tyndall
   2. BLouis Pasteur
   3. CJoseph Lister
   4. DCharles Chamberland

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   Answer: D · Charles Chamberland

   Charles Chamberland, who worked with Pasteur, built the first steam pressure sterilizer in 1879. He also designed the Chamberland candle, an unglazed porcelain filter that holds back bacteria. Filtrates that stayed infective after passing such filters led to the discovery of viruses. John Tyndall gave his name to intermittent steaming (tyndallisation).

   • A: Tyndall is known for intermittent steaming, not the autoclave.
   • B: Pasteur directed the laboratory, but these devices were Chamberland's work.
   • C: Lister introduced carbolic acid antisepsis in surgery.

3. Question 3 · Hard

   In a Rideal-Walker test, phenol 1:95 shows growth in the 2.5- and 5-minute subcultures but not at 7.5 or 10 minutes. The test disinfectant gives: 1:400 growth at 2.5 min only; 1:500 growth at 2.5 and 5 min; 1:600 growth at 2.5, 5 and 7.5 min. Using the dilutions that kill in 7.5 but not 5 minutes, the coefficient is:

   1. A4.2
   2. B6.3
   3. C0.19
   4. D5.3

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   Answer: D · 5.3

   The end point is the dilution that sterilizes the suspension at 7.5 minutes but not at 5 minutes. For phenol this is 1:95, and for the test disinfectant it is 1:500. The coefficient is 500 / 95 = 5.3, so the disinfectant is about five times as active as phenol in water. The test uses Salmonella Typhi and no organic matter, so it overrates many disinfectants.

   • A: 4.2 uses the 1:400 dilution, which killed by 5 minutes.
   • B: 6.3 uses the 1:600 dilution, which needed 10 minutes.
   • C: 0.19 inverts the ratio.

4. Question 4 · Medium

   In the Spaulding scheme, a disinfectant that kills vegetative bacteria, mycobacteria, most viruses and fungi, but not reliably bacterial spores, is classed as:

   1. ALow-level
   2. BA chemical sterilant
   3. CIntermediate-level
   4. DHigh-level

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   Answer: C · Intermediate-level

   Spaulding grades disinfectants by the hardest organisms they kill. Intermediate-level agents, such as alcohols, phenolics and some chlorine solutions, kill mycobacteria but not spores reliably. High-level agents kill everything except large numbers of spores, and sterilants kill spores with long contact. Killing of M. tuberculosis is the marker that separates intermediate from low level.

   • A: Low-level disinfectants do not kill mycobacteria.
   • B: A sterilant destroys all microbes, including spores.
   • D: High-level disinfectants also kill most spores with adequate contact time.

5. Question 5 · Medium

   The Chick-Martin test was devised to improve on the Rideal-Walker test mainly because it:

   1. ATests the disinfectant with organic matter present
   2. BMeasures killing of spores instead of vegetative cells
   3. CCompares the disinfectant with phenol at timed intervals
   4. DSamples diluted disinfectant actually in use on the ward

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   Answer: A · Tests the disinfectant with organic matter present

   The Rideal-Walker test is done in water, so it overrates disinfectants such as phenolics that are inactivated by organic matter. The Chick-Martin test adds dried yeast or faeces to the test mixture, which is closer to real conditions. Both compare the disinfectant with phenol against a test organism. The in-use test of Kelsey and Maurer is the one that samples solutions from the ward.

   • B: Both tests use vegetative bacteria, not spores.
   • C: Timed comparison with phenol is the Rideal-Walker method itself.
   • D: Sampling in-use solutions is the Kelsey-Maurer in-use test.

6. Question 6 · Easy

   Spores of which organism are used as the biological indicator for sterilization by ionizing (gamma) radiation?

   1. ABacillus pumilus
   2. BGeobacillus stearothermophilus
   3. CBacillus atrophaeus
   4. DClostridium sporogenes

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7. Question 7 · Hard

   During an autoclave cycle, the pressure gauge reads 15 lb/in² (103 kPa) above atmospheric, but the chamber thermometer reads only 115 °C. The most likely explanation is that:

   1. AThe boiler was filled with too much water
   2. BAir was not fully expelled before the vent was closed
   3. CThe gauge is reading lower than the true pressure
   4. DThe chamber contains superheated, dry steam

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8. Question 8 · Hard

   A hot air oven is loaded with wrapped glass pipettes, set to 160 °C and timed for 2 hours from switch-on. The chart shows the load reached 160 °C after 80 minutes. A Bacillus atrophaeus spore strip from the load grows. The most likely cause is that:

   1. AThe holding time began before the load was at 160 °C
   2. BBacillus atrophaeus is the wrong indicator for dry heat
   3. CGlass pipettes cannot be sterilized by dry heat
   4. DThe oven should have been opened at once to cool

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9. Question 9 · Hard

   A load carries 10^6 spores, and the spores' D value at 121 °C is 1.5 minutes. How long must the load be held at 121 °C to reach a sterility assurance level of 10^-6?

