Predicting Oral Beta-lactam susceptibilities against Streptococcus pneumoniae

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.