Detection of echinocandin-resistant Candida glabrata in blood cultures spiked with different percentages of FKS2 mutants [Susceptibility]

Infections caused by the co-existence of C. glabrata echinocandin-resistant- and echinocandin-susceptible cells may be possible and the detection of FKS mutants when the proportions of FKS mutants are underrepresented poses a problem. We assessed the role of EUCAST and methods directly performed on positive blood cultures – Etest (ET_DIR) and anidulafungin-containing agar plates – for detecting resistance in C. glabrata isolates containing different amounts of echinocandin-susceptible/resistant Candida glabrata isolates.

We studied ten pairs of C. glabrata involving parental echinocandin-susceptible and isogenic echinocandin-resistant FKS mutant isolates. Three inocula per pair (1-5 x 10³, 1-5 x 10², and 10-50 CFU/mL) were prepared spanning suspensions with different amounts of susceptible/resistant isolates (9/1, 5/5, and 1/9). Suspensions were spiked in BACTEC bottles and incubated until positive and compared the three methods.

The EUCAST showed echinocandin resistance when the bottles were spiked with resistant isolates at 5/5 and 1/9 proportions; the suspensions with a 9/1 proportion of resistant isolates resulted susceptible in three pairs. We observed with the ET_DIR, resistance to both echinocandins in all pairs (resistance to micafungin and anidulafungin, MICs ≥ 0.064 mg/L and ≥ 0.125 mg/L, respectively) and a double ring of growth inhibition in two pairs. The anidulafungin-containing plates showed fungal growth in the 90 spiked blood cultures at 48 hours. Testing of echinocandin susceptibility with the ET_DIR directly on blood positive bottles is a reliable and rapid method to detect echinocandin resistance in C. glabrata. On the other hand, resistance can be missed with the EUCAST method when resistant-isolates are underrepresented.

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