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Medium-dependent variation in bactericidal activity of antibiotics against susceptible Staphylococcus aureus
Abstract:
Staphylococcus aureus resistant to bactericidal activity of antibiotics caused sepsis in three patients. Bacteriological and clinical responses were not achieved until serum and tissue fluid levels of administered antibiotics exceeded the minimum bactericidal concentration (MBC) of the infecting organism. Fifteen clinical isolates of S. aureus were tested in brain heart infusion broth and Mueller-Hinton broth for the MBC of gentamicin, vancomycin, clindamycin, oxacillin, cefazolin, and cephalothin. Results showed significant eightfold or greater broth-dependent differences in the MBC of at least one antibiotic against 87% (13/15) of strains tested. The MBC was unpredictable and varied with the strain, antibiotic, and medium used. No controlled studies are available to indicate the clinical significance of the MBC demonstrated in different media. The necessity for treating serious infection with bactericidal drugs has not yet been established; however, in septicemia such as that caused by bacterial endocarditis, bacteriostatic antibiotics have generally failed to eradicate the infection, whereas bactericidal agents have often been curative. Therefore, in patients unresponsive to usual antistaphylococcal therapy, we suggest that MBC testing be performed in at least two media and that treatment be instituted with antibiotics demonstrating the lowest MBC in all media used.
Insights
Antibiotic resistance in Staphylococcus aureus can cause sepsis. Minimum bactericidal concentration (MBC) testing in multiple media is crucial for effective treatment when standard therapies fail.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Antibiotic-resistant Staphylococcus aureus poses a significant threat, causing severe infections like sepsis.
- Treatment failure occurred in patients until antibiotic levels exceeded the minimum bactericidal concentration (MBC) of the pathogen.
Purpose of the Study:
- To investigate the variability of MBCs for S. aureus against common antibiotics.
- To assess the impact of different culture media on MBC determination.
- To provide recommendations for optimizing antibiotic selection in resistant S. aureus infections.
Main Methods:
- Minimum bactericidal concentrations (MBCs) were determined for 15 S. aureus clinical isolates.
- Testing involved gentamicin, vancomycin, clindamycin, oxacillin, cefazolin, and cephalothin.
- MBCs were assessed using two different broth media: brain heart infusion broth and Mueller-Hinton broth.
Main Results:
- Significant, eightfold or greater broth-dependent differences in MBCs were observed for at least one antibiotic against 87% of tested strains.
- MBC values were unpredictable, varying significantly based on the bacterial strain, antibiotic, and culture medium used.
- No controlled studies currently validate the clinical significance of MBCs determined in different media.
Conclusions:
- The choice of culture medium significantly impacts MBC results for S. aureus, highlighting the need for standardized testing.
- In cases of severe or unresponsive S. aureus infections, performing MBC testing in multiple media is recommended.
- Treatment should prioritize antibiotics demonstrating the lowest MBC across all tested media to ensure efficacy.