Cefotaxime: in vitro activity and tentative interpretive standards for disk susceptibility testing

Area of Science:

• Clinical microbiology
• Antimicrobial susceptibility testing
• Pharmacology

Background:

• Cefotaxime is a third-generation cephalosporin antibiotic.
• Evaluating cefotaxime's antimicrobial spectrum and optimizing susceptibility testing methods are crucial for effective clinical use.

Purpose of the Study:

• To compare the in vitro activity of cefotaxime with cephalothin against recent clinical isolates.
• To establish reliable disk diffusion breakpoints for cefotaxime susceptibility testing.

Main Methods:

• Antimicrobial activity was assessed against 9,412 clinical isolates.
• Disk diffusion-MIC regression analyses were performed using 5- and 30-microgram cefotaxime disks on 420 isolates.
• Breakpoint determination utilized the error rate-bounded method and regression analysis.

Main Results:

• Cefotaxime demonstrated 8-64 times greater activity against Enterobacteriaceae and 2-4 times greater activity against Pseudomonas aeruginosa compared to cephalothin.
• Cefotaxime showed 0.5-0.125 times the activity of cephalothin against staphylococci.
• Tentative MIC breakpoints for cefotaxime were proposed: susceptible ≤8 µg/ml, resistant >32 µg/ml.
• The 30-microgram disk provided the best discrimination for susceptibility categories.
• Recommended zone diameter breakpoints: susceptible ≥23 mm, indeterminate 15-22 mm, resistant ≤14 mm.

Conclusions:

• Cefotaxime exhibits a distinct spectrum of activity, being more potent against Gram-negative bacteria and less potent against staphylococci than cephalothin.
• The 30-microgram cefotaxime disk diffusion test is recommended for accurate susceptibility categorization.