An in Vivo Model for Evaluation of the Postantibiotic Effect

Abstract

A new experimental model to evaluate the postantibiotic effect (PAE) in vivo was developed using subcutaneously implanted tissue cages in rabbits with normal host defence mechanisms. The rabbits received benzylpenicillin i.v. in a dose giving a free penicillin concentration of 10 X MIC in the tissue cage fluid (TCF). A log-phase suspension of group A streptococci was injected into the tissue cages exposing them to penicillin in vivo. After 2 h bacterial samples were withdrawn, treated with penicillinase and transferred to 2 tissue cages in untreated rabbits. Simultaneously, unexposed streptococci were implanted in 2 other cages in the same animals. By repeated sampling of TCF, growth curves of the streptococci exposed to penicillin and the controls could be compared and a PAE of 1.6-2.4 h demonstrated. The PAE was of the same magnitude as that found in vitro. The model has several advantages for the demonstration of PAE in vivo: repeated samplings are easy to perform percutaneously, the effect of subinhibitory antibiotic concentrations are avoided, interindividual variations are eliminated since each animal is its own control, and the experiments can be performed in animals with undisturbed host defence mechanisms.

Penicillin Pharmacodynamics in Four Experimental Pneumococcal Infection Models

“…Three to 4 weeks after the implantation, the tissue cages had sealed with a thin layer of connective tissue and filled with clear, yellowish tissue cage fluid (TCF). Earlier studies have shown that the TCF albumin/serum albumin ratio corresponds very well to the skin albumin/plasma albumin ratio of 0.45 found in humans (3,23).…”

“…However, C max was much higher in serum than in lungs, with a ratio of 2/1 to 3/1. In contrast, the half-lives were considerably longer in lungs than in serum (20%), resulting in comparable T ϾMIC s. However, in the tissue cage, due to a large volumeto-surface area ratio, an even antibiotic concentration at the infection site is achieved during the induction phase (23). The ␤-lactams accumulated slowly in TCF, and the C max in TCF was lower than that in serum, but the half-life was much longer.…”

“…For the mouse models the bioassay organism was Micrococcus luteus ATCC 9341 and the growth medium was Mueller-Hinton agar (Difco Laboratories), and for the rabbit model the bioassay organism was Bacillus stearothermophilus ATCC 3032 and the growth medium was tryptose-glucose agar (23). Penicillin was dissolved in pooled normal mouse serum or in 50% rabbit serum for preparation of the standard curves, and the concentrations were derived from the standard curves.…”

“…A 5-cm incision was then made in the midline, and four well-separated pouches were bluntly dissected in the subcutaneous layer. An autoclaved cylindrical steel-net cage with a volume of 4 ml was implanted in each pouch (23). The incision was closed with sutures.…”

“…The four test strains, for which the MICs were different, then in the logarithmic phase of growth, were diluted 10 Ϫ2 in phosphate-buffered saline (PBS), and thereafter, 0.4 ml of the dilutions was injected into the four different cages, one strain in each cage, giving a starting inoculum in the cages of approximately 5 ϫ 10 5 CFU/ml. Since the C max of penicillin in the tissue cages is reached approximately 1 h after injection, antibiotic therapy was initiated 1 h before inoculation (23). The penicillin was administered by intravenous injection in the ear vein of one dose of 4, 7.5, 15, 75, or 150 mg of penicillin per kg of body weight.…”