Antifungal Susceptibility Profile

Australian antifungal susceptibility data for Candida and Cryptococcus from invasive infections (2008-2010) using the CLSI M27-A3 microbroth susceptibility standard for yeasts.

The in vitro antifungal susceptibility data, MICs (ug/ml) at which 90% (MIC90) of the strains were inhibited and the range of MICs for are summarised below.  

1. Echinocandins against Candida. Note the echinocandins are not active against Cryptococcus.

2. Flconazole, voriconazole and posaconazole against Candida and Cryptococcus.

3. Amphotericin B, 5-Fluorocytosine and itraconazole against Candida and Cryptococcus.

All isolates of C. albicans, C. tropicalis and C.krusei were susceptible to all three echinocandins.  Only two isolates each of C. glabrata and C. parapsilosis had high MIC’s (>2 mg/ml) to Caspofungin, while 10 isolates of C. parapsilosis also had high MIC’s to anidulafungin and micafungin.  Two isolates of C. dubliniensis had high MIC’s (>2 mg/ml) to all three echinocandins.  These results are similar to those published by other studies that show the very low MICs typical for wild-type strains of C. albicans, C. glabrata, C. tropicalis, C. krusei and C. kefyr and the higher MICs typical of C. parapsilosis, C. guilliermondii, and C. lusitaniae for all three echinocandins.

All isolates tested were susceptible to amphotericin B.  Resistance to fluconazole was detected predictably in isolates of C. krusei and C. glabrata.  The majority of C. glabrata strains and some isolates of Cryptococcus were found to be susceptible dose dependent (SDD) to fluconazole (MICs 16-32 ug/ml).  Resistance to itraconazole (MIC >1 ug/ml) was more wide spread and also there were a greater number of isolates in the susceptible dose dependent range (MICs 0.25-0.5 ug/ml).  This occurred predominantly in isolates of C. glabrata, C. parapsilosis and C. krusei and in some isolates of Cryptococcus.  Voriconazole resistance (MIC >4 ug/ml) was limited to a few isolates of C. glabrata; all voriconazole resistant C. glabrata stains showed cross resistance to fluconazole and itraconazole.  Suggested likely resistance to posaconazole (MIC's >2 ug/ml) occurred in 161/303 (53%) of C. glabrata isolates and in 1 isolate of C. tropicalis.

These results are in keeping with other studies that show intrinsic resistance to antifungal agents by Candida and Cryptococcus species is largely predictable based on accurate identification of the organism.  Resistance to amphotericin B and the echinocandins is uncommon, while resistance to the azoles is largely confined to isolates of C. glabrata and C. krusei.

Australian antifungal susceptibility data for Candida isolates from recurrent vulvovaginal candidiasis (2007-2009) using the CLSI M44-A2 disk susceptibility standard for yeasts.

A total of 270 Candida isolates from patients with a clinical history of recurrent vulvovaginal candidiasis were tested against fluconazole, clotrimazole and nystatin.  Candida albicans accounted (82%) and C. glabrata (13%) accounted for 95% of the isolates.  Resistance rates for all the agents tested were very low; 10 isolates against fluconazole (7 C. glabtata and 3 C. krusei), 1 C. glabrata isolate against clotrimazole, and 1 C. albicans against Nystatin).  All isolates of C. albicans were susceptible to fluconazole and clotrimazole, however as expected 20% of C. glabrata isolates were resistant to fluconazole, although all were susceptible to Nystatin.  A previous USA study of patients with suspected vulvovaginal candidiasis reported the incidence of C. albicans as 70% and that for C. glabrata as 19%14.  These authors also reported a low fluconazole resistance rate of 3.7% for C. albicans and a similar expected higher rate of 15.2% for C. glabrata.  In conclusion, species identification was predictable of the antifungal susceptibility.  Candida albicans remains the dominant pathogen in vulvovaginal candidiasis and antifungal drug resistance does not appear to be a factor in determining recurrent infection.  Finally, the role of non-albicans Candida species in vulvovaginal candidiasis remains uncertain and further studies are warranted.

Antifungal susceptibility profiles for common human pathogenic fungi.

Susceptibility based on at least 75% of clinical isolates being susceptible. Due to the marked influence of methodology on MIC values for different antifungals and lack of correlation between in vitro and in vivo results, the results of in-vitro studies should be interpreted with caution. This table is based on current treatment regimes and in vitro susceptibility data from both the literature and the WCH Mycology Unit. Currently, the most reliable data is for fluconazole and itraconazole against Candida infections; the least reliable data is for the moulds.

S = Susceptible; SDD = Susceptible but Dose Dependent; R = Resistant; V = Variable; I = Intermediate; N = No data.

Susceptible - Dose Dependent is a new category recently described by the NCCLS subcommittee on antifungal susceptibility testing for isolates where susceptibility is dependent on achieving the maximal possible blood level of the antifungal agent. For fluconazole doses of 400 mg/day or more may be required for adults with normal renal function and habitus. For itraconazole plasma concentration of >0.5 ug/ml may be required for optimal response. [see Rex et al. CID 1997; 24:235-247].

AmB = Amphotericin B; Flu = Fluconazole; Vori = Voriconazole; Itra = Itraconazole; Keto = Ketoconazole; Terb = Terbinafine [* topical only]; 5FC = 5-Fluorocytosine; Griseo =Griseoflvin.

Yeasts

Dermatophytes

Moulds

Dimorphic moulds

Zygomycetes

Breakpoints [Rex et al. CID 1997; 24:235-47].

Amphotericin B MIC

Susceptible < 1 ug/ml
Intermediate 2 ug/ml
Resistant > 4 ug/ml

Comments: MIC's for Candida species tightly cluster at 0.25-1.0 ug/ml. The detection of resistance using the NCCLS M27A susceptibility test method for Amphotericin B may be unreliable on RPMI media. An MIC of > 1 ug/ml is likely to be resistant. Antibiotic Medium 3 may enhance the detection of resistance, but this medium is not standardised and substancial lot-to-lot variability is possible.

Fluconazole MIC

Susceptible < 8 ug/ml
Susceptible Dose dependent 16-32 ug/ml
Resistant > 64 ug/ml

Itraconazole MIC

Susceptible < 0.125 ug/ml
Susceptible Dose dependent 0.25-0.5 ug/ml
Resistant > 1 ug/ml

5 Fluorocytosine MIC

Susceptible < 4 ug/ml
Intermediate 8-16 ug/ml
Resistant >32 ug/ml

Comments: Breakpoints based on historical data and drug pharmacokinetics.