   1. A12 minutes
   2. B18 minutes
   3. C8 minutes
   4. D9 minutes

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10. Question 10 · Medium

    Milk pasteurised by the holder method (63 °C for 30 minutes) is checked for adequate heating by:

    1. AA colour change in a Browne's tube
    2. BA negative alkaline phosphatase test
    3. CKilling of a Geobacillus spore strip
    4. DNo bacterial growth on culture of the milk

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11. Question 11 · Medium

    After ethylene oxide sterilization, plastic devices are held in an aerator before use. Why?

    1. AThe gas must be activated by warm air to work
    2. BThe spores need extra time to die after exposure
    3. CToxic residual gas must diffuse out of the plastic
    4. DMoisture added to the chamber must be dried off

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12. Question 12 · Medium

    Why is a horizontal laminar-flow clean bench unsuitable for opening cultures of Mycobacterium tuberculosis?

    1. AIt lets unfiltered room air reach the culture
    2. BIts HEPA filter cannot trap mycobacteria
    3. CIt blows filtered air out towards the worker
    4. DIts airflow is too weak to carry aerosols

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13. Question 13 · Easy

    Edward Jenner's 1796 method of protecting against smallpox used material taken from:

    1. AAttenuated cultures grown in the laboratory
    2. BDried spinal cord of infected rabbits
    3. CCowpox lesions on a milkmaid's hand
    4. DDried scabs of a smallpox patient

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14. Question 14 · Medium

    Which bacterium, chosen for its very small cell size, is the challenge organism used to validate 0.22 µm sterilizing membrane filters?

    1. ABrevundimonas diminuta
    2. BBacillus atrophaeus
    3. CBacillus pumilus
    4. DGeobacillus stearothermophilus

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15. Question 15 · Medium

    When Christensen's urea agar is prepared, the urea solution is:

    1. AAutoclaved with the base at 121 °C for 15 minutes
    2. BSteamed with the base on three days
    3. CFilter-sterilized and added to the cooled, autoclaved base
    4. DHeated in a hot air oven before mixing

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16. Question 16 · Hard

    A heat-sensitive endoscope camera is wrapped in cotton linen and loaded into a hydrogen peroxide gas plasma sterilizer. The cycle aborts repeatedly, though other loads run normally. The most likely cause is:

    1. AThe camera's plastic melts at the cycle temperature
    2. BThe load needs a longer aeration phase afterwards
    3. CThe cotton linen absorbs the hydrogen peroxide
    4. DThe peroxide cassette is past its expiry date

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17. Question 17 · Medium

    After sterilizing glassware in a hot air oven, the door is not opened until the temperature falls below about 60 °C, because:

    1. AThe spores need extra time to die
    2. BThe thermostat resets only when cool
    3. CSudden cooling may crack the glassware
    4. DOpening early lets steam condense inside

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18. Question 18 · Medium

    Dry heat kills microorganisms mainly by:

    1. ACoagulation of proteins by condensing steam
    2. BFormation of thymine dimers in DNA
    3. COxidative damage to cell constituents
    4. DAlkylation of amino and sulphydryl groups

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19. Question 19 · Easy

    Salvarsan, the arsenical drug for syphilis that its discoverer called a 'magic bullet', was developed by:

    1. ASelman Waksman
    2. BAlexander Fleming
    3. CGerhard Domagk
    4. DPaul Ehrlich

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20. Question 20 · Medium

    Spores of Bacillus atrophaeus (formerly B. subtilis var. niger) are the biological indicator for which pair of sterilizing methods?

    1. AMoist heat and hydrogen peroxide plasma
    2. BDry heat and ethylene oxide
    3. CMoist heat and gamma radiation
    4. DGamma radiation and filtration

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21. Question 21 · Easy

    Under the Bio-Medical Waste Management Rules 2016, human anatomical waste such as a placenta or tissue biopsy is disposed of by:

    1. ALandfill without treatment
    2. BAutoclaving and shredding
    3. CIncineration
    4. DChemical disinfection and drain disposal

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22. Question 22 · Easy

    Live Nipah virus, the cause of repeated outbreaks in Kerala, may be cultured only at which biosafety level?

    1. ABSL-4
    2. BBSL-1
    3. CBSL-2
    4. DBSL-3

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23. Question 23 · Easy

    Hand washing in chlorinated lime solution to cut deaths from puerperal fever in a maternity ward was introduced by:

    1. AJoseph Lister
    2. BJohn Snow
    3. CIgnaz Semmelweis
    4. DLouis Pasteur

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24. Question 24 · Hard

    A spore suspension of 10^6 spores/mL is heated at 121 °C. After 10 minutes, a viable count shows 10^2 spores/mL. What is the D value at 121 °C?

    1. A1.7 minutes
    2. B4 minutes
    3. C2.5 minutes
    4. D0.4 minutes

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25. Question 25 · Easy

    Patient serum for the VDRL test is heated at 56 °C for 30 minutes before testing in order to:

    1. AClear a lipaemic sample
    2. BSterilize the serum
    3. CDestroy rheumatoid factor
    4. DInactivate complement

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