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ABSTRACTDuring 1994 and 1995, 157 isolates of Streptococcus pyogenesfrom patients with invasive disease were consecutively collected in the San Francisco Bay area to determine the frequency of antimicrobial resistance. Susceptibility testing was performed according to the guidelines of the National Committee for Clinical Laboratory Standards by the disk method and by broth microdilution. For comparison of susceptibility patterns, an additional 149 strains were randomly collected from patients with pharyngitis. For San Francisco County, 32% of the isolates from invasive-disease-related specimens but only 9% of the isolates from throat cultures from the same period were resistant to erythromycin (P= 0.0007). Alameda County and Contra Costa County had rates of resistance of =10% from isolates from all cultures. When the data were analyzed by hospital, the San Francisco County Hospital had a statistically higher rate of erythromycin resistance (39%) among the strains from serious infections compared to those from other counties (P = <0.0003). For tetracycline, high rates of resistance were observed in San Francisco County for both isolates from patients with invasive disease (34%) and pharyngitis (21%) in the same period. Using pulsed-field gel electrophoresis, two clones, one at the San Francisco County Hospital and a second in the entire area, were identified. The latter clone exhibited resistance to bacitracin. Of 146 strains that were tested by microdilution, all were susceptible to penicillin. Clindamycin resistance was not seen among the erythromycin-susceptible strains, but two of the 39 erythromycin-resistant strains were also resistant to clindamycin. An additional 34 strains showed resistance to clindamycin when exposed to an erythromycin disk in the double-disk diffusion test, suggesting that the mechanism of erythromycin resistance is due to an ermgene. This study demonstrates a high rate of resistance to macrolides and tetracycline among S. pyogenesisolates in San Francisco County and shows that macrolide resistance is more common in strains from patients with invasive disease than in strains from those with pharyngitis.

Microdilution MIC test trays containing 11 beta-lactam antibiotics in Mueller-Hinton broth at pH 7.31 or 6.80 were prepared and stored at 4, -10, -25, and -70 degrees C. The drugs tested were ampicillin, ticarcillin, mezlocillin, piperacillin, azlocillin, cefazolin, cefotaxime, moxalactam, cefoperazone, ceftriaxone, and imipenem. MICs for Staphylococcus aureus ATCC 29213, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 were determined at weekly intervals for up to 1 year. The data from the MIC determinations showed the stability of antimicrobial activity over time to be -70 degrees C greater than -25 degrees C approximately 4 degrees C much greater than -10 degrees C. The relative stability at 4 degrees C as compared with that at -10 degrees C cannot be explained by desiccation, as determined by changes in broth sodium concentrations. The relative instability at -10 degrees C may have been caused in part by a temperature fluctuation, resulting in intermittent freezing and thawing of the antibiotics. Some of the drugs appeared to be more stable when diluted in broth at pH 6.80, but endpoints were more difficult to read. Cefazolin and cefoperazone were stable at all four storage temperatures. Cefotaxime, moxalactam, and ceftriaxone also were relatively stable. The other drugs showed moderately rapid to rapid deterioration at each temperature except -70 degrees C. Storage at -25 degrees C is suitable for up to 3 months for many, but not all, beta-lactams; -10 degrees C appears to be unsuitable. Storage at -70 degrees C is recommended.