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Predicting Oral Beta-lactam susceptibilities against Streptococcus pneumoniae
Mark E Murphy1,2, Eleanor Powell3, Joshua Courter4
1Division of Infectious Diseases, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, USA.
Background:
Oral beta-lactam antimicrobials are not routinely tested against Streptococcus pneumoniae due to presumed susceptibility based upon penicillin minimum inhibitory concentration (MIC) testing. Currently, Clinical and Laboratory Standards Institute provides comments to use penicillin MIC ≤0.06 to predict oral cephalosporin susceptibility. However, no guidance is provided when cefotaxime MIC is known, leading to uncertainty with interpretation. The purpose of this study was to evaluate cefotaxime and penicillin MICs and their respective correlation to oral beta-lactam categorical susceptibility patterns.
Methods:
249 S. pneumoniae isolates were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF) and then tested by broth microdilution method to penicillin, cefotaxime, amoxicillin, cefdinir, cefpodoxime, and cefuroxime.
Results:
Using Clinical and Laboratory Standards Institute (CLSI) non-meningitis breakpoints for cefotaxime, 240/249 isolates were classified as susceptible. Of the cefotaxime susceptible isolates, 23% of the isolates are misrepresented as cefdinir susceptible. Amoxicillin correlated well with penicillin MIC breakpoints with only 1 discordant isolate out of 249.
Conclusion:
The correlation between amoxicillin and penicillin creates a very reliable predictor to determine categorical susceptibility. However oral cephalosporins were not well predicted by either penicillin or cefotaxime leading to the possible risk of treatment failures. Caution should be used when transitioning to oral cephalosporins in cefotaxime susceptible isolates, especially with higher cefotaxime MICs.
Insights
Penicillin MICs reliably predict amoxicillin susceptibility in Streptococcus pneumoniae. However, neither penicillin nor cefotaxime MICs accurately predict oral cephalosporin susceptibility, risking treatment failure.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- Routine testing of oral beta-lactam antimicrobials against Streptococcus pneumoniae is uncommon, relying on penicillin minimum inhibitory concentration (MIC) for susceptibility predictions.
- Clinical and Laboratory Standards Institute (CLSI) guidance suggests using penicillin MIC ≤0.06 to predict oral cephalosporin susceptibility, but lacks recommendations for cefotaxime MIC interpretation.
- This uncertainty highlights a need to evaluate the correlation between cefotaxime and penicillin MICs and their predictive value for oral beta-lactam susceptibility patterns.
Purpose of the Study:
- To evaluate the correlation between cefotaxime and penicillin minimum inhibitory concentrations (MICs).
- To assess the predictive accuracy of these MICs for oral beta-lactam categorical susceptibility patterns in Streptococcus pneumoniae.
Main Methods:
- Identified 249 Streptococcus pneumoniae isolates using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-ToF).
- Determined minimum inhibitory concentrations (MICs) via broth microdilution for penicillin, cefotaxime, amoxicillin, cefdinir, cefpodoxime, and cefuroxime.
Main Results:
- Using CLSI non-meningitis breakpoints, 240 out of 249 (96.4%) S. pneumoniae isolates were classified as cefotaxime susceptible.
- Among cefotaxime-susceptible isolates, 23% were inaccurately predicted as cefdinir susceptible.
- Amoxicillin MICs demonstrated strong correlation with penicillin MIC breakpoints, with only one discordant isolate (0.4%) out of 249.
Conclusions:
- Amoxicillin MICs serve as a reliable predictor for determining categorical susceptibility in Streptococcus pneumoniae.
- Neither penicillin nor cefotaxime MICs accurately predict susceptibility to oral cephalosporins, indicating a potential risk of treatment failure.
- Caution is advised when prescribing oral cephalosporins for cefotaxime-susceptible isolates, particularly those with higher cefotaxime MICs.
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