bug sex

[doctorFred]

ok, this has come up on both an nbme and usmleworld, and it seemed obvious to me both times, and both times i got it wrong.

you have two different bacteria, A and B. A is resistant to drug X, B is resistant to drug Y. you mix them together, and now they're resistant to both drugs. you repeat the experiment, except this time you add DNase to the plate (specifically stating that the bacterial cells are impermeable to DNase, and now, neither colony grows. this is..
conjugation
mutation
transduction
transformation
transposition

i say transformation every time, and the answer is always conjugation. i really don't get it. DNase degrades DNA, right? so if there was naked DNA that they were taking up to confer resistance to the abx, it should be eliminated by adding DNase to the plate. how is conjugation the right answer? conjugation doesn't take place "outside" the cell. please tell me what i'm missing here.

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[MSKalltheway]

Huh? How is the answer conjugation? Your logic is correct from what I see. I remember that UW question too and I actually remember transformation was correct. If you use DNAse to get rid of the DNA on the plate from lysed bacteria, there should be no more resistance from bacteria picking up the free DNA.

 

[Doctor4Life1769]

If these bugs are impermeable or resistant to DNAse, then how would adding DNAse cause the bugs to not grow when DNAse is added?

Maybe the Q-stem is wanting you to answer the type of antibiotic gene transfer that comes about when mixing these 2 bugs together? That would be conjugation, right?

I haven't done any of these QBanks yet (I take the Step I in a year), but I figured I would take a crack at this to see if my rationale is correct. However, I don't have the actual Q stem in front of me either.

 

[medmom]

My initial response would be conjugation because the naked DNA is not being taken up by the bacteria. Rather they are exchanging DNA material via conjugation.

 

[Instatewaiter]

but wouldnt the sex pili of conjugation protect the plasmid from the DNase.

I had a similar question in Uworld and the answer was transformation.

 

[peter90036]

gosh... its just SEX ...its not that complicated,
HOW can you get it wrong ?