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Importance of medium in demonstrating penicillin tolerance by group B streptococci
Abstract:
A total of 30 clinical isolates of group B streptococci were studied for penicillin tolerance in vitro. Minimal inhibitory and bactericidal concentrations of penicillin were determined simultaneously in three test media which have been used for group B streptococci, tryptose phosphate, Mueller-Hinton, and Todd-Hewitt broths, using a logarithmic-phase inoculum of 10(5) colony-forming units per ml. Minimal inhibitory concentrations in the three media did not differ significantly. However, minimal bactericidal concentrations were significantly higher in tryptose phosphate broth (mean, 1.04 mug/ml) than in Mueller-Hinton broth (0.22 mug/ml) or Todd-Hewitt broth (0.15 mug/ml). Similarly, ratios of minimal bactericidal to minimal inhibitory concentrations were significantly greater in tryptose phosphate broth than in Mueller-Hinton or Todd-Hewitt broth. After incubation in tryptose phosphate broth for an additional 24 h, the minimal bactericidal concentration consistently fell to levels which were only twice or equal to the minimal inhibitory concentration. This study illustrates the importance of the medium in the demonstration of penicillin tolerance and of controlling laboratory variables in the susceptibility testing of group B streptococci with penicillin.
Insights
Penicillin tolerance in group B streptococci varies significantly based on the culture medium used. Tryptose phosphate broth showed higher penicillin tolerance compared to Mueller-Hinton or Todd-Hewitt broths.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Group B Streptococcus (GBS) is a significant pathogen, particularly in neonates.
- Penicillin is the primary antibiotic for GBS infections.
- Understanding penicillin tolerance is crucial for effective treatment.
Purpose of the Study:
- To investigate the in vitro penicillin tolerance of clinical isolates of GBS.
- To evaluate the impact of different culture media on penicillin tolerance determination.
Main Methods:
- Studied 30 clinical GBS isolates.
- Determined penicillin's minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs).
- Tested in three media: tryptose phosphate, Mueller-Hinton, and Todd-Hewitt broths.
Main Results:
- MICs of penicillin were similar across all tested media.
- MBCs were significantly higher in tryptose phosphate broth compared to Mueller-Hinton and Todd-Hewitt broths.
- The ratio of MBC to MIC was significantly greater in tryptose phosphate broth, indicating higher tolerance.
Conclusions:
- The choice of culture medium significantly influences the demonstration of penicillin tolerance in GBS.
- Laboratory variables, especially the medium, must be controlled for accurate penicillin susceptibility testing of GBS.
- Further research may be needed to optimize susceptibility testing protocols for GBS.