Abstracts

AGAR DIFFUSION TESTS WITH CEFUROXIME DISKS FOR PREDICTING CEFTRIAXONE SUSCEPTIBILITY AMONG ISOLATES OF STREPTOCOCCUS PNEUMONIAE.

Cícero A.G. Dias1,3^* * Corresponding author. Mailind address: Fundação Faculdade Federal de Ciências Médicas de Porto Alegre, Rua Sarmento Leite, 245, CEP 90050-170, Porto Alegre, RS. Fax (+5551) 227-1899, E mail: oliver@pampa.tche.br , Ivonyr A. Kader¹, Pedro d’Azevedo^1,3, Silvana Superti², Dirce Alves², Gislaine Olm¹

¹Microbiologia e Virologia, Fundação Faculdade Federal de Ciências Médicas de Porto Alegre, Porto Alegre, RS, Brasil. ²Laboratório Central de Análises Clínicas da Irmandade Santa Casa de Misericórdia de Porto Alegre, Porto Alegre, RS, Brasil. ³Instituto de Microbiologia Prof. Paulo de Góes, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ

Submitted: February 02, 1998; Returned to authors for corrections: May 25, 1998;

Approved: July 23, 1998

SHORT COMMUNICATION

ABSTRACT

The performance of agar diffusion tests using disks of cefuroxime (30µg) for predicting ceftriaxone susceptibility in 33 isolates of Streptococcus pneumoniae was studied. All 7 resistant isolates to ceftriaxone (MIC ³1.0 µg/ml) exhibited zones of inhibition <28mm. The procedure can be easily adapted to clinical laboratories.

Key words: Screening test, resistance, pneumococcus.

Infections due to Streptococcus pneumoniae with resistance to third generation cephalosporins are being detected worldwide (2,4,6,7,11,12). Isolates of S. pneumoniae showing this behavior limit treatment options, especially in infections located in the central nervous system. Very few information is available about resistance to third generation cephalosporins among S. pneumoniae isolated in Brazil. Recently, however, intermediate resistance to ceftriaxone was detected in 2.8% (5/175) strains of S. pneumoniae isolated in the south of Brazil (3). Lack of more information about resistance of S. pneumoniae against third generation cephalosporin in Brazil may be, at least in part, due to the fact that the determination of minimal inhibitory concentration (MIC) of each particular isolate is necessary to discriminate between resistance (intermediate or frank) and susceptibility. The National Committee for Clinical Laboratory Standards (NCCLS) (9) does not recommend agar diffusion tests for ceftriaxone and cefotaxime for S. pneumoniae. Some investigators have studied the performance of agar diffusion tests using different cephalosporins to predict ceftriaxone and cefotaxime resistance among S. pneumoniae (5,8). According to a more recent study by Barry and Fuchs (1), 30µg cefuroxime disks would be preferred to predict susceptibility of S. pneumoniae to both cefotaxime and ceftriaxone. Such authors proposed that strains showing inhibition zones ³28 mm in diameter would be predictably susceptible to both drugs.

Our objective in the present study was to investigate the use of cefuroxime disks in agar diffusion tests for predicting ceftriaxone susceptibility among S. pneumoniae stains isolated in Porto Alegre, RS.

A total of 33 isolates of S. pneumoniae isolated in Porto Alegre, RS, was studied. All isolates showed intermediate (MIC between 0.12 and 1.0 µg/ml) or frank resistance (MIC>1.0 µ.g/ml) to penicillin. Agar diffusion tests using 30 µg cefuroxime disks were carried out on Mueller-Hinton agar with 5% defibrinated sheep blood, as recommended by the NCCLS (9). All isolated had theirs MICs to ceftriaxone determined by agar dilution tests on Mueller-Hinton agar with 5% defibrinated sheep blood containing different drug concentrations (in a range between 0.25 and 8.0 µg/ml) (10). Plates of both agar diffusion and agar dilution tests were read after 20-24 hours of incubation at 35ºC in CO₂.

We observed six isolates with MICs of 1.0 µg/ml (intermediate resistance) and one with MIC of 4.0 µg/ml (frank resistance) to ceftriaxone. All isolates with resistance to ceftriaxone showed inhibition zones <28 mm around 30 µg cefuroxime disks. Among those isolates with MICs < 1.0 µg/ml (susceptible) to ceftriaxone, 23/26 presented zones of inhibition <28 mm in the agar diffusion test. In three susceptible isolates, however, an inhibition zone < 28 mm was observed (Table 1). So, applying Barry and Fuchs’ (1) criteria to our results, we could say that all ceftriaxone resistant strains would have been detected by agar diffusion tests using 30 µg cefuroxime disks and would deserve MIC determination. On the other hand, only 3/26 (11.5%) susceptible strains were classified as "resistant" (zone < 28 mm). Although the study developed by Barry and Fuchs had tested a much larger number of isolates (n=199) containing a larger proportion of resistant strains than our study, the results are quite comparable. In that study, for instance, 18/143 (12.6%) ceftriaxone susceptible strains had inhibition zones < 28 (false "resistant"), as compared to 11.5% observed in our study.

The test using a surrogate disk of cefuroxime appears to be safe for predicting ceftriaxone susceptibilities of S. pneumoniae. This procedure can be adapted to the routine of clinical laboratories and the use of this test use would contribute to a better understanding about ceftriaxone resistance of S. pneumoniae in Brazil. However, we strongly suggest that strains with zones of inhibition <28mm must be submitted to a dilution procedure to determine MIC for confirmation of resistance and to discriminate between intermediate and frank resistance to ceftriaxone, in order to provide reliable susceptibility test results for the correct therapeutic assessment.

ACKNOWLEDGMENTS

The authors thank to Maria Beatriz Cirne Lima Guedes and Rosângela Agne for technical assistance and to the Quality Control Laboratory of the School of Pharmacy of Universidade Federal do Rio Grande do Sul for providing the ceftriaxone used in agar dilution tests.

RESUMO

Teste de difusão em ágar usando discos de cefuroxima para predizer suscetibilidade à ceftriaxona em Streptococcus pneumoniae

O desempenho do teste de difusão em ágar usando-se discos de cefuroxima (30 µg) para predizer suscetibilidade à ceftriaxona foi estudado em 33 cepas de Streptococcus pneumoniae. As 7 amostras resistentes à ceftriaxona (CIM ³1.0 µg/ml) apresentaram zonas de inibição < 28mm. O teste pode ser facilmente adaptado a laboratórios clínicos.

Palavras-chave: triagem, resistência, pneumococo.

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