Doing the antibacterial test, the unknown colony grows around the plate. E. coli and antibacterial regent were mixed with LB, PBS and EDTA.
Not likely the contamination was from the LB, PBS or EDTA. When diluting the sample the contaminated colony reduced as well (pic not attached here).
This sometimes happens to me when I'm using a pin-replicator to do my agar dilution AST. Although it looks like you spotted manually, I found that when the surface tension of the drop of culture breaks (either from a pipette tip or pin-replicator) that sometimes the culture flicks away from the spot and causes those single colonies that you're seeing. Perhaps that could be the problem, if your plate wasn't contaminated by something else of course! Whenever I'm running important stuff or time consuming experiments I always have at least one negative control plate to make sure its not the media/plate/BSC etc...
If you want to chat more, just let me know. Best of luck!
Are the contaminating cultures on the surface of the agar or embedded into the solid medium?
All it takes is one "overspray" event to make colonies grow (bubble popping, tip ejected too close to the plate, etc.).
It seems because of contaminated tips and/or extra water droplets on surface of the medium used.
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