Pharmacodynamic study of beta-lactams alone and in combination with beta-lactamase inhibitors against Pseudomonas aeruginosa possessing an inducible beta-lactamase.

OBJECTIVES

The antimicrobial efficacies of beta-lactams alone and in combination with beta-lactamase inhibitors were investigated by applying a rabbit tissue cage model against a strain of Pseudomonas aeruginosa with an inducible AmpC (iAmpC) beta-lactamase.

METHODS

Two sterilized golf Wiffle balls were surgically implanted in the rabbit dorsal cervical area. After 4 weeks, Wiffle balls had filled with tissue cage fluid (TCF), in which 2 mL of 10(6) cfu/mL of the test isolate were inoculated. To achieve the same T > MIC as in humans, 400 mg/kg of the beta-lactams alone and in combination was administered twice a day via subcutaneous injection. The dosing regimens were as follows: piperacillin alone, 4 g piperacillin/0.5 g tazobactam; ticarcillin alone, 3 g ticarcillin/0.1 g clavulanate; and 3 g ticarcillin/ 0.3 g clavulanate.

RESULTS

The changes in bacterial counts (log cfu/mL) after the 3 day treatments were as follows: 1.03 +/- 0.97 (control), -1.31 +/- 0.61 (piperacillin), -2.81 +/- 0.53 (4 g piperacillin/0.5 g tazobactam), -1.61 +/- 0.68 (ticarcillin), -3.42 +/- 0.75 (3 g ticarcillin/0.1 g clavulanate) and -1.65 +/- 1.47 log cfu/mL (3 g ticarcillin/0.3 g clavulanate). AmpC induction by high-dose clavulanate was observed in rabbit TCF, and was confirmed by the in vitro induction study.

CONCLUSIONS

The study indicated that tazobactam significantly enhanced the antibacterial activity of piperacillin against iAmpC P. aeruginosa; clavulanate had synergy with the antibacterial activity of ticarcillin at low concentration, but had no effect on ticarcillin at high concentration due to AmpC induction by clavulanate.