ASRB NET and ARS / SMS / STO (Agricultural Scientists Recruitment Board) · India · Agricultural Scientists Recruitment Board
Original practice questions written to the published syllabus · not past papersHow bacteria and fungi are built, grown, identified and killed, and how bacteria become resistant to antimicrobials.
In the exams: ICAR AICE-JRF core group 06; ICAR AIEEA-PG 13 (Microbiology); PRC Microbiology & Public Health 14%; China microbiology and immunology 35 (its largest single component); TNPSC Micro/VPH/Preventive Medicine 30
How our questions for ASRB NET and ARS / SMS / STO spread across the units of General Veterinary Microbiology. Difficulty: 16 easy, 42 medium, 28 hard.
| Mechanisms of antimicrobial resistance | 15 | |
| Bacterial genetics | 10 | |
| Sterilisation | 9 | |
| General mycology | 9 | |
| Bacterial structure | 8 | |
| Bacterial classification | 8 | |
| Bacterial growth | 8 | |
| Bacterial identification | 7 | |
| Disinfection | 7 | |
| Culture methods | 5 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Transfer of bacterial genes from one bacterium to another by a bacteriophage is called:
A. Transduction
In transduction a phage packages host DNA and injects it into the next host. Generalised transduction can move any gene; specialised transduction moves genes next to the prophage insertion site. Lysogeny is the state of a phage genome integrated as a prophage, not the transfer itself.
Further reading: Quinn et al., Veterinary Microbiology and Microbial Disease, 2nd ed.
The Luria-Delbrueck fluctuation test showed that phage-resistant mutants of E. coli:
D. Arise spontaneously and randomly before exposure, and the phage only selects them
Parallel cultures gave a wide variance in resistant counts ('jackpots'), which fits random mutation during growth before selection, not induction by the phage. The same logic explains why antimicrobial resistance mutants pre-exist in large populations.
Further reading: Madigan et al., Brock Biology of Microorganisms, 16th ed.
A load carries 10^6 spores of a test organism whose D-value at 121 degrees C is 1.5 minutes. How long must it be held at 121 degrees C to reach a sterility assurance level of 10^-6?
C. 18 minutes
Log reductions needed = log(10^6) - log(10^-6) = 6 - (-6) = 12. Time = 12 x D = 12 x 1.5 = 18 min. Stopping at 10^0 (6 logs x 1.5 = 9 min) is the common error: a SAL of 10^-6 means a one-in-a-million chance of one survivor, 6 logs beyond a single survivor.
Further reading: Rutala, Weber and HICPAC, CDC Guideline for Disinfection and Sterilization in Healthcare Facilities
Which statement about prions is correct?
C. They resist standard autoclave cycles and need extended or special inactivation
Prions (the agents of scrapie and BSE) are the most resistant infectious agents on the Spaulding-type resistance ladder. They survive standard 121 degrees C cycles, alcohols and many disinfectants; recommended procedures use prolonged high-temperature steam and/or strong sodium hydroxide or hypochlorite, as set out in national guidance.
Further reading: Rutala, Weber and HICPAC, CDC Guideline for Disinfection and Sterilization in Healthcare Facilities
Under the Spaulding classification, a flexible endoscope that contacts intact mucous membranes should, at minimum, receive:
B. High-level disinfection
Critical items (entering sterile tissue or vessels) must be sterile; semi-critical items (contacting mucous membranes or non-intact skin), such as endoscopes, need at least high-level disinfection; non-critical items (intact skin) need low or intermediate level. Cleaning always comes first.
Further reading: Rutala, Weber and HICPAC, CDC Guideline for Disinfection and Sterilization in Healthcare Facilities
Ethyl alcohol is a more effective disinfectant at about 70% than at 100% because:
B. Water is needed for protein denaturation and slows evaporation, prolonging contact time
Alcohols act by denaturing proteins and dissolving lipids, which proceeds poorly without water. Neither concentration is sporicidal, so alcohol is not a sterilant.
Further reading: Quinn et al., Veterinary Microbiology and Microbial Disease, 2nd ed.
Enterococci carrying vanA are resistant to vancomycin because they:
D. Build peptidoglycan precursors ending in D-Ala-D-Lac, which vancomycin binds poorly
Vancomycin normally binds the D-Ala-D-Ala terminus of the pentapeptide. The van operon replaces it with D-Ala-D-Lac (vanA, vanB) or D-Ala-D-Ser (some other van types), reducing binding. 23S methylation is the erm mechanism.
Further reading: CLSI VET01/VET01S; WOAH Terrestrial Manual ch. 2.1.1; Giguere et al., Antimicrobial Therapy in Veterinary Medicine, 5th ed.
An E. coli isolate from a calf is resistant to third-generation cephalosporins, and this resistance is reversed by clavulanic acid. The most likely mechanism is:
C. An extended-spectrum beta-lactamase (ESBL), such as a CTX-M enzyme
ESBLs hydrolyse extended-spectrum cephalosporins but are inhibited by clavulanate; CTX-M types are the commonest worldwide in animals and people. AmpC enzymes also hydrolyse these drugs but are poorly inhibited by clavulanate, which is the distinguishing point.
Further reading: CLSI VET01/VET01S; WOAH Terrestrial Manual ch. 2.1.1; Giguere et al., Antimicrobial Therapy in Veterinary Medicine, 5th ed.
The combination of a sulfonamide with trimethoprim is synergistic because the two drugs:
D. Block two successive enzymes in the same folate synthesis pathway
Sulfonamides compete with para-aminobenzoic acid at dihydropteroate synthase; trimethoprim inhibits bacterial dihydrofolate reductase further down the same pathway. The sequential blockade is often bactericidal where each alone is bacteriostatic.
Further reading: Quinn et al., Veterinary Microbiology and Microbial Disease, 2nd ed.
Brooder pneumonia, with yellowish nodules in the lungs and air sacs of young chicks, is caused mainly by:
D. Aspergillus fumigatus
Chicks inhale large numbers of A. fumigatus conidia from mouldy litter or contaminated hatcheries. Candida albicans causes crop mycosis ('thrush'), and Microsporum gallinae (now Lophophyton gallinae) causes favus of the comb.
Further reading: Quinn et al., Veterinary Microbiology and Microbial Disease, 2nd ed.
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