Acute respiratory infections with penicillin-resistant strains of Streptococcus pneumoniae and a 13-lactamaseproducing strain of Haemophilus influenzae were established in neutropenic weanling rats. By use of nonsurgical intrabronchial instillation of the bacteria suspended in molten agar, reproducible, acute respiratory infections suitable for experimental antibiotic efficacy studies were established.Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, and penicillin-resistant strains have been isolated with increasing frequency in recent years (1) and have been shown to be resistant to a wide range of ,B-lactam and macrolide antibiotics (5). Haemophilus influenzae is recognized as the second most common pathogen, after S. pneumoniae, with approximately 30% of the strains producing 3-lactamase (4). Consequently, there is a need for models of infection to predict efficacy against these emerging respiratory pathogens. Although a mouse model of H. influenzae infection does exist (3), establishing suitable respiratory infection models in rats has proven difficult because of the lack of virulence of these organisms, in particular, penicillin-resistant S. pneumoniae, in this species. One of the commonly used methods for producing respiratory infections in rats involves introduction of agar beads enmeshed with an organism, e.g., Pseudomonas aeruginosa, via tracheotomy, was described by Cash et al. (2). A rat model of prolonged pulmonary infection with nontypeable H. influenzae was also developed by using this technique (6), and more recently, the model was modified by using a suspension of the organism as a semisolid agar gel (8), although inoculation required surgical intervention and resulted in an infection which was chronic. 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), but even under these conditions, we were unable to produce infections with penicillin-resistant S. pneumoniae and H. influenzae. In the studies reported here, a ,B-lactamase-producing clinical isolate of H. influenzae H128 (MICs: amoxicillin, 32 jig/ml; amoxicillin-clavulanic acid [2:1], 0.5 and 0.25 ,ug/ml, respectively) and a penicillin-resistant non-,-lactamase-producing strain of S. pneumoniae N1387 (amoxicillin MIC, 2 ,ug/ml) were used. (Oxoid) maintained molten at 40°C by nonsurgical intrabronchial instillation via intratracheal intubation (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. Serial dilutions were plated onto chocolate agar (H. influenzae) or blood agar (S. pneumo...

Amoxycillin/clavulanic acid and clavulanic acid have been previously reported to demonstrate bactericidal activity in tissue culture studies against intracellular Legionella pneumophila. A rat model of legionellosis was therefore developed for the purpose of assessing the efficacy of these agents against L. pneumophila in vivo. Therapy by the subcutaneous route was started 12 h after infection when the majority of the bacteria observed in lavage fluid were residing in alveolar macrophages. Treatment with amoxycillin was ineffective in reducing the bacterial counts of L. pneumophila in lung homogenates whereas amoxycillin/clavulanic acid displayed bactericidal effects of the same order as the control antibiotic, erythromycin. Further in-vivo studies are planned to assess the clinical relevance of these findings.

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,...

No abstract

High doses of amoxicillin, equivalent to those produced by 500- and 750-mg oral doses in humans (area under the plasma concentration-time curve), were effective against a penicillin-resistant strain ofStreptococcus pneumoniae in an experimental respiratory tract infection in immunocompromised rats; this superior activity confirms the results of previous studies. An unexpected enhancement of amoxicillin’s antibacterial activity in vivo against penicillin-resistant and -susceptible S. pneumoniaestrains was observed when subtherapeutic doses of amoxicillin were coadministered with the β-lactamase inhibitor potassium clavulanate. The reason for this enhancement was unclear since these organisms do not produce β-lactamase. The differential binding of clavulanic acid and amoxicillin to penicillin-binding proteins may have contributed to the observed effects.