Activity of cefepime against ceftazidime-resistant gram-negative bacilli using low and high inocula

J Antimicrob Chemother. 1995 Jun;35(6):765-73. doi: 10.1093/jac/35.6.765.


Cefepime is a broad-spectrum cephalosporin that is reported to have enhanced activity against ceftazidime-resistant Gram-negative bacilli. In this study the effects of varying inoculum size on in-vitro susceptibility to cefepime and other selected antimicrobial agents were determined by agar dilution MICs and in time-kill studies. Among strains of Pseudomonas aeruginosa (n = 55) and Enterobacter spp (n = 56) that had previously been identified as ceftazidime-resistant, 73% and 96% were susceptible to cefepime (MIC < or = 16 mg/L), respectively, when tested with an inoculum of 10(4) cfu. However, with an inoculum of 10(7) cfu, 98% and 100% of strains were resistant, respectively. Furthermore, the bactericidal activity of cefepime against ceftazidime-resistant isolates was also inoculum-dependent. In time-kill studies, bactericidal action was obtained only at the lowest concentration of organisms (10(4) cfu/mL). beta-Lactamase extracted from an isolate of P. aeruginosa that demonstrated an inoculum effect had a lower affinity for cefepime than for ceftazidime. Overall, cefepime proved to be more resistant to hydrolysis by the beta-lactamase. However, differences in kinetics of the beta-lactamase against cefepime or ceftazidime do not appear to be of consequence in determining susceptibility of P. aeruginosa and Enterobacter spp. at high bacterial densities, since most strains with chromosomally-mediated beta-lactamase are highly resistant.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cefepime
  • Ceftazidime / pharmacology*
  • Cephalosporin Resistance*
  • Cephalosporins / pharmacology*
  • Enterobacter / drug effects
  • Enterobacter cloacae / drug effects
  • Gram-Negative Bacteria / drug effects*
  • Gram-Negative Bacteria / enzymology
  • Kinetics
  • Microbial Sensitivity Tests
  • Pseudomonas aeruginosa / drug effects
  • beta-Lactamases / metabolism


  • Cephalosporins
  • Cefepime
  • Ceftazidime
  • beta-Lactamases