The trailing end point phenotype in antifungal susceptibility testing is pH dependent

Area of Science:

• Medical Mycology
• Antimicrobial Susceptibility Testing
• Clinical Laboratory Science

Background:

• Azole antifungal drug susceptibility testing presents interpretation challenges.
• Trailing growth in Candida species leads to ambiguous Minimum Inhibitory Concentrations (MICs).
• The low-high phenotype, characterized by trailing growth, complicates resistance determination.

Purpose of the Study:

• To investigate the influence of medium composition and pH on trailing growth in Candida species.
• To characterize the in vitro behavior of isolates exhibiting the low-high phenotype.
• To identify optimal conditions for accurate azole antifungal susceptibility testing.

Main Methods:

• Microdilution susceptibility testing was performed comparing RPMI 1640 and yeast nitrogen broth.
• The effect of medium pH on trailing growth was assessed by adjusting pH levels.
• Buffering capacity was evaluated using MOPS and citrate buffers.

Main Results:

• Trailing growth in RPMI 1640 was resolved in a minimal medium at pH 4.5.
• The low-high phenotype was influenced by medium pH, with trailing occurring at higher pH values.
• Adjusting the pH of RPMI 1640 reduced trailing without altering MICs for known susceptible/resistant phenotypes.

Conclusions:

• Medium pH is a critical factor influencing trailing growth in azole antifungal susceptibility testing.
• Adjusting medium pH can eliminate trailing, improving the interpretation of MICs for Candida species.
• Optimizing pH may enhance the accuracy of in vitro susceptibility testing for azole antifungals.