Adding antibiotics to poured plates - (Jan/08/2016 )
Hi. I need to use some LB plates with Carbenicillin and Chloramphenicol selection plates. I have LB carbenicillin plates prepared but do not have any that include chloramphenicol. I've never added a solution of antibiotic to the surface of plates before. Is this common practice?
It's possible but a little sketchy. You calculate the volume of the plate (about 25 ml usually) and the amount of antibiotic you would add to that amount of medium. Then dilute to about 100 ul total volume and spread evenly on the plate. Ideally you should let the plate age for a bit to allow diffusion of the antibiotic into the plate. The plates usually end up being a bit too wet -- they can be dried by leaving them open with the lids partially off in a sterile BSC.
If it were me, I would make new plates with Carbenicillin and Chloramphenicol added before pouring. This way may be more reliable. I am afraid the concentration of the added antibiotic will not be even and could lead to more problems.
If you do add fresh antibiotic, you need to work quickly when you spread it, otherwise you will generate different concentrations of antibiotic. Probably better to prepare fresh.