For the clear plaques, I can understand it's a complete lysis, no bacteria are growing in that aera any more. But how to explain the turbid plaques? Is it the phage lysed some of the bacteria, but the rest got resistant to the phage, which led to a incomplete lysis? And how about the halo plaques?
And why the plaques have certain size, but cannot enlarge limitless?
It depends a bit on the life cycle of the specific phage you are dealing with. Turbid plaques are usually produced by lysogenic phage such as lambda. In some of the cells the phage may lysogenize instead of continuing the lytic cycle, and if this happens with high enough frequency the plaque will look 'turbid'. Other times (I am guessing this is what you call 'halo plaques') phage replication does not lyse the cell, but slows their growth noticeably enough that you can distinguish a plaque in solid medium; a typical example are filamentous phages.
As for why phage plaques do not grow to lyse the entire plate, one of the reasons is that most phages cannot productively infect stationary phase cells, so once the bacterial lawn reaches that stage it's game over for the phage.
lysogens begin to grow in the center of the plaque, giving the plaque a turbid, bulls-eye appearence.
Isolate cells (by limiting dilution, streaking or whatever) from the turbid area of the plaque and check them for resistance to infection and for phage production. A colony resistant to superinfection that does not produce infectious phage on its own is most likely a lysogen.
Or you can also do PFGE of chromosomal DNA of the infected cells and probe it with a phage-derived DNA or RNA fragment (i.e. a Southern blot).
Alejandro Martin Colony resistant to superinfection that does not produce infectious phage can be resistant to phage?? it is not always a lysogen i guess??
Vijay Singh,
Can you reword your question? Looks like you are asking something interesting but I don't quite understand it...
Agree with Alejandro Martin's explanations.
As well, worth considering, that lysogen-independent spontaneous mutations in host genes coding for cell surface receptors of phage will occur, contributing to turbid plaque appearance.
Alejandro Martin its just my explanation that the resistant colony may produce turbid plaques, its not always lysogen.
Vijay Singh, yes, you are right, lysogeny is only one of several different explanations. As pointed out by Richard Goldstein the turbidity might arise also from spontaneous mutation or antigenic phase variation of the initial receptor used for phage attachment/entry, but these are less frequent events (those are the resistant colonies you observe, for instance, when leaving in the incubator a plate where the bacterial lawn has been wiped out by a lytic phage).
I agree that a mixture of resistant cells and sensitive cells in the lawn will lead to the appearance of turbid plaques. However all the plaques would look the same, I would not expect that you see some clear plaques and some turbid plaques on the same plate, but I'm not sure whether that is what you are seeing or not. Other possible explanations are of course lysogeny and also note that some phages are normally just not virulent, for example filamentous bacteriophages
some phages do not lyse bacteria efficiently beyond the log phase. it is possible that some of the phages adhered at a later point in the metabolism of the bacteria and produced weaker zones of clearing.
When spot assay is used, there is a possibility that the phage has enzymes that can act on the polysaccharide layer surrounding the bacterium and modify the appearance of the bacterial in Petri dishes.
In the isolation of bacteriophages, the plaques were seen within 4 to 5 hours of incubation and on further incubation turbidity was observed. When the phages were diluted, at 10 power 9 and greater individual large plaques were visible with colonies in centre.
When the phage showing turbidity was considered for determination of activity against other strains of E.coli , ""clear plaques"" against 7 strains were observed. What could be the explanation for this result.
likely the "colonies in centre" were lysogenic host cells containing prophage.
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