TPR MCAT Review Test 1 C/P #24

[ThujaOccidentalis]

Are QACs more likely to be effective against Gram-(–) or Gram-(+) bacteria?

A. Gram-(+) bacteria because they are encapsulated by two cellular membranes and a thin peptidoglycan cell wall.
B. Gram-(–) bacteria because they are encapsulated by two cellular membranes and a thin peptidoglycan cell wall.
C. Gram-(+) bacteria because they possess a single phospholipid membrane and a thick peptidoglycan cell wall.
D. Gram-(–) bacteria because they possess a single phospholipid membrane and a thick peptidoglycan cell wall.

From the passage, we are told that QACs disrupt the plasma membrane of bacteria.

C is the correct answer, with the reasoning being it is easier to disrupt one membrane of Gram-(+) than two of Gram-(-), which I agree with. But won't the Gram-(+) bacteria's much thicker peptidoglycan cell wall also act as a hinderance of QACs from reaching the membrane?

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[bobeanie95]

I don't know what a QAC is but I'm assuming it's an antibiotic. It helps to remember that during a bacterial infection drugs/enzymes will want to target compounds that are unique to the bacteria, such as the peptidoglycan cell wall, think for example lysozyme. Destroying the cell wall would burst the bacteria. So even though the peptidoglycan layer is thicker, it is usually the target compound, and in gram (+) bacteria it's pretty much out in the open for attack.

 

[ThujaOccidentalis]

Given in the passage, QAC is a broad spectrum antibiotic. The mechanism of action is to disrupt/break the plasma membrane of bacteria.

This leads to the given reasoning on the answer key, which is disrupting 1 membrane is easier than disrupting 2. My question is: although broaching one is easier than broaching two, isn't it harder for the antibiotic to get through the thicker wall before even reaching the single membrane it needs to pop? Is that better than popping two membranes but only have to deal with a much thinner wall?

"As stated in the passage, QACs are considered broad-spectrum antibiotics and thus need to work against a structure present in all bacterial cells – a cell membrane. Effectiveness would likely be increased if there was one membrane to broach instead of two (choice C is the best answer among the choices; eliminate choice B)."