Table 2.

Induction of AmpC expression in P. aeruginosaby cefoxitin, clavulanate, and tazobactam

StrainDisk approximation induction results for clavulanic acidCephalosporinase activity in cell-free sonic extractsa
Uninduced controlClavulanate at 2 μg/mlClavulanate at 8 μg/mlClavulanate at 16 μg/mlClavulanate at 50 μg/mlCefoxitin at 50 μg/mlTazobactam at 30 μg/ml
P. aeruginosa1Positive2 ± 02 ± 222 ± 4b, c36 ± 6b230 ± 80b146 ± 6b3 ± 4
P. aeruginosa13Positive8 ± 68 ± 220 ± 2b, c36 ± 8b128 ± 8b147 ± 26b7 ± 2
P. aeruginosa246Positive9 ± 47 ± 2143 ± 22b177 ± 16b285 ± 24b310 ± 46b5 ± 4
P. aeruginosa242Negative2 ± 21 ± 02 ± 21 ± 298 ± 14b98 ± 14b2 ± 2
P. aeruginosa164Negative4 ± 62 ± 23 ± 22 ± 025 ± 2b90 ± 18b1 ± 2
  • a Cephalosporinase activity (nanomoles of cephalothin hydrolyzed per minute per milligram of protein) in sonic extracts of logarithmic-phase cultures treated for 2 h with normal saline (uninduced control), clavulanate (2, 8, 16, or 50 μg/ml), cefoxitin (50 μg/ml), or tazobactam (30 μg/ml). Data represent the mean ± 2 SDs for duplicate experiments.

  • b Cephalosporinase activity was significantly increased compared to that for uninduced controls.

  • c Cephalosporinase activity was significantly decreased compared to that for cultures treated with 16 μg of clavulanate per ml.