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in my molecular bio class we were asked a trick question:

If the bacteria has a plasmid with that grants it ampicillin ressistance, can the bacteria survive if placed in penicillin?

I gave it a thought and said that it cant, since amp differs from pen by an extra amino acid that could harm the bacteria, the answer corresponds with the class material but amp is known as the broad spectrum antibiotic.... so should i go with the class flow lured by the experimental assumption and assume it dies or persist on whether it will survive? i'm really confused at this point, any input is appreciated

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Neither of the two answers provided give the full answer, both touch on it, from different perspectives, but neither address it completely.

Given your description, it sounds as though the question is being asked in the context of a lab strain.

Plasmids for ampicillin resistance carry the AmpR gene on them which encodes for the enzyme beta-lactamase.

Beta-lactamase hydrolyses the beta-lactam ring, which is the 4-membered ring that has the carbonyl group and the nitrogen in the ring.

It then decarboxylates the carboxy group that results from the hydrolysis, removing the threat to the bacteria's ability to make cell walls, as Chris explains.

As both ampicillin and penicillin are closely related beta-lactam antibiotics, and have the same beta-lactam ring moiety which is the substrate of beta-lactamase, both molecules are neutralized by the enzyme and the bacterial cells will divide and not be inhibited from forming a cell wall.

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  • $\begingroup$ The down votes without explanation make no sense at all. Both the answer that @Chris gave and the answer that I gave are factually correct. Ampicillin and Penicillin are Beta-lactam antibiotics and resistance to them is provided by the beta-lactamase enzyme. As the question said plasmid and not F-factor, we talking about transformation and not conjugation. In the lab, you use a Plasmid with AmpR to confer resistance, and pretty much any of the common beta-lactam antibiotics will be resisted by transformants. $\endgroup$ – AMR Oct 4 '15 at 4:02
  • $\begingroup$ Out of interest: Why the downvote? What is wrong with this answer, what needs to be changed? $\endgroup$ – Chris Oct 4 '15 at 11:05
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Short answer: Your assumption is wrong and bacteria, which are resistant to penicillin are are also resistant to ampicillin.

Long answer: There are different classes of antibiotics, which have different mechanisms of action. Penicillin (and also ampicillin) belong to the class of beta-lactam antibiotics which are an irreversible inhibitor of the bacterial transpeptidase, which is needed for the proper formation of the bacterial cell wall.

There is a number of different natural occuring penicillins as well as some which have been generated by modifications of the general structure. The general chemical formula for these is shown below (from the Wikipedia article), the different penicillins differ only in the "-R"-group shown in the molecule (in the upper left):

enter image description here

If you now compare Penicillin G (which occurs naturally and is still used) and ampicillin side-by-side, you will see the the difference is not very big, basically only one amino group:

Penicillin GAmpicillin Penicillin G on the left, Ampicillin on the right.

The difference of this one amino group makes ampicillin more polar and allows it to penetrate the cell wall of some gram negative bacteria (the main action goes against gram positive bacteria).

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  • $\begingroup$ The down votes without explanation make no sense at all. Both the answer that @Chris gave and the answer that I gave are factually correct. Ampicillin and Penicillin are Beta-lactam antibiotics and resistance to them is provided by the beta-lactamase enzyme. As the question said plasmid and not F-factor, we talking about transformation and not conjugation. In the lab, you use a Plasmid with AmpR to confer resistance, and pretty much any of the common beta-lactam antibiotics will be resisted by transformants. $\endgroup$ – AMR Oct 4 '15 at 4:02
  • $\begingroup$ Out of interest: Why the downvote? What is wrong with this answer, what needs to be changed? $\endgroup$ – Chris Oct 4 '15 at 11:05
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My first reaction to your question was that it would depend on the mechanism of resistance. As Chris pointed out, ampicillin differs from penicillin only by the addition of an amino group on the amide alpha carbon. While the similarity may suggest that resistance to one confers resistance to the other, the amino group does provide a means to distinguish between the two and, through the happenstances of evolution, I could imagine cases where bacteria evolved resistance to one antibiotic while remaining susceptible to the other. This paper describes penicillin resistant but ampicillin susceptible Enterococcus faecalis strains isolated from clinical samples. While this describes somewhat of the opposite scenario as your question, I don't see any reason to say unequivocally that ampicillin resistance implies penicillin resistance.

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  • $\begingroup$ For lab strains, with the AmpR gene encoding beta-lactamase on a plasmid it does, and based on the wording of the question I am going to say that it is referring to lab strains on to Amp or Pen plates.. The article you linked to does not give an explanation of the method of resistance. It could very well be that these bacteria have a different form of resistance than beta-lactamase, and it could be that ampicillin greater polarity, as Chris pointed out, makes it more effective against these bacteria. $\endgroup$ – AMR Oct 4 '15 at 0:55

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