Model System To Evaluate the Effect ofampDMutations on AmpC-Mediated β-Lactam Resistance

Abstract

Mutations within the structural gene ofampDcan lead to AmpC overproduction and increases in β-lactam MICs in organisms with an inducibleampC. However, identification of mutations alone cannot predict the impact that those mutations have on AmpD function. Therefore, a model system was designed to determine the effect ofampDmutations on ceftazidime MICs using an AmpD⁻mutantEscherichia colistrain which produced an inducible plasmid-encoded AmpC.ampDgenes were amplified by PCR from strains ofE. coli,Citrobacter freundii, andPseudomonas aeruginosa. Also, carboxy-terminal truncations ofC. freundii ampDgenes were constructed representing deletions of 10, 21, or 25 codons. AmplifiedampDproducts were cloned into pACYC184 containing induciblebla_ACT-1-ampR. Plasmids were transformed intoE. colistrains JRG582 (AmpD⁻) and K-12 259 (AmpD⁺). The strains were evaluated for a derepressed phenotype using ceftazidime MICs. Some mutatedampDgenes, including theampDgene of a derepressedC. freundiiisolate, resulted in substantial decreases in ceftazidime MICs (from >256 μg/ml to 12 to 24 μg/ml) for the AmpD⁻strain, indicating no role for these mutations in derepressed phenotypes. However,ampDtruncation products andampDfrom a partially derepressedP. aeruginosastrain resulted in ceftazidime MICs of >256 μg/ml, indicating a role for these gene modifications in derepressed phenotypes. The use of this model system indicated that alternative mechanisms were involved in the derepressed phenotype observed in strains ofC. freundiiandP. aeruginosa. The alternative mechanism involved in the derepressed phenotype of theC. freundiiisolate was downregulation ofampDtranscription.