Dear Aunt Yersinia,
What are advantages and disadvantages of pour and spread plates?
It is refreshing to see such wisdom – knowing that there are at least two methods of pouring plates – in a young man.
While all molecular biologists know or have at least seen a ”spread plate” aka Petri dish with some agar in it, a “pour plate” is quite exotic.
For spread plates you make a series of dilutions of your bugs in a buffer, put a certain volume on an agar surface and spread them using a spreader or glass beads. The bacteria then form colonies on the agar surface. The number of colonies in 1 ml of initial medium equals the number on the plate times volume and dilution.
For the pour plates you dilute bacteria the same way, but add them to, and mix with, the melted medium and then pour the mixture into your plates. The medium solidifies; the colonies grow throughout the volume of the plate.
Pour plates have some disadvantages:
- you need a separate container for each dilution of the bugs;
- you need to carefully control the temperature of the medium: too hot will kill your bags, too cold will create clumps of congealed agar, which can sometimes be mistaken for colonies;
- you have to make sure that your colonies spread evenly in the medium;
- if your colonies overlap in 3D, you may have difficulty in picking them without mixing.
Although you don’t need spreaders, I see only one real advantage of pour plates in comparison with standard spread plates:
If you bacteria don’t like oxygen, they would be better off in the media, where there is less oxygen, than on the agar surface. But this will not substitute for proper anaerobic conditions, as there will still be some oxygen dissolved in the medium. In other words, anaerobes will not grow anyway and microaerophiles will grow on the agar surface just as well.
In the end, I am a big believer in “the wisdom of crowds”. If pour plates were better overall, more people would use them or at least have heard about them.