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Antimicrobial Agents and Chemotherapy, August 2003, p. 2499-2506, Vol. 47, No. 8
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.8.2499-2506.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Selection of Resistant Streptococcus pneumoniae during Penicillin Treatment In Vitro and in Three Animal Models

Jenny Dahl Knudsen,^1,2* Inga Odenholt,³ Helga Erlendsdottir,⁴ Magnus Gottfredsson,^4,5 Otto Cars,⁶ Niels Frimodt-Møller,² Frank Espersen,² Karl G. Kristinsson,⁴ and Sigurdur Gudmundsson⁵

Department of Clinical Microbiology, Rigshospitalet,¹ Microbiological Research and Development, Statens Serum Institut, Copenhagen, Denmark,² Department of Infectious Diseases, University Hospital, Malmö, Sweden,³ Departments of Microbiology,⁴ Internal Medicine, Landspitalinn (University Hospital), Reykjavík, Iceland,⁵ Department of Infectious Diseases, University Hospital, Uppsala, Sweden⁶

Received 12 July 2002/ Returned for modification 17 February 2003/ Accepted 10 May 2003

Pharmacokinetic (PK) and pharmacodynamic (PD) properties for the selection of resistant pneumococci were studied by using three strains of the same serotype (6B) for mixed-culture infection in time-kill experiments in vitro and in three different animal models, the mouse peritonitis, the mouse thigh, and the rabbit tissue cage models. Treatment regimens with penicillin were designed to give a wide range of T_>MICs, the amounts of time for which the drug concentrations in serum were above the MIC. The mixed culture of the three pneumococcal strains, 10⁷ CFU of strain A (MIC of penicillin, 0.016 µg/ml; erythromycin resistant)/ml, 10⁶ CFU of strain B (MIC of penicillin, 0.25 µg/ml)/ml, and 10⁵ CFU of strain C (MIC of penicillin, 4 µg/ml)/ml, was used in the two mouse models, and a mixture of 10⁵ CFU of strain A/ml, 10⁴ CFU of strain B/ml, and 10³ CFU of strain C/ml was used in the rabbit tissue cage model. During the different treatment regimens, the differences in numbers of CFU between treated and control animals were calculated to measure the efficacies of the regimens. Selective media with erythromycin or different penicillin concentrations were used to quantify the strains separately. The efficacies of penicillin in vitro were similar when individual strains or mixed cultures were studied. The eradication of the bacteria, independent of the susceptibility of the strain or strains or the presence of the strains in a mixture or on their own, followed the well-known PK and PD rules for treatment with ß-lactams: a maximum efficacy was seen when the T_>MIC was >40 to 50% of the observation time and the ratio of the maximum concentration of the drug in serum to the MIC was >10. It was possible in all three models to select for the less-susceptible strains by using insufficient treatments. In the rabbit tissue cage model, a regrowth of pneumococci was observed; in the mouse thigh model, the ratio between the different strains changed in favor of the less-susceptible strains; and in the mouse peritonitis model, the susceptible strain disappeared and was overgrown by the less-susceptible strains. These findings with the experimental infection models confirm the importance of eradicating all the bacteria taking part in the infectious process in order to avoid selection of resistant clones.

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* Corresponding author. Mailing address: Damstein 7B, DK-2720 Vanloese, Denmark. Phone: 45 3874 9930. Fax: 45 3874 9932. E-mail: jdk{at}ssi.dk.

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Antimicrobial Agents and Chemotherapy, August 2003, p. 2499-2506, Vol. 47, No. 8
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.8.2499-2506.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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