Cefpirome susceptibility testing evaluates the effectiveness of cefpirome, a broad-spectrum cephalosporin antibiotic, against bacterial isolates. Cefpirome is particularly useful for treating Gram-negative infections and some Gram-positive infections.

Testing Procedures

Preparation:

• Bacterial Isolate: Use a pure culture of the microorganism.
• Inoculum Preparation: Standardize the bacterial suspension to match the 0.5 McFarland turbidity standard, ensuring consistent test results.

Disk Diffusion Method:

• Agar Plate: Plate Mueller-Hinton agar with the standardized bacterial suspension.
• Cefpirome Disk: Place a cefpirome-impregnated disk onto the agar surface.
• Incubation: Incubate the plate at 35-37°C for 16-18 hours.
• Measurement: After incubation, measure the diameter of the inhibition zone around the disk.
• Interpretation: Compare the zone diameter to established susceptibility breakpoints:
  • Susceptible: ≥ 23 mm
  • Intermediate: 16-22 mm
  • Resistant: ≤ 15 mm These breakpoints can vary based on specific guidelines from organizations like CLSI (Clinical and Laboratory Standards Institute).

Broth Microdilution Method:

• Preparation: Prepare a series of cefpirome concentrations in a microtiter plate, typically with two-fold dilutions.
• Inoculation: Add the standardized bacterial suspension to each well containing cefpirome.
• Incubation: Incubate at 35-37°C for 16-24 hours.
• Reading Results: Determine the minimum inhibitory concentration (MIC), which is the lowest concentration of cefpirome that prevents visible bacterial growth.
• Interpretation: Compare MIC values to established breakpoints to classify the isolate as:
  • Susceptible
  • Intermediate
  • Resistant Standard MIC breakpoints for cefpirome are typically:
  • Susceptible: MIC ≤ 4 µg/mL
  • Intermediate: MIC = 8 µg/mL
  • Resistant: MIC ≥ 16 µg/mL

Automated Systems (e.g., VITEK, Phoenix):

• Procedure: Automated systems can be used for cefpirome susceptibility testing by using predefined panels containing cefpirome.
• Incubation and Reading: The system automatically incubates and measures bacterial growth, providing susceptibility results based on pre-established algorithms and breakpoint criteria.