Etest for antifungal susceptibility testing of yeasts

Aromatic Diboronic Acids as Effective KPC/AmpC Inhibitors

Over 30 compounds, including para-, meta-, and ortho-phenylenediboronic acids, ortho-substituted phenylboronic acids, benzenetriboronic acids, di- and triboronated thiophenes, and pyridine derivatives were investigated as potential β-lactamase inhibitors. The highest activity against KPC-type carbapenemases was found for ortho-phenylenediboronic acid 3a, which at the concentration of 8/4 mg/L reduced carbapenems' MICs up to 16/8-fold, respectively. Checkerboard assays revealed strong synergy between carbapenems and 3a with the fractional inhibitory concentrations indices of 0.1-0.32. The nitrocefin hydrolysis test and the whole cell assay with E. coli DH5α transformant carrying blaKPC-3 proved KPC enzyme being its molecular target. para-Phenylenediboronic acids efficiently potentiated carbapenems against KPC-producers and ceftazidime against AmpC-producers, whereas meta-phenylenediboronic acids enhanced only ceftazidime activity against the latter ones. Finally, the statistical analysis confirmed that ortho-phenylenediboronic acids act synergistically with carbapenems significantly stronger than other groups. Since the obtained phenylenediboronic compounds are not toxic to MRC-5 human fibroblasts at the tested concentrations, they can be considered promising scaffolds for the future development of novel KPC/AmpC inhibitors. The complexation of KPC-2 with the most representative isomeric phenylenediboronic acids 1a, 2a, and 3a was modeled by quantum mechanics/molecular mechanics calculations. Compound 3a reached the most effective configuration enabling covalent binding to the catalytic Ser70 residue.

Antifungal Susceptibly Testing by Concentration Gradient Strip Etest Method for Fungal Isolates: A Review

Antifungal susceptibility testing is an important tool for managing patients with invasive fungal infections, as well as for epidemiological surveillance of emerging resistance. For routine testing in clinical microbiology laboratories, ready-to-use commercial methods are more practical than homemade reference techniques. Among commercially available methods, the concentration gradient Etest strip technique is widely used. It combines an agar-based diffusion method with a dilution method that determinates a minimal inhibitory concentration (MIC) in µg/mL. Many studies have evaluated the agreement between the gradient strip method and the reference methods for both yeasts and filamentous fungi. This agreement has been variable depending on the antifungal, the species, and the incubation time. It has also been shown that the gradient strip method could be a valuable alternative for detection of emerging resistance (non-wild-type isolates) as Etest epidemiological cutoff values have been recently defined for several drug-species combinations. Furthermore, the Etest could be useful for direct antifungal susceptibility testing on blood samples and basic research studies (e.g., the evaluation of the in vitro activity of antifungal combinations). This review summarizes the available data on the performance and potential use of the gradient strip method.

Comparative Analysis of Disc Diffusion and E-test with Broth Micro-dilution for Susceptibility Testing of Clinical Candida Isolates Against Amphotericin B, Fluconazole, Voriconazole and Caspofungin

BACKGROUND

Antifungal susceptibility testing remains an area of intense interest because of the increasing number of clinical isolates resistant to antifungal therapy. Clinical and Laboratory Standards Institute has proposed reference broth micro dilution (BMD) method for susceptibility testing. The reference method is time-consuming and poorly suited for the routine clinical laboratory setting. Agar-based susceptibility testing methods, disk diffusion (DD) method and the E-test method can be an easier, reliable and less time consuming alternative for the BMD method.

AIM

To compare the results of Amphotericin B, fluconazole, voriconazole, and Caspofungin susceptibility testing by DD, and the E-test method with the CLSI reference method for clinical Candida isolates.

MATERIALS AND METHODS

Broth Microdilution (BMD), E-test and Disk diffusion testing of the various clinical Candida isolates was performed in accordance with CLSI documents. The results obtained were analysed and compared.

RESULTS

The categorical agreement for Amphotericin B, fluconazole, voriconazole, and Caspofungin susceptibility results by E-test and DD method was 65.2%, 67.4%; 100%, 82.6%; 100%, 100%; 100%, 97.8% respectively.

CONCLUSION

The agar-based E-test and disk diffusion methods are reliable alternatives to the BMD method for Candida isolates when test susceptible to fluconazole, voriconazole, and Caspofungin, however the susceptibility testing results must be interpreted with caution in case of Amphotericin B.

Rhodotorula fungemia: two cases and a brief review

Rhodotorula is emerging as an important cause of nosocomial and opportunistic infections. We present two cases of Rhodotorula mucilaginosa fungemia diagnosed over a period of 3 months at our hospital. The first case was of a pre-term neonate in the neonatal ICU who presented with respiratory failure and sepsis. The second involved an adult female who had been injured in a road traffic accident requiring an operation for a hematoma and was later shifted to the medical ICU. For a new hospital like ours, finding two cases of Rhodotorula fungemia within a span of 3 months prompted us to describe them in this report. These cases emphasize the emerging importance of Rhodotorula mucilaginosa as a pathogen and the importance of identification and MIC testing for all fungal isolates recovered from the blood stream.

In vitro susceptibility testing of Candida and Aspergillus spp. to voriconazole and other antifungal agents using Etest®: results of a French multicentre study

Minimum inhibitory concentrations (MICs) of the antifungal agent voriconazole were determined using the Etest and compared with those of amphotericin B, itraconazole and fluconazole using 1986 clinical isolates of Candida spp. Voriconazole MICs were also compared with those of amphotericin B and itraconazole using 391 clinical isolates of Aspergillus spp. Voriconazole was found to have more potent activity and lower MIC values than amphotericin B, itraconazole and fluconazole against C. albicans, C. tropicalis, C. parapsilosis and C. kefyr. Against C. glabrata and C. krusei, voriconazole was more active than either of the other two azole antifungals but had similar activity to amphotericin B. For species of Aspergillus, MIC values of voriconazole were lower than those of amphotericin B and itraconazole against A. fumigatus and A. flavus, and were similar to those of amphotericin B against A. niger. Against A. terreus, MIC values for voriconazole and itraconazole were similar. A. terreus is known to be resistant to amphotericin B, and this was reflected in higher MIC values compared with those of voriconazole and itraconazole. Voriconazole therefore compares very favourably with other antifungal agents against a large number of clinical isolates of Candida and Aspergillus spp.

Correlation between E-test, disk diffusion, and microdilution methods for antifungal susceptibility testing of fluconazole and voriconazole

The activities of fluconazole and voriconazole against isolates of Candida spp. (n = 400) were tested by the E-test, disk diffusion, and the National Committee for Clinical Laboratory Standards (NCCLS) M27-A2 broth microdilution-based reference methods. More than 96% of isolates found to be susceptible to fluconazole by the reference method were identified as susceptible by the agar-based methods. Lesser degrees of correlation with the reference method were seen for isolates identified as resistant by the agar-based methods. Interpretive categories are not available for voriconazole, but results qualitatively similar to those for fluconazole were seen. The agar-based E-test and disk diffusion methods are reliable alternatives to the NCCLS M27-A2 reference microdilution method for isolates that test susceptible to fluconazole.

Interlaboratory evaluation of the Etest for antifungal susceptibility testing of dermatophytes

A three-site interlaboratory reproducibility evaluation of the Etest concentration gradient strip method for testing antifungal susceptibilities was conducted using 30 strains of dermatophytes