LB plates for Beta-galactosidase assay

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  • add X-gal, antibiotic & IPTG once the LB/agar mix has cooled-down, then mix thoroughly (invert many times) & plate (aliquot agar mix into separate petri dishes). X-gal is light-sensitive; wrap the plates in foil when they've solidified.
  • if you've never done the alpha-fragment blue/white assay before, make sure that your Ecoli strain doesn't have the full lacZ gene)
  • I usually aim for a final 70ug/mL X-gal concentration and an 80uM IPTG (u = micro) concentration (others seem to manage fine with lower concentrations, but I haven't had time to test this).
  • Suspend X-gal powder in DMSO (to 25mg/mL; add 4mL DMSO to 100mg invitrogen X-gal bottle (available at biostores) & mix thoroughly).
  • Make stock IPTG is 0.5M (in H2O), but this is 6,250X above 80uM, so you shouldn't make it this concentrated unless you make tons of plates at once.

(Contributed by Alfonso Farrugio)

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