Beta lactamase cleaves the ring in beta-lactam antibiotics such as those from the penicillin and cephalosporin families rendering these cell wall synthesis inhibitors inactive. This raises the concentrations needed to inhibit growth causing a resistance phenotype. If your E. coli are ampicillin resistant you can always use other families of antibiotics such as tetracycline, kanamycin, streptomycin, vancomycin, methylparaben and malachite green.

You can use carbenecillin instead of ampicillin for selection to reduce satellite colonies.