Unpredictable In Vitro Killing Activity of Amphotericin B against Four Candida auris Clades

Abstract

Candida auris is an emerging multiresistant yeast against which amphotericin B (AMB) is still the first therapeutic choice in certain clinical situations (i.e., meningitis, endophthalmitis, and urinary tract infections). As data about the in vitro killing activity of AMB against C. auris clades are lacking, we determined MICs, minimum fungicidal concentrations (MFCs), and killing activity of AMB against 22 isolates representing the 4 major C. auris clades (South Asian n = 6; East Asian n = 4; South African n = 6, and South American n = 6). MIC values were ≤1 mg/L regardless of clades; MFC ranges were, 1-4 mg/L, 2-4 mg/L, 2 mg/L, and 2-8 mg/L for South Asian, East Asian, South African, and South American clades, respectively. AMB showed concentration-, clade-, and isolate-dependent killing activity. AMB was fungicidal at 1 mg/L against two of six, two of four, three of six, and one of six isolates from the South Asian, East Asian, South African, and South American clades, respectively. Widefield fluorescence microscopy showed cell number decreases at 1 mg/L AMB in cases of the South Asian, East Asian, and South African clades. These data draw attention to the weak killing activity of AMB against C. auris regardless of clades, even when MICs are low (≤1 mg/L). Thus, AMB efficacy is unpredictable in treatment of invasive C. auris infections.

Keywords: Candida auris; amphotericin B; in vitro; killing rate; time–kill.

Figure 1

Time–kill plots of amphotericin B against Candida auris isolates 174 (A), type strain (NCPF 13029 = CBS 10913) (B), 182 (C), and I-172 (D), belonging to the South Asian, East Asian, South African, and South American clades, respectively.

Figure 2

Killing rate values of amphotericin B against 22 clinical isolates belonging to the South Asian (A), East Asian (B), South African (C), and South American (D) Candida auris clades. Positive and negative k values indicate the decrease and increase, respectively, in viable cell numbers. Error bars are omitted for better visualization of the graphics.

Figure 3

Candida auris cells (isolates 196, South Asian clade and I-156, South American clade) treated with 0.5 and 1 mg/L amphotericin B and examined with widefield fluorescence microscopy. Drug-free controls were included. Micrographs were taken after 24 h. Blue cells indicate viable cells. In the case of isolate 196 (left), cell numbers decreased at 1 mg/L. For isolate I-156 (right), large aggregates with up to 50 cells were seen at 1 mg/L. Dead cells were never detected. Bar: 10 µm.