Bacterial genetics

11 Kerala MSc MLT Entrance practice questions on Bacterial genetics, part of Microbiology. The first 11 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 · Hard

   In a neonatal ICU, carbapenem-resistant Klebsiella pneumoniae, Escherichia coli and Enterobacter cloacae are isolated from five babies. Typing shows that the isolates of each species are unrelated strains, but all carry blaNDM-1 on the same large plasmid with a transfer region. The most likely way resistance spread is:

   1. ATransformation with DNA from dead cells
   2. BIndependent mutation in each organism
   3. CConjugal transfer of the plasmid between species
   4. DClonal spread of a single resistant strain

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   Answer: C · Conjugal transfer of the plasmid between species

   The same plasmid in unrelated strains of three species points to horizontal gene transfer. A large plasmid with a transfer region is conjugative, so it moves by direct cell-to-cell contact, often in the gut. Clonal spread would give one strain, and independent mutation would not produce the identical acquired gene on the same plasmid. Infection control must target the plasmid as well as the strains.

   • A: Transformation is inefficient in Enterobacterales and does not fit a large plasmid.
   • B: blaNDM-1 is an acquired gene, not a mutation of native genes.
   • D: Typing showed unrelated strains, so this is not one clone.

2. Question 2 · Medium

   Lederberg's replica-plating experiment showed that streptomycin-resistant mutants of E. coli:

   1. AArose spontaneously before the drug was present
   2. BWere induced by contact with streptomycin
   3. CAcquired resistance genes from other bacteria
   4. DArose only after long exposure to low drug levels

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   Answer: A · Arose spontaneously before the drug was present

   Colonies on a drug-free master plate were printed onto plates with streptomycin. Resistant colonies appeared at the same positions on each replica, and the matching master colonies were resistant without ever meeting the drug. So the mutations had occurred spontaneously and the drug only selected them. This is why resistant mutants are present in any large bacterial population, such as a TB cavity.

   • B: The drug selects resistant mutants; it does not induce them.
   • C: No donor bacteria were present in the experiment.
   • D: Resistance was present in master colonies never exposed to the drug.

3. Question 3 · Easy

   Griffith's experiment, in which heat-killed capsulated pneumococci made live non-capsulated pneumococci lethal to mice, first showed:

   1. ALysogenic conversion
   2. BTransformation
   3. CTransduction
   4. DConjugation

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   Answer: B · Transformation

   Griffith found that a factor from dead capsulated pneumococci turned live non-capsulated cells into capsulated, virulent ones. Avery, MacLeod and McCarty later showed that this factor was DNA. Transformation is the uptake of free DNA from the environment. It needs no cell contact and no phage.

   • A: Lysogenic conversion is a change caused by a prophage.
   • C: Transduction needs a bacteriophage to carry DNA.
   • D: Conjugation needs direct contact between living cells.

4. Question 4 · Medium

   A non-toxigenic strain of Corynebacterium diphtheriae becomes toxigenic after infection with beta-phage. This change is called:

   1. ATransformation
   2. BLysogenic conversion
   3. CTransposition
   4. DGeneralized transduction

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   Answer: B · Lysogenic conversion

   The tox gene is carried by the beta-phage, not by the bacterial chromosome. When the phage becomes a prophage, the bacterium expresses the toxin gene, and it keeps this property only while it stays lysogenic. Other examples are the erythrogenic toxin of Streptococcus pyogenes and botulinum toxins C and D. This is why toxigenicity must be tested, not assumed from identification alone.

   • A: Transformation is uptake of free DNA.
   • C: Transposition moves a DNA element within a cell.
   • D: Generalized transduction carries random bacterial genes, not phage genes.

5. Question 5 · Medium

   In a cross between an Hfr strain and an F- strain, the recipient usually remains F-. Why?

   1. AThe rest of the F factor is transferred last, and mating stops first
   2. BOnly plasmids, not chromosomal genes, are transferred
   3. CF- cells destroy any F factor DNA they receive
   4. DThe Hfr strain has lost its F factor

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   Answer: A · The rest of the F factor is transferred last, and mating stops first

   In an Hfr strain the F factor is integrated into the chromosome. Transfer starts within the F factor and carries chromosomal genes first, with the rest of the F factor at the very end. Mating pairs nearly always break apart before the whole chromosome is transferred. The recipient gains chromosomal genes at high frequency but rarely becomes F+ or Hfr.

   • B: Hfr crosses transfer chromosomal genes at high frequency.
   • C: Recipients do not routinely destroy incoming F DNA.
   • D: The Hfr strain keeps its F factor, integrated in the chromosome.

6. Question 6 · Easy

   In an R plasmid, the resistance transfer factor (RTF) is the part that:

   1. AIntegrates the plasmid into the chromosome
   2. BCarries the genes for drug resistance
   3. CCodes for the enzymes that destroy drugs
   4. DCarries the genes for conjugal transfer

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7. Question 7 · Easy

   Transposons ('jumping genes') differ from plasmids in that transposons:

   1. ACannot replicate on their own outside a replicon
   2. BCan never carry genes for antibiotic resistance at all
   3. CNeed a bacteriophage to move between cells
   4. DAre always made of single-stranded DNA

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8. Question 8 · Medium

   The Ames test screens chemicals for possible carcinogenicity by measuring how often they:

   1. AInduce prophage release from lysogenic E. coli
   2. BKill wild-type Salmonella growing on rich agar
   3. CRevert histidine-requiring Salmonella to independence
   4. DTransfer resistance plasmids between Salmonella

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9. Question 9 · Hard

   A 35-year-old man from Malappuram has cough for 6 weeks. Sputum cartridge-based NAAT (Xpert MTB/RIF) reports: M. tuberculosis detected, rifampicin resistance detected. He has had no previous treatment. How has this strain most likely acquired rifampicin resistance?

   1. APlasmid carrying a rifampicin-inactivating enzyme
   2. BPhage transfer of a resistant RNA polymerase beta gene
   3. CTransposon carrying a rifampicin efflux pump
   4. DPoint mutation in the rpoB gene on the chromosome

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10. Question 10 · Medium

    Methicillin resistance in Staphylococcus aureus (MRSA) is due to the mecA gene. This gene codes for:

    1. AAn efflux pump that actively expels beta-lactam drugs from the cell
    2. BA beta-lactamase that hydrolyses methicillin and oxacillin
    3. CA penicillin-binding protein with low affinity for beta-lactams
    4. DA thickened cell wall that blocks entry of beta-lactams

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11. Question 11 · Hard

    Two auxotrophic E. coli strains each give no colonies on minimal medium when plated alone. Mixed together, they give prototroph colonies. Prototrophs still appear when DNase is added to the mixture, and also when the two strains are grown in the two arms of a U-tube separated by a filter that stops bacteria but lets viruses through. Which mechanism of gene transfer explains these results?

    1. AConjugation
    2. BTransduction
    3. CTransformation
    4. DSpontaneous mutation

    Answer and explanation in LearnFree account