Carbenicillin susceptibility testing is a microbiological procedure used to determine the effectiveness of carbenicillin, a beta-lactam antibiotic, against specific bacterial isolates. The test evaluates whether bacteria are resistant or sensitive to carbenicillin, providing critical information for appropriate antibiotic treatment.

Testing Procedures

• Preparation:
  • Bacterial Isolate: Obtain a pure culture of the bacterium to be tested.
  • Inoculum Preparation: Adjust the bacterial suspension to a specific turbidity, usually equivalent to a 0.5 McFarland standard.
• Disk Diffusion Method:
  • Agar Plate: Plate Mueller-Hinton agar with the standardized bacterial suspension.
  • Carbenicillin Disk: Place a carbenicillin-impregnated disk onto the agar surface.
  • Incubation: Incubate the plate at 35-37°C for 16-18 hours.
  • Measurement: Measure the diameter of the inhibition zone surrounding the disk.
  • Interpretation: Compare the zone size to standardized susceptibility breakpoints to classify the bacteria as susceptible, intermediate, or resistant.
• Broth Microdilution Method:
  • Preparation: Prepare a series of carbenicillin concentrations in a microtiter plate.
  • Inoculation: Add the bacterial suspension to each well containing the antibiotic.
  • Incubation: Incubate at 35-37°C for 16-24 hours.
  • Reading Results: Determine the minimum inhibitory concentration (MIC) by identifying the lowest concentration of carbenicillin that inhibits visible bacterial growth.
  • Interpretation: Compare MIC values to established susceptibility ranges to categorize the isolate as susceptible, intermediate, or resistant.

Quality Control

• Controls: Use reference strains with known carbenicillin susceptibility profiles to ensure accuracy and reliability of the test.
• Standards: Follow guidelines from organizations like CLSI (Clinical and Laboratory Standards Institute) for standardized testing procedures and breakpoint criteria.

Carbenicillin susceptibility testing is essential for determining the efficacy of carbenicillin against bacterial pathogens, guiding effective treatment, and managing resistance patterns. Accurate testing and interpretation are crucial for optimal patient outcomes and effective infection management.