Tolerance to amphotericin B in clinical isolates of Candida tropicalis

A broth microdilution method was used for testing amphotericin B against 33 clinical isolates of Candida tropicalis. All isolates were in vitro susceptible to the polyene (MIC [minimal inhibitory concentration] ≤1.0 μg/mL). However, when the isolates were cultured in a medium containing amphotericin B at a concentration of 1.5 μg/mL, a wide interstrain variation of growth rate was observed. Five isolates (15%) proved to be highly tolerant to the drug and grew at a frequency ranging from 1 × 10 −1 to 2 × 10 −2. Twenty-three isolates (70%) grew at a frequency ranging from 1 × 10 −5 to 1 × 10 −8. The remaining five isolates (15%) failed to grow in drug-containing medium. In general, this growth variation was not associated with amphotericin B MICs displayed by the single isolates. In addition, the strains grown in drug-containing medium did not represent amphotericin B–resistant mutants, as shown by the maintenance of MICs similar to those of their respective parent isolates. Killing experiments conducted in selected isolates confirmed a variation of fungicidal activity of amphotericin B. To see whether this phenomenon was associated with a variation of amphotericin B response in vivo, we established an experimental model of systemic murine candidiasis in CD1 mice by intravenous injection of cells belonging to Candida tropicalis 3147 (growth rate at a frequency of 1 × 10 −1 in amphotericin B medium) and Candida tropicalis 4055 (no growth). Low (0.3 mg/kg/day) and high (1 mg/kg/day) doses of amphotericin B were both effective at reducing the fungal burdens in the kidneys of mice infected with either strain ( p, 0.01 to 0.02). However, whereas the burden of mice infected with isolate 3147 and treated with the polyene at 0.3 mg/kg/day was reduced by 1.2 ± 0.25 (mean ± standard deviation) log 10 cfu/g compared to untreated mice, the same dosing regimen yielded a burden reduction of 2.6 ± 0.07 log 10 cfu/g in mice infected with isolate 4055 ( p < 0.001). Similarly, amphotericin B at 1 mg/kg/day yielded a burden reduction of 1.8 ± 0.20 vs. 2.5 ± 0.30 log 10 cfu/g in mice infected with isolates 3147 and 4055, respectively ( p < 0.001). Our data revealed a variable pattern of tolerance to amphotericin B among isolates of Candida tropicalis and showed that this phenomenon might influence the rate of organ clearance during therapy.