Bacterial infection

[basophilic]

Which of the following is true in a patient presenting with bacterial infection?

A)The causative agent most likely possesses two lipid bilayer membranes.

B)The causative agent is protected from osmotic damage by a glucose polymer.

The question stem was worded as shown above. I understand that A connotes a Gram-negative bacterium; this was the right answer.
But isn't B also true of bacteria? I know cell walls protect plant cells from osmotic damage, but doesn't that apply to bacteria as well?
Or is A the better answer because it corresponds primarily to bacteria whereas B is true but isn't specific to bacteria?

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[Quinoline]

I don't think gram positive bacteria have 2 lipid bilayers. Is it implied somewhere else that the infection is caused by a gram negative bacteria?

Gram positive and negative bacteria have peptidoglycan layers and gram-negative bacteria also have a layer with lipopolysaccharide. Cellulose is a glucose polymer, but peptidoglycan is made from glucose-derived sugars, so I'm not sure if they'd classify as a strict 'glucose polymer'.

 

[basophilic]

The passage was about heme biosynthesis and potential enzyme defects in that pathway, so nothing about the bacteria was implied.

Your explanation about peptidoglycan vs. cellulose makes sense in terms of making B a worse answer, but you wouldn't say it's a false statement, right? It is true that the cell wall protects bacteria from bursting or shriveling?

I guess I'll keep this in mind: instead of thinking "if X, then not Y" I should put the answer choices on a spectrum from worst to best.

 

[mcatjelly]

I guess I'll keep this in mind: instead of thinking "if X, then not Y" I should put the answer choices on a spectrum from worst to best.

Oh my gosh, absolutely. The MCAT often tests your ability to pick the best answer; not just the right one.

 

[amy_k]

i wouldnt classify peptidoglycan as a glucose polymer, it is a polymer of alternating amino acids and sugars, i think thats the issue with B. Both Gram positive and gram negative have an inner lipid bilayer plasma membrane.

 

[kingtal0n]

Well I am curious why wouldn't it be a staph infection, a gram positive invader very common on skin surfaces. Why do we assume it has to be gram negative?

And I agree that when I think of glucose protective polymers, the one that jumps right to mind is cellulose, a rigid plant cell protector that would stop a cell from bursting due to osmotic pressure. I feel like the derivatives of glucose in bacterial cell walls or membranes are more known for their immunological, community, transport, sensing, and activity sites.

 

[amy_k]

i think that B is ruled out based on the fact that bacterial cells do not have a glucose-polymer cell wall so B could not apply to a bacterial infection. That leaves us with the possibility that it is either a gram positive or gram negative bacteria, gram positive has one lipid bilayer whereas gram negative has 2. Since A says the infection is most likely caused by a gram negative bacteria (with 2 lipid bilayer membranes) it seems to be relying on the assumption that gram negative bacteria are more likely to cause infection in humans since they are harder for the immune system to fight, and/or more likely to be toxic to humans due to the second lipid bilayer, and thus generally more pathogenic than gram positive.

 

[amy_k]

Well I am curious why wouldn't it be a staph infection, a gram positive invader very common on skin surfaces. Why do we assume it has to be gram negative?

And I agree that when I think of glucose protective polymers, the one that jumps right to mind is cellulose, a rigid plant cell protector that would stop a cell from bursting due to osmotic pressure. I feel like the derivatives of glucose in bacterial cell walls or membranes are more known for their immunological, community, transport, sensing, and activity sites.

you are totally right that it could be gram positive bacteria, like staph, but that the answer A implies the disease is "likely to be" caused by gram negative seems to indicate that gram positive bacteria do not usually cause a serious infection in healthy individuals. Staph bacteria are found on the skin and in mucous membranes of healthy individuals and are not usually harmful.

Regardless, this question is quite vague in itself, did it come with a passage?

 

[kingtal0n]

Staph bacteria are found on the skin and in mucous membranes of healthy individuals and are not usually harmful.

I feel like staph is under-rated, I mean there is the whole MRSA right? Not as common, but a very potential deadly threat to us. And then there is acne related bacterial infections, and you can literally search up 9999 pictures of skin staph infections which appear relatively common. I feel like downplaying the normal flora is fine- we know they are there but ignore them mostly. But even the non-invasive staph can be troublesome and require immune clearance if they get in somehow, even just a normal every-day pore. Then you have pus forming etc..

 

[amy_k]

I feel like staph is under-rated, I mean there is the whole MRSA right? Not as common, but a very potential deadly threat to us. And then there is acne related bacterial infections, and you can literally search up 9999 pictures of skin staph infections which appear relatively common. I feel like downplaying the normal flora is fine- we know they are there but ignore them mostly. But even the non-invasive staph can be troublesome and require immune clearance if they get in somehow, even just a normal every-day pore. Then you have pus forming etc..

you are totally right and i can relate to the serious threat of staph infections personally, and have heard of a terrifying case where an apparently healthy woman used her friend's makeup brush and contracted a rare and severe staph infection which left her partially paralyzed. but the general consensus (at least for mcat purposes) is that the average healthy person with a functioning immune system and no other influencing factors such as high blood sugar is not threatened by staph, and such cases as this are thankfully rare, at least for now!