Potential unreliability of nitrofurantoin disks in susceptibility testing

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• The Kirby-Bauer disk diffusion method is a standard for antimicrobial susceptibility testing (AST).
• Accurate AST is crucial for guiding antibiotic therapy and combating antimicrobial resistance.
• Nitrofurantoin is a key antibiotic for treating urinary tract infections, often caused by Proteus species.

Purpose of the Study:

• To evaluate lot-to-lot variability in commercial nitrofurantoin disks used in Kirby-Bauer testing.
• To investigate the cause of discrepancies in zone diameters observed with different nitrofurantoin disk lots.
• To assess the impact of disk variability on the accurate susceptibility categorization of Proteus mirabilis.

Main Methods:

• Utilized 50 strains of Proteus mirabilis for susceptibility testing.
• Employed three distinct lots of commercial nitrofurantoin disks.
• Measured zone diameters and analyzed antimicrobial content and eluate pH for each disk lot.

Main Results:

• One lot of nitrofurantoin disks produced zone diameters 3–6 mm larger than the other two lots.
• This discrepancy led to a significant number of Proteus mirabilis strains being incorrectly classified as susceptible.
• Differences in antimicrobial content did not explain the observed variability; higher eluate pH from the unreliable disks correlated with larger zones.

Conclusions:

• Lot variability in nitrofurantoin disks can compromise the accuracy of Kirby-Bauer antimicrobial susceptibility testing.
• Higher eluate pH from certain disk lots promotes faster nitrofurantoin diffusion, leading to erroneously large inhibition zones.
• Standardization of nitrofurantoin disk manufacturing is essential to ensure reliable AST results and appropriate patient treatment.