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

• Published in: Journal of antimicrobial chemotherapy 1995-06, Vol.35 (6), p.765-773
• Main Authors: JOHNSON, C. C, LIVORNESE, L, GOLD, M. J, PITSAKIS, P. G, TAYLOR, S, LEVISON, M. E
• Format: Article
• Language: English
• Subjects: Antibacterial agents; Antibiotics. Antiinfectious agents. Antiparasitic agents; beta-Lactamases - metabolism; Biological and medical sciences; Cefepime; Ceftazidime - pharmacology; Cephalosporin Resistance; Cephalosporins - pharmacology; Enterobacter; Enterobacter - drug effects; Enterobacter cloacae - drug effects; Gram-Negative Bacteria - drug effects; Gram-Negative Bacteria - enzymology; Kinetics; Medical sciences; Microbial Sensitivity Tests; Pharmacology. Drug treatments; Pseudomonas aeruginosa; Pseudomonas aeruginosa - drug effects
• ISSN: 0305-7453; 1460-2091
• 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.