Studies on Determination of Minimum Bactericidal Concentrations of Penicillin-G for β-Hemolytic Streptococci

Abstract

Minimum bactericidal concentrations (MBC) of penicillin-G were determined for 94 isolates of 0-hemolytic streptococci: 37 group A; 40 group B; 6 group C; and 11 group G. Our experiments focused on the influence of technical variable factors on the outcome of the MBC test such as inoculum preparation, manner of inoculation, technique of sampling for survivors, growth phase of inoculum, final concentration of inoculum, and time of incubation.
The results were summarized as follows:
1. The mean MBC, ranged from 0.0145μg/ml (group A) to 0.084μg/ml (group B). The MBC/MIC ratio ranged from 1-to 4-fold (3 strains: 2 group A, 1 group C), showing no penicillin-G tolerance.
2. Logarithmic-phase inocula used were prepared by incubating 10 ml Mueller-Hinton broth (MHB) in water bath at 37°C for 4 to 5 h after adding 0.01 ml of 24 h-incubated cultures.
3. 0.1 ml of logarithmic-phase inocula were released on the surface of penicillin-containing MHB (1 ml) without shaking or splashing.
4. It was important to resuspend the organisms by vortexing 4 h before tubes were sampled so that any organisms contaminating glassware above the meniscus were exposed to antibiotic.
5. No carry-over effect was observed with 0.01 ml samples.
6. Effect of bacterial growth phase on outcome of MBC test was not confirmed.
7. All tests used final volumes of 1.1 ml of MHB containing 10⁵ cells/ml and 24-hincubation time.