Bactericidal effects of ticarcillin-clavulanic acid against Legionella pneumophila pneumonia in immunocompromised weanling rats

Abstract

A model of acute LegioneUla pneumophila pneumonia in neutropenic weanling rats was developed as a means of assessing the efficacies in vivo of the ,I-lactams ticarcillin, ticarcillin-clavulanic acid, and clavulanic acid, agents active against the organism in vitro. Weanling rats were dosed with cyclophosphamide 3 days before and immediately prior to infection by intrabronchial intubation with L. pneumophila. The bacteria persisted in the lungs of untreated animals at high counts (5.0 to 7.0 loglo CFU/g of lung tissue) for up to 168 h after infection, and the histological characteristics of the infection were similar to those of the disease in humans. Transmission electron micrography revealed the presence of L. pneumophila multiplying within alveolar macrophages. Therapy with ticarcillin was ineffective in reducing the bacterial numbers in the lung tissue, whereas ticarcillinclavulanic acid and clavulanic acid were active, producing bactericidal effects similar to those of erythromycin. The ticarcillin-clavulanic acid combination was significantly more efficacious (P < 0.01) than corresponding doses of clavulanic acid alone. Synergistic activity between ticarciflin and clavulanic acid against L. pneumophila has been demonstrated in vivo, and the combination showed activity similar to that of erythromycin.The 1-lactamase inhibitor clavulanic acid displays only a low order of antibacterial activity against most pathogenic bacteria, but an exception is Legionella pneumophila, which is susceptible to low concentrations of the compound (16). In addition, synergy has been demonstrated between P-lactams and clavulanic acid in vitro (8, 10), and both the inhibitor and the combination of amoxicillin and clavulanic acid have been shown to produce bactericidal effects against intracellular L. pneumophila in tissue culture studies in vitro (17) and in immunocompetent rats (13).These findings suggested that studies with antibiotics against experimental L. pneumophila infections were warranted, but initial studies with clavulanic acid combinations showed that the conventional guinea pig model of legionellosis was unsuitable for this purpose because of the sensitivity of this animal species to P-lactam antibiotics. For this reason, we have developed an experimental model of L. pneumophila pneumonia with neutropenic weanling rats, suitable for the evaluation of ,-lactams and other agents.The studies reported here were designed to investigate the activity of clavulanic acid and the combination of ticarcillin plus clavulanic acid against L. pneumophila in vitro and in the neutropenic rat model of legionellosis. In vitro susceptibility tests. MICs were determined by serial dilution of the compounds into buffered charcoal-yeast extract agar (Oxoid) (1). Inocula were prepared by harvesting growth of L. pneumophila on agar plates after 72 h of incubation into 1 ml of Mueller-Hinton broth (BBL) and adjusting the turbidity to that of a 0.5 McFarland barium sulphate turbidity standard. Volumes (0.003 ml) were inoculated onto each plate,...

Comparative activities of piperacillin and tazobactam against clinical isolates of Legionella spp

“…Several P-lactam antibiotics actually demonstrate activity against Legionella spp. in vitro on various media (9,11), but a number of these are ineffective when examined against intracellular Legionella strains in vitro or in animal models of infection with these organisms (3,13,14,16). A notable exception to these observations is the fact that both clavulanic acid alone and combinations of this agent with either amoxicillin or ticarcillin have demonstrated activity against cell-associated Legionella pneumophila in vitro and in experimental animal models (13,14,16).…”

“…In these studies, neither amoxicillin nor ticarcillin alone showed activity against cell-associated organisms. Furthermore, in an animal model, the combination of clavulanate with either amoxicillin or ticarcillin proved comparable to erythromycin in eradication of organisms within pulmonary alveolar macrophages (13,14). A small but significant advantage was noted for the combination of ticarcillin with clavulanic acid compared with clavulanate alone (13).…”

“…Both in tissue culture (16) and in a weanling rat model of infection (13,14), clavulanic acid has proven effective in reducing numbers of intracellular L. pneumophila organisms. In these studies, neither amoxicillin nor ticarcillin alone showed activity against cell-associated organisms.…”

Development of experimental respiratory infections in neutropenic rats with either penicillin-resistant Streptococcus pneumoniae or beta-lactamase-producing Haemophilus influenzae

“…To date, we can find no model of H. influenzae type b pneumonia in rats, and the method described here is a modification of a simple, nonsurgical technique (9) used previously to produce acute, shortterm respiratory infections in immunocompetent weanling rats with penicillin-susceptible S. pneumoniae (11). In later studies, immunocompromised weanling rats were used to enhance infection with Legionella pneumophila (10) (9). Lungs (mean weight, 1 g) from five rats from each treatment group and the untreated group were sampled at various times after infection and homogenized in 1 ml of nutrient broth in a Colworth stomacher for 1 min.…”

“…Rats infected with H. influenzae H128 received an oral dose of 0.5 ml of amoxicillin at 100 mg/kg or amoxicillin-clavulanate at 100 and 50 mg/kg, respectively, and rats infected with S. pneumoniae N1387 received amoxicillin at 100, 200, or 350 mg/kg. Doses were selected to produce an area under the plasma concentration-time curve of weanling rats similar to that produced in humans following administration of a standard oral dose (10). Therapy commenced at 24 h postinfection and continued three times a day (at 1000, 1600, and 2200 h) for 3 days.…”