Differential distributions in tissues and efficacies of aztreonam and ceftazidime and in vivo bacterial morphological changes following treatment

Concentration-dependency of beta-lactam-induced filament formation in Gram-negative bacteria

Ceftazidime and cefotaxime are beta-lactam antibiotics with dose-related affinities for penicillin-binding protein (PBP)-3 and PBP-1. At low concentrations, these antibiotics inhibit PBP-3, leading to filament formation. Filaments are long strands of non-dividing bacteria that contain enhanced quantities of endotoxin molecules. Higher concentrations of ceftazidime or cefotaxime cause inhibition of PBP-1, resulting in rapid bacterial lysis, which is associated with low endotoxin release. In the present study, 37 isolates of Escherichia coli, Klebsiella spp., Pseudomonas aeruginosa and Acinetobacter spp. were studied over a 4-h incubation period in the presence of eight concentrations of ceftazidime or cefotaxime. As resistance of Gram-negative bacteria is an emerging problem in clinical practice, 14 isolates of E. coli and Klebsiella pneumoniae that produced extended-spectrum beta-lactamases (ESBLs) were also investigated. Morphological changes after exposure to the beta-lactam antibiotics revealed recognisable patterns in various bacterial families, genera and isolates. In general, all isolates of Enterobacteriaceae produced filaments within a relatively small concentration range, with similar patterns for E. coli and K. pneumoniae. Pseudomonas and Acinetobacter spp. produced filaments in the presence of clinically-relevant concentrations of both antibiotics as high as 50 mg/L. In all genera, filament-producing capacity was clearly related to the MIC. Ceftazidime induced filament production in more isolates and over wider concentration ranges than did cefotaxime. Interestingly, ESBL-producing isolates were not protected against filament induction. The induction of filament production may lead to additional risks during empirical treatment of severe infections.

[Is aztreonam could become "a challenger" for treatment of Gram negative infections? May be an answer from microbiological data: in vitro study including 36,485 strains of enterobacteria and Pseudomonas]

During 6 years, 2000 to 2006, we looked for evolution of antimicrobial resistance to beta-lactams of clinical isolates from patients with Gram negative infections. In vitro susceptibilities to piperacillin+tazobactam, ceftazidime, and aztreonam were followed along the period. Enterobacteriaceae of group 1 to 3 represent 24,884 strains and Pseudomonas spp. 11601. The global sensitivity of aztreonam was 85%, 87.1% for ceftazidime and 88.8% for pieracilline-tazobactam. Of course, aztreonam spectrum of activity is dedicated to Gram negative bacteria only but it is known to have a great penetration ratio in tissues and a half-live observed up to 6 times higher than in serum in experimental models. Antimicrobial rotation strategy is recommended in the prudent use of antibiotics. It seems that in our hospital a consistent increase of the use of aztreonam might be possible and proposed without increasing risk for infection control practices according to the microbiological laboratory results.