Viral Plaque Question. Please help!

[LaFleur]

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"A viral plaque is a visible structure formed within a cell culture...The bacteriophage viruses replicate and spread, thus generating regions of cell destructions known as plaques.
These plaques can sometimes be detected visually using colony counters, in much the same way as bacterial colonies are counted; however, they are not always visible to the naked eye, and..."

So this means that if you're given a chart of dilutions and # of plaques formed on the plate you can calculate #phage/mL in much the same way you would calculate cells/mL of a liquid culture of bacteria if you were given the dilution and the number of colonies formed on the plate, because you assume one phage made the plaque you're looking at. Am I right?

Also, how does viral plaque assaying work? (ex. http://en.wikipedia.org/wiki/File:M._smegmatis_plaque.jpg) I understand the basics of it, but more specificaly I'm wondering why there's a lawn of bacteria and not just colonies formed in this technique, as I probably would have expected.

The other thing I was wondering was why one plaque doesn't lyse the entire plate (why plaques don't increase in size indefinitely)? (this is the most important question I don't understand)

I'm sorry if this isn't exactly DAT related, but these things came up in my genetics class, and when I asked my professor she didn't provide an adequate explanation (and it's not really covered in the notes or the book...it's on a problem set she handed out in class >.<).

Thanks so much!

 

[LaFleur]

Ok wait...I might have just come up with an explanation for the bolded question (after simmering on it for 2 days lol)!!!
The plaques don't increase in size indefinitely because the bacteria deplete nutrients and eventually stop growing, which means they're not reproducing, and since bacteriophages can't reproduce on their own, when the bacteria aren't reproducing, the bacteriophages can't reproduce either. 👍?

The other question I still don't understand. 😕

 

[sfoksn]

You get a lawn because you let those bacteria proliferate with enough nutrients to cover up the whole petri dish surface, which creates a "lawn" of bacteria.

Plaques do lyse through the whole lawn if you leave it like that. They grow indefinitely if you leave them alone. You want to analyze the plaque locations and numbers before the phages eat through the whole lawn. 😀

Bacteria don't run out of nutrients. They are planted on a relatively thick (compared to the bacteria) layer of nutrients and they have enough to survive for quite a while.

 

[ToothTickler]

You get a full lawn of bacteria because you have a LOT of bacteria and you're giving it optimal conditions to grow. When you get colonies, you're usually sampling something that might not be very saturated with microbes (or you've purposely diluted it to get colony isolation), where as plaque assaying, you are basically overkilling your plate with bacteria. This is so you can make sure any spots lacking bacterial growth is due to phage.

And technically, you're not calculating # phage per dilution, but "plaque-forming-units." But that's being ridiculously nit-picky 😉