Susceptibility to ��-lactams and expression of ampC and creD genes in the studied mutants.

aPAO1, wild-type reference strain; PA��C, ampC knockout mutant of PAO1; PA��R, ampR knockout mutant of PAO1; PA��D, ampD knockout mutant of PAO1; PA��E, ampE knockout mutant of PAO1; PA��DE, ampD-ampE knockout mutant of PAO1; PA��DDh2Dh3, ampD triple (ampD-ampDh2-ampDh3) knockout mutant of PAO1; PA��DDh2Dh3R, ampR knockout mutant of PA��DDh2Dh3; PA��dacB, dacB knockout mutant of PAO1; 1A1 (DacB W273X), DacB W273X in vitro spontaneous mutant of PAO1; 1A1��C, ampC knockout mutant of 1A1; 1A1��R, ampR knockout mutant of 1A1; 1A1��D, ampD knockout mutant of 1A1; 1A1��E, ampE knockout mutant of 1A1;2A2 (DacB frameshift), in vivo spontaneous mutant of PAO1 containing a deletion of nts 1074�C1078 in dacB; PA��creBC, creBC knockout mutant of PAO1; PA��DDh2Dh3creBC, creBC knockout mutant of PA��DDh2Dh3creBC; 1A1��creBC, creBC knockout mutant of 1A1; 2A2��creBC, creBC knockout mutant of 2A2; 1A1��creD, creD knockout mutant of 1A1.bCAZ, ceftazidime; CEP, cefepime; PIP, piperacillin; PIP/TZ, piperacillin-tazobactam; ATM, aztreonam; IMP, imipenem.cClinical Laboratory Standards Institute (CLSI) susceptibility breakpoints: CAZ, CEP, and ATM��8 ��g/ml; PIP and PIP/TZ��64 ��g/ml; IMP��4 ��g/ml.dRelative amount of ampC or creD mRNA compared to PAO1 basal level��standard deviation. Induction experiments were carried out with 50 ��g/ml cefoxitin.eNA, not applicable; ND, not determined.