nadiabarati
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Bacterial strain X is resistant to ampicillin and sensitive to gentamicin. Bacterial strain Y is resistant to gentamicin and sensitive to ampicillin. Bacterial strains X and Y are grown in mixed culture in medium without antibiotics, then the culture is plated on medium containing both ampicillin and gentamicin. Bacterial colonies grow on the plates. In a second experiment, DNase is added to the mixed culture medium. When this mixed culture is plated on medium containing both antibiotics, no colonies grow. Assuming that bacterial cells are impermeable to DNase, which of the following processes best explains these observations?
A. Conjugation
B. Mutation
C. Transduction
D. Transformation
E. Transposition
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| doc179
Forum Guru
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I think its D
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| mazinger
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I would go for A..
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original mazinger z
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| nadiabarati
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So if DNAse has beed added to the environment and destroyed the DNA so how the bacteria got resistence to the antibiotic to that they were already sensitive! So it's not transformation, because there is not any DNA to be gotten by bacteria!!!! to become resistent.
in conjugation you mean at the same time they just exchange resistent genes with each other, both sided??
I think mutation can be reasonable. Please someone clarify the case
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| doc179
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"Bacterial strains X and Y are grown in mixed culture in medium without antibiotics,"
"In a second experiment, DNase is added to the mixed culture medium"
pay attention to these word they have added DNAase to the mixed culture when X and Y were resistant to just one antibiotic each
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| mazinger
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Yup I noticed my error, you're right, thought it was (read it was) permeable, but it was my fault.. I always tend to read so fast that I miss very important data... Its definetly D
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original mazinger z
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| thatsme
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doc179, I Think, we still have to consider B. It seems to make more sense.
Mutation is a change in DNA base sequence of an organism. They can cause nchanges in enzyme activity, antibiotic suseptibilty......
The bacteria stairn in the question is impermeable to DNase, which means, no action against the bacterial DNA. If after staining there is no growth in the second experience, I think there is an alteration of the bacterial DNA which is mostly due to mutation.
Transformation seems to be clear for me. Am I still wrong?
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| thatsme
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Transformation seems to NOT be clear for me. Am I still wrong?
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| doc179
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Tranformation is the most likely event that occurred.
Mutation is theoretically possible but very improbable.
Okay, let me lead you thru this........
First there were some ampR bacteria and some GentaR bacteria. When co-cultured, there are always some bacteria dying (and thus spilling their cellular contents into the medium) and bacteria have this tendency to pick up extraneous pieces of DNA from the medium. This is transformation. So some Genta resistant bacteria picked up some AmpR or vice versa. This happens very often. So when this culture was then plated on Amp+Genta media, a few of them that had taken up the useful pieces of DNA from the medium had what it takes to survive.
Lets see the conditions mentioned again. DNAase was added to the media in which they were grown without antibiotics. (When any bacteria died and spilled their insides into the media, their DNA was destroyed. So other bacteria did not have a chance to take up any useful extraneous DNA. AmpR bacteria could not pick up GentaR DNA, GentaR bacteria could not pick up any AmpR DNA). No transformation took place. And when this mixture was plated on media with both antibiotics, there were no bacteria that could survive the challenge.
WHY I THINK IT IS NOT MUTATION:
1. If it is mutation occurring that confers resistance or causes the lack of it, then it should not be in any way be influenced by the presence or absence of DNAse in the culture medium.
2. ColonieS (plural!) of bacteria were found in the plate with two antibiotics. That means more than one bacteria needs to undergo the same random mutational change that miraculously confers the changes in DNA sequence that will enable survival in the new media with two antibiotics. How likely is it that many bacteria get the same random mutational change that confers exactly what they need to survive in the new medium? That is a very low likelihood change. Not absolutely impossible, but very improbable.
In the usmle, one needs to pick the MOST LIKELY answer, if it is not crisp and clearly one answer. So transformation is the better answer.
Edited by doc179 on 03/11/06 - 08:00 AM. Reason: better wording
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| thatsme
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thanks. I see now
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