I have noticed this for a long time, but never really found an explanation for it: when I seed plates with say 50 uL OP50 bacteria, why do the bacteria always grow at highest concentration on the edge of the “bacterial mat”?
When I add a drug to the agar that has colour and next day seed the plate with OP50, the following day the colour also seems to be most intense at the edge of the mat. I guess this has also got to do with how OP50 grows on the edge…
part of the answer can be mechanical, e.g. the way you spread the plate using your spreader or dropping pipette…such that more OP50 are spread/located initially to the periphery of the plate.
HOWEVER, all things considered, when the plate is spread evenly, then the leading (peripheral) edge of the bacterial spread will grow quickest as they encounter less competition for resources…a bit like in a single colony.
As for the coloured drug infusions, I guess this is just because the bacteria at the edge are more active and therefore take up more of the drug…so it appears that the drug is concentrated at the edge?