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Updated: Aug 5, 2026

Mutagenesis and Functional Selection Protocols for Directed Evolution of Proteins in E. coli
Published on: March 16, 2011
Selection of resistant mutants of Citrobacter freundii by second and third-generation cephalosporins and imipenem
Abstract:
Using a single-step selection procedure, resistant mutants could be obtained from three clinical isolates of Citrobacter freundii with two second-generation and four third-generation cephalosporins but not with imipenem. All mutants showed a drastically increased beta-lactamase activity and were cross-resistant to all the cephalosporins examined. Combinations of cloxacillin with the cephalosporins were markedly synergistic, suggesting the principal role of the cephalosporinase in the resistance of these mutants.
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