Cefazolin susceptibility testing determines the effectiveness of cefazolin, a first-generation cephalosporin antibiotic, against bacterial pathogens. Here’s a technical overview:

• Testing Methods:
  • Broth Microdilution: Cefazolin is serially diluted in a broth medium and inoculated with the bacterial isolate. The minimum inhibitory concentration (MIC) is the lowest concentration of cefazolin that inhibits visible bacterial growth.
  • Agar Dilution: Cefazolin is mixed into an agar medium at varying concentrations. After inoculating with the bacterial strain, the MIC is identified by the lowest concentration of cefazolin that prevents bacterial growth on the agar surface.
  • Disk Diffusion (Kirby-Bauer Method): A disk impregnated with cefazolin is placed on an agar plate inoculated with the bacteria. The zone of inhibition around the disk is measured to assess susceptibility. This method provides a qualitative assessment of whether cefazolin inhibits bacterial growth.
• Standards and Guidelines:
  • Clinical and Laboratory Standards Institute (CLSI): Provides detailed guidelines for cefazolin susceptibility testing, including standard procedures for broth microdilution and disk diffusion. CLSI establishes breakpoints for MIC values and zone diameters to interpret susceptibility, intermediate susceptibility, or resistance.
  • European Committee on Antimicrobial Susceptibility Testing (EUCAST): Offers alternative guidelines and breakpoints for cefazolin susceptibility testing. EUCAST’s breakpoints might differ slightly from those of CLSI.
• Interpretation of Results:
  • Susceptible: The MIC falls within the range where cefazolin is expected to be effective at standard dosing regimens, or the zone of inhibition around the disk meets the criteria for susceptibility.
  • Intermediate: The MIC is at a level where cefazolin may be effective only under specific conditions, such as higher doses, or the zone of inhibition is borderline.
  • Resistant: The MIC exceeds the range where cefazolin is considered effective, or the zone of inhibition is below the threshold indicating resistance.
• Quality Control:
  • Control strains such as Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 are used to ensure the accuracy and reliability of susceptibility testing. CLSI and EUCAST guidelines provide acceptable ranges for these control organisms to verify test performance.
• Clinical Relevance:
  • Cefazolin susceptibility testing is critical for selecting appropriate treatment for infections caused by Gram-positive bacteria, including Staphylococcus aureus and Streptococcus species. It helps guide effective therapy and manage antibiotic resistance, ensuring optimal patient outcomes.

Cefazolin susceptibility testing is essential for appropriate antibiotic selection and resistance management, contributing to effective infection control and patient care.