In vitro interaction of fluconazole and trimethoprim–sulfamethoxazole against Candida auris using ETEST and checkerboard methods

Abstract

Candida auris was discovered in 2009 and has rapidly emerged as a serious public health threat with cases reported in over 20 countries worldwide. As of May 8, 2020, the Centers for Disease Control and Prevention reported a total of 1122 US cases. C. auris is often multidrug resistant, leaving few options for treatment. Sulfonamides are known to inhibit a bacterial enzyme involved in folate synthesis and may also inhibit yeast organisms by a similar mechanism. The combination of trimethoprim and sulfamethoxazole is more commonly used than either drug alone. The objective of this study was to evaluate the combination of fluconazole and trimethoprim–sulfamethoxazole against C. auris. Minimum inhibitory concentrations (MICs) of fluconazole and trimethoprim–sulfamethoxazole were determined by ETEST and broth microdilution for 11 C. auris strains. Fluconazole MICs (µg/mL) were 4–>256 by ETEST and 2–>256 by broth microdilution (73% resistant); trimethoprim–sulfamethoxazole MICs were >32 by ETEST and 32–>128 by broth microdilution (no interpretive guidelines for C. auris). Using our MIC: MIC ETEST method and a checkerboard method, we investigated the interaction of fluconazole and trimethoprim–sulfamethoxazole against all isolates. These interactions were analyzed by calculating the summation fractional inhibitory concentration with synergyof ≤0.5, additivity of >0.5–1.0, indifference of >1–4, and antagonism of >4. The combination of fluconazole and trimethoprim–sulfamethoxazole revealed synergy with three (27%) and additivity with one (9%) isolate. Indifference was found for the remaining seven (64%) isolates. With the checkerboard method, synergy was seen in 1/11 (9%) isolates with fluconazole (½ MIC) plus trimethoprim–sulfamethoxazole (1/64 MIC); additivity, in 7/11 (64%) isolates with fluconazole (1/8 MIC–1×MIC) plus trimethoprim–sulfamethoxazole (1/128 MIC–½ MIC); and indifference in 3/11 (27%) isolates. Regardless, in vitro interactions may or may not correlate with clinical outcomes. Synergy testing with additional drug combinations and isolates should be performed.