growth inhibition by X-gal? - (Feb/18/2008 )

I tried to see LacZ activity of E.coli MC4100 (whole lac operon was preserved. very similar to K12) on plates, so I dotted E. coli MC4100 broth culture on LB plate spreaded with X-gal.

However, I saw very small and white dots of this strain, while positive control (E.coli DH5a + pUC19) showed good growth and blue coloration.
I don't know what happened to my E.coli or X-gal.

Does X-gal inhibit growth of wild-type E. coli? Does anybody have similar experiences?

Thank you for your answers.

Do you have any antibiotics in your agar?