Cefixime susceptibility testing evaluates the effectiveness of cefixime, a third-generation cephalosporin, against bacterial pathogens. This testing helps guide appropriate antibiotic therapy for infections caused by susceptible bacteria. Here’s a detailed technical overview:

Testing Methods

• Broth Microdilution: Cefixime is diluted in a series of concentrations within a broth medium. After inoculating the medium with the bacterial isolate, the minimum inhibitory concentration (MIC) is determined as the lowest concentration of cefixime that inhibits visible bacterial growth.
• Agar Dilution: Cefixime is incorporated into an agar medium at different concentrations. After inoculating the agar with the bacterial strain, the MIC is identified as the lowest concentration that prevents visible growth on the agar surface.
• Disk Diffusion (Kirby-Bauer Method): A disk impregnated with cefixime is placed on an agar plate inoculated with the bacteria. The diameter of the zone of inhibition around the disk is measured to determine susceptibility. This method provides a qualitative assessment based on predefined breakpoints.

Standards and Guidelines

• Clinical and Laboratory Standards Institute (CLSI): Provides guidelines for cefixime susceptibility testing, including procedures for broth microdilution and disk diffusion. CLSI specifies MIC breakpoints and zone diameter criteria to classify bacteria as susceptible, intermediate, or resistant.
• European Committee on Antimicrobial Susceptibility Testing (EUCAST): Offers guidelines for cefixime testing with its own set of breakpoints and procedures. EUCAST guidelines might differ slightly from CLSI’s in terms of breakpoints and testing protocols.

Interpretation of Results

• Susceptible: The MIC is within the range indicating that cefixime is effective at standard dosages, or the zone of inhibition meets the susceptibility criteria. This suggests that cefixime is likely to be effective for treating infections caused by the tested bacteria.
• Intermediate: The MIC falls into a range where cefixime might be effective under specific conditions, such as at higher doses, or the zone of inhibition is borderline. This indicates that the efficacy of cefixime could be variable and may require careful consideration in treatment.
• Resistant: The MIC exceeds the effective range for cefixime, or the zone of inhibition is below the resistance threshold. This indicates that cefixime is unlikely to be effective against the bacterial strain.

Quality Control

• Control Strains: Use of control strains such as Escherichia coli ATCC 25922 and Klebsiella pneumoniae ATCC 700603 is essential to ensure the accuracy and reliability of susceptibility testing. CLSI and EUCAST guidelines specify acceptable ranges for these control organisms to verify the performance of the testing methods.

Clinical Relevance

• Treatment Guidance: Cefixime is commonly used to treat infections caused by Gram-negative bacteria, including Neisseria gonorrhoeae, Haemophilus influenzae, and Escherichia coli. Accurate susceptibility testing ensures appropriate use of cefixime, helping to effectively manage these infections.
• Resistance Management: Given the rise of antibiotic resistance, accurate susceptibility testing for cefixime is crucial for guiding therapy and managing resistance patterns. It helps in optimizing treatment choices and improving patient outcomes.

Cefixime susceptibility testing is essential for ensuring the effective use of this antibiotic in treating infections caused by Gram-negative bacteria. By determining the MIC or measuring the zone of inhibition, clinicians can make informed decisions about using cefixime, ensuring optimal patient care and managing antibiotic resistance.