Detection of Echinocandin-Resistant in Blood Cultures Spiked with Different Percentages of Mutants.

Infections caused by the coexistence of echinocandin-resistant and echinocandin-susceptible cells may be possible, and the detection of mutants when the proportions of mutants are underrepresented poses a problem. We assessed the role of EUCAST and methods directly performed on positive blood cultures-Etest (ET) and anidulafungin-containing agar plate assays-for detecting resistance in isolates containing different amounts of echinocandin-susceptible and -resistant isolates. We studied 10 pairs of isolates involving parental echinocandin-susceptible isolates and isogenic echinocandin-resistant mutant isolates. Three inocula per pair (1 × 10 to 5 × 10, 1 × 10 to 5 × 10, and 10 to 50 CFU/ml) spanning suspensions with different amounts of susceptible/resistant isolates (9/1, 5/5, and 1/9 proportions for each the three inocula) were prepared. The suspensions were spiked in Bactec bottles and incubated until they were positive, and the three methods were compared. The EUCAST method showed echinocandin resistance when the bottles were spiked with susceptible/resistant isolates at 5/5 and 1/9 proportions; the results for the suspensions with a 9/1 proportion of susceptible/resistant isolates were susceptible for three pairs. We observed with the ET resistance to both echinocandins in all pairs (resistance to micafungin and anidulafungin; MICs, ≥0.064 mg/liter and ≥0.125 mg/liter, 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 h. Testing of echinocandin susceptibility with the ET directly on the positive blood culture bottles is a reliable and rapid method to detect echinocandin resistance in On the other hand, resistance can be missed with the EUCAST method when resistant isolates are underrepresented.