Steps of viral infection of a bacterium!

[theshanthan]

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I am sorry if I am not allowed to post a specific question. But this is a question I found while I was studying form one of the mcat guides.

Prior to infecting a bacterium, a bacteriophage must:

a. reproduce, making copies of the phage chromosome.
b.integrate its genome into the bacterial chromosome.
c. penetrate the bacterial cell wall completely.
d.attach to a receptor on the bacterial cell membrane.

I thought the answer was C.
Rationale: Bacteria have cell wall. Without breaking through them they can not reach the cell membrane. But the right answer is given is a D. I am confused. Am I missing something? This isn't the first mistake I though I found in the book, but I don't want to be as arrogant as to say that the book is wrong. I just want to be right, because I don't want the damn book to mess me up!

 

[sbuxaddict]

Pretty sure the steps are: (from TPRH)

1) Attachment/adsorption, meaning binding to the exterior of a bacterial cell.
2) Penetration/eclipse; aka injection of viral genome.
3) Entrance into lytic or lysogenic cycle.

I think the question is basically asking what happens first in order for infection to occur, not what happens right before infecting it. But that's just my take on it and how I'd interpret the question.

What book are you using, btw, since you said you found other mistakes?

 

[Ssina]

Pretty sure the steps are: (from TPRH)

1) Attachment/adsorption, meaning binding to the exterior of a bacterial cell.
2) Penetration/eclipse; aka injection of viral genome.
3) Entrance into lytic or lysogenic cycle.

I think the question is basically asking what happens first in order for infection to occur, not what happens right before infecting it. But that's just my take on it and how I'd interpret the question.

What book are you using, btw, since you said you found other mistakes?

👍
what if it was a gram negative bacterium with an outer membrane, you have to attached to the specific receptor first before it can inject it's DNA content through the cell wall. I think it's emphasizing the fact that there should be a specific receptor for that virus, which leads to tissue/cell specificity, rather than indiscriminately infecting any cell

 

[Pisiform]

Bacteriophage cannot penetrate cell itself. It rather injects it genome inside the cell by penetration and attachment ...

 

[MedPR]

I am sorry if I am not allowed to post a specific question. But this is a question I found while I was studying form one of the mcat guides.

Prior to infecting a bacterium, a bacteriophage must:

a. reproduce, making copies of the phage chromosome.
b.integrate its genome into the bacterial chromosome.
c. penetrate the bacterial cell wall completely.
d.attach to a receptor on the bacterial cell membrane.

I thought the answer was C.
Rationale: Bacteria have cell wall. Without breaking through them they can not reach the cell membrane. But the right answer is given is a D. I am confused. Am I missing something? This isn't the first mistake I though I found in the book, but I don't want to be as arrogant as to say that the book is wrong. I just want to be right, because I don't want the damn book to mess me up!

The key is attachment. I did this in EK last night and in the lecture, it clearly states that the first step is adsorption to the bacteria. Go back and read carefully and you'll see it.

Bacteriophage cannot penetrate cell itself. It rather injects it genome inside the cell by penetration and attachment ...

This is also in the EK lecture. Bacteriophages only inject genome (through its tail), as opposed to other viruses which can be endocytosed into a eukaryotic cell.

 

[theshanthan]

I think I am getting the gist of what you guys are saying. But I still can't seem to wrap muy head around, even if it was a gram-negative bacteria, the receptor proteins should not stick out of the capsule? or the peptidoglycan cell wall even if it thin. So, to reach the receptor proteins the phage would first have to go through the first line of defense that is the cell wall.

The other possibility is they are talking about the step just before infection! If so it makes sense that the receptor protein adsorbence is the right answer.

 

[theshanthan]

but for that to happen, it has to first go through the cell wall right? I am just confused at this point! so, the receptor proteins stick out of the capsule and the cell wall?

 

[theshanthan]

The key is attachment. I did this in EK last night and in the lecture, it clearly states that the first step is adsorption to the bacteria. Go back and read carefully and you'll see it.



This is also in the EK lecture. Bacteriophages only inject genome (through its tail), as opposed to other viruses which can be endocytosed into a eukaryotic cell.

but for that to happen, it has to first go through the cell wall right? I am just confused at this point! so, the receptor proteins stick out of the capsule and the cell wall?

 

[chiddler]

but for that to happen, it has to first go through the cell wall right? I am just confused at this point! so, the receptor proteins stick out of the capsule and the cell wall?

If the cell wall covers the membrane bound receptors, then the membrane bound receptors won't be doing much since they are sealed off.

The most likely situation is that there are receptors on the outside of the bacterial cell wall.

 

[theshanthan]

If the cell wall covers the membrane bound receptors, then the membrane bound receptors won't be doing much since they are sealed off.

That would make sense! but seeing as the cell wall is not a lipid bilayer or a mosaic. It just seems that it would be hard for receptor proteins to stick out. I mean the peptidoglycan wall doesn't don't have any integral proteins! as far as I know. At this point I don't know what to think. I googled the question but all the results are pointing me towards a cell membranes receptor proteins.

I will just ask my pi tomorrow.

The most likely situation is that there are receptors on the outside of the bacterial cell wall.

Then option D should say cell wall and not cell membrane! Just food for thought

 

[theshanthan]

@sbuxaddict

If you did not figure out from the past discourse! it is examkrackers biology. They are riddled with errors. I mean the first question in the ochem book has errors!

 

[MedPR]

but for that to happen, it has to first go through the cell wall right? I am just confused at this point! so, the receptor proteins stick out of the capsule and the cell wall?

Gram - have an outer membrane outside the cell wall.

Bacteriophage must adsorb before it can infect. The same is true for eukaryote viruses that attach to the receptors before endocytosing.

 

[theshanthan]

Gram - have an outer membrane outside the cell wall.

Bacteriophage must adsorb before it can infect. The same is true for eukaryote viruses that attach to the receptors before endocytosing.

But that is not the cell membrane! If what I have learnt from MCAT practice tests, pay attention to every word, never take anything for granted.

and I agree it should adsorb but to adsorb it should have access to the cell membrane that is my point! so to gain access should it not go through the cell wall?

May be I am just going on tangent, I will just remember the answer I guess!

 

[Ssina]

not really, it just needs to stick to the cell wall and then injects it into the cell

 

[MedPR]

But that is not the cell membrane! If what I have learnt from MCAT practice tests, pay attention to every word, never take anything for granted.

and I agree it should adsorb but to adsorb it should have access to the cell membrane that is my point! so to gain access should it not go through the cell wall?

May be I am just going on tangent, I will just remember the answer I guess!

c. penetrate the bacterial cell wall completely.
d.attach to a receptor on the bacterial cell membrane.

Are both bad answers. C is even worse because the bacteriophage never penetrates the cell wall, or cell membrane, or any part of the bacterium. It adsorbs, and injects it's genome. The MCAT, like every other multiple choice test is about picking the best answer.

Answer D is obviously better than answer C.

 

[MedPR]

not really, it just needs to stick to the cell wall and then injects it into the cell

The point he's trying to make is that gram- bacteria have an outer membrane, so a bacteriophage wouldn't necessarily adsorb directly to the cell wall

 

[theshanthan]

c. penetrate the bacterial cell wall completely.
d.attach to a receptor on the bacterial cell membrane.

Are both bad answers. C is even worse because the bacteriophage never penetrates the cell wall, or cell membrane, or any part of the bacterium. It adsorbs, and injects it's genome. The MCAT, like every other multiple choice test is about picking the best answer.

Answer D is obviously better than answer C.

That helped. I see how that is the best answer. It is true, it never even penetrates!

 

[CaptainScience]

Base on what I learned in bacterial pathogenesis last semester, a bacteriophage must first interact with protein receptors on the host cell. That action then cause the bacteriophage to inject enzyme into the host that breaks down the cell well in order to enter the bacteria.

 

[SLC]

You'll learn early on in microbiology (either now or as a med-student) that viruses can display a great deal of tissue tropism. They often attach to a specific receptor on a cell, lending to the phenomenon of having a viral infection that's fairly specific to one tissue (or type of tissue) in the body.

This obviously doesn't apply directly to the question originally asked, since bacteria don't have tissues. But the concept applies to humans as well as bacteria, just that in humans there are lots of different populations of cells to infect, and to a virus, not all are created equal.