Experimental infections for the evaluation of beta-lactamase resistance

Abstract

The antibacterial activity and pharmacokinetics of the P-lactamase-stable cephalosporin cefuroxime and the gram-negative 3-lactamase-susceptible cephalosporin cefazolin were compared in two contrasting infection models in which Proteus morganii 82, which produces chromosomally mediated 3-lactamase, was the pathogen. In the rat paw model, characterized by high numbers of localized bacteria, cefazolin was destroyed at the site of infection and consequently did not produce a therapeutic response. In the mouse intraperitoneal model cefazolin was also inactive, despite peritoneal concentrations being unaffected by high counts of the 3-lactamase-producing P. morganii in the body cavity. In contrast the pharmacokinetics of cefuroxime was unaffected by the presence of the Ilactamase-producing P. morganii, and good therapeutic responses were seen in both models.During the past few years numerous cephalosporins have been introduced which have a broader antibacterial spectrum than earlier cephalosporins and greater stability in the presence of bacterial P-lactamases. The latter characteristic is important because of the increasing incidence of 3-lactamase-producing clinical isolates. It is therefore useful to have suitable experimental models for the correct evaluation of this characteristic. In vitro techniques, e.g., stability of a new P-lactam antibiotic in the presence of partially purified bacterial ,B-lactamases or the effects of bacterial inoculum size on the minimal inhibitory concentration, give some indication of the P-lactamase stability of a new antibiotic. However, in vitro conditions which are optimal for antibiotic-bacteria interactions do not necessarily extrapolate to patients where factors such as the number of bacterial cells at a site of infection and the tissue distribution of the antibiotic have an important bearing on the outcome of therapy.In this study we have evaluated, in two experimental animal models, the influence of Plactamase on the tissue distribution and the rate of bacterial killing by the P-lactamase-stable cephalosporin cefuroxime (3) and the gram-negative, 3-lactamase-sensitive cephalosporin cefazolin. MATERIALS AND METHODSAntibiotics. Cefuroxime sodium salt was prepared by Glaxo; cefazolin (Totacef; Bristol Laboratories) was purchased.Bacterial strain. The strain of Proteus morganii 82 used in the experiments was characterized by high minimal inhibitory concentrations of both cefuroxime (62 pg/ml) and cefazolin (>125 .g/ml). P. morganii 82 produced a chromosomally mediated P-lactamase with an isoelectric point of 7.6 as determined by isoelectric focusing on polyacrylamide gel (2). This enzyme, in the experimental conditions of the iodometric method used, hydrolyzed cefazolin (216 pmol/h), but no hydrolysis of cefuroxime was detectable (4). Paw infection model. Twenty-four Charles River CD (Cobs) male rats weighing 200 g were used. Three groups of four animals were infected by inoculating into the plantar dermis of the right hind paw 0.1 ml of a 10-fold-concentrated overnight culture o...

Reduction of cephamycin concentrations at the infection site in mice with experimental peritoneal infection caused by cephalosporinase-producing bacteria

“…However, there has been no report dealing with the stability of cephamycins to cephalosporinases in vivo, except that by Xerii (7). In this study, to clarify the in vivo stability of cephamycins to cephalosporinases, we measured the concentrations of cephamycins (cefoxitin [4], cefmetazole [6], and cefbuperazone [5]) and cefazolin, a cephalosporinasesusceptible cephalosporin, in peritoneal fluid of mice with experimental peritoneal infection caused by each of two bacteria producing different types of cephalosporinase.…”