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Antimicrob Agents Chemother. 1979 August; 16(2): 190-197
Copyright © 1979, American Society for Microbiology. All Rights Reserved.

Antimicrobial Susceptibility Testing of Pneumococci: Determination of Kirby-Bauer Breakpoints for Penicillin G, Erythromycin, Clindamycin, Tetracycline, Chloramphenicol, and Rifampin

M. R. Jacobs{dagger}, Y. Mithal, R. M. Robins-Browne, M. N. Gaspar and H. J. Koornhof

Department of Microbiology, School of Pathology of the South African Institute for Medical Research and the University of the Witwatersrand, Johannesburg, South Africa

ABSTRACT

Antimicrobial susceptibility testing of pneumococci is now essential to monitor for the presence of resistance to agents such as the penicillins, macrolides, lincomycins, chloramphenicol, and tetracycline. In this study, clinical isolates of a selection of resistant South African strains were tested for antimicrobial susceptibility by minimal inhibitory concentration (MIC) determination and by a modified Kirby-Bauer disk diffusion technique, using Mueller-Hinton medium supplemented with 5% horse blood. Disk diffusion breakpoints were determined for penicillin G, erythromycin, clindamycin, tetracycline, chloramphenicol, and rifampin. Reliable results were obtained on disk diffusion for all these agents except for penicillin G. With 6-µg penicillin G disks, zones of strains with intermediate penicillin susceptibility overlapped those of sensitive and resistant strains. With 5-µg methicillin disks, clearer separation of strains based on susceptibility to penicillin G occurred. Strains with zones of <35 mm around penicillin G disks and <25 mm around methicillin disks should have penicillin G MICs determined to confirm their resistance to penicillin G. In view of the potential for pneumococci to be resistant to the agents used in this study, antimicrobial susceptibility of all clinically significant isolates should be determined.

FOOTNOTES

{dagger} Present address: Institute of Pathology, Case Western Reserve University, Cleveland, OH 44106.

Antimicrob Agents Chemother. 1979 August; 16(2): 190-197
Copyright © 1979, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Jacobs, M., Bajaksouzian, S, Palavecino-Fasola, E., Holoszyc, H., Appelbaum, P. (1998). Determination of penicillin MICs for Streptococcus pneumoniae by using a two- or three-disk diffusion procedure [In Process Citation]. J. Clin. Microbiol. 36: 179-183 [Abstract] [Full Text]  
  • Pallares, R., Linares, J., Vadillo, M., Cabellos, C., Manresa, F., Viladrich, P. F., Martin, R., Gudiol, F. (1995). Resistance to Penicillin and Cephalosporin and Mortality from Severe Pneumococcal Pneumonia in Barcelona, Spain. NEJM 333: 474-480 [Abstract] [Full Text]  
  • Rauch, A. M., O'Ryan, M., Van, R., Pickering, L. K. (1990). Invasive Disease due to Multiply Resistant Streptococcus pneumoniae in a Houston, Tex, Day-care Center. Arch Pediatr Adolesc Med 144: 923-927 [Abstract]  


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