In vitro demonstration of a true post-beta-lactamases inhibitor effect (PLIE) of clavulanic acid on Klebsiella pneumoniae and Haemophilus influenzae.

In vivo, serum concentrations of beta-lactamase inhibitors measured during the last part of the dosing interval are below the levels associated with i n vitro activity. Nevertheless, beta-lactam plus beta-lactamase inhibitor c ombinations remain active in vivo throughout the dosing interval. One of th e many reasons for this contradiction may be the PLIE, The PLIE can be eval uated only in the light of the postantibiotic effect (PAE). Also, accurate determination of the PLIE requires a careful investigation of all bacterial regrowth delays (BRDs) inherent to the technical procedures used. The purp ose of the study reported herein was to determine the true in vitro PLIE of clavulanic acid (CA) against two beta-lactamase-producing strains, a Klebs iella pneumoniae strain (amoxicillin [AMX] MIC > 256 mg/L; CA MIC=64 mg/L; and AMX+CA MIX=4 mg/L) and a Haemophilus influenzae strain (AMX MIC=32 mg/L ; CA>32 mg/L; AMX-CA=1 mg/L). For each strain, a stationary phase inoculum of 10(7) was preexposed for 2 h to either CA alone or CA + AMX in various c oncentrations. Dilution to 10(-2) or 10(-3) was performed to eliminate the CA and/or AMX after the preexposition phase. Hourly bacterial counts were d one between 0 and 8 h and after 24 h. Control cultures exposed to AMX after dilution showed a growth delay possibly ascribable to the time needed for bacteria to produce a large enough amount of beta-lactamases. control exper iments were done to unequivocally differentiate PLIE from PAE and BRD. The true PLIE values thus obtained ranged from 0 to 4.5 h for K. pneumoniae and from 0 to 15 h for H. influenzae. For both strains, a PLIE was demonstrate d after exposure to CA alone.