Cytometric approach for a rapid evaluation of susceptibility of Candida strains to antifungals.

ABSTRACT

OBJECTIVE: To achieve a fast and reliable determination of the susceptibility of Candida strains to amphotericin B (Am B), fluconazole (Flu) and 5-fluorocytosine (5-FC), using cytometric methods as an alternative to the classical dilution method. METHODS: Twenty-three clinical isolates of Candida with different susceptibility patterns were treated for 1 h with two concentrations each of Am B (2 and 8 mg/L), Flu (8 and 64 mg/L) and 5-FC (4 and 32 mg/L), followed by staining with three different fluorochromes, under conditions previously defined through an optimisation study. These were 1 mg/L propidium iodide (PI)/10(6) cells for 30 min at 30 degrees C (a marker that only penetrates cells with severe lesions of the membrane); 0.5 microM FUN-1/10(6) cells for 30 min at 30 degrees C (a fluorescent probe which after entering the yeast cell is converted, by metabolically active yeasts, from a diffuse cytosolic pool with a yellow-green fluorescence into red cylindrical intravacuolar structures) and 0.25 microM of JC-1/10(6) cells for 15 min at 37 degrees C (a monomer that changes reversibly from green to red the J-aggregates, with the increased membrane potential). About 50 000 yeast cells were analysed by flow cytometry (FCM), at FL3 (red, 620 nm) for PI and FL2 (yellow-green, 575 nm) for FUN-1 and the ratio of FL3 to FL1 was determined (red, 620 nm/green, 525 nm) for JC-1; 200 cells of each suspension were also analysed by epifluorescence microscopy (EPM). Viability studies were performed in parallel to count the number of colony forming units. RESULTS: Susceptible (S) strains exposed to Am B and stained with JC-1 showed a dose-dependent decrease in the mitochondrial potential, i.e. a decreased ratio between red/green fluorescence by FCM and a decrease in J-aggregates by EPM. Neither FUN-1 nor PI was useful in the study of Am B activity. Susceptibility to Flu and 5-FC could be detected with FUN-1 staining metabolic changes were detected by an increase in yellow-green intensity of fluorescence by FCM or a decrease of cylindrical intravacuolar structure formation by EPM, although no decrease in total viability was registered. Staining with JC-1 could predict resistance to both drugs, but did not allow distinction between sensitive dose-dependent strains (S-DD) or intermediate (I) resistance to Flu or 5-FC, respectively, from S strains. PI did not stain Candida cells treated with Flu or 5-FC under our experimental conditions. CONCLUSION: Susceptibility patterns of Candida strains to Am B can be determined by using JC-1, and to Flu and 5-FC by using FUN-1. PI was not a useful probe with which to study the effect of such antifungals under the conditions described here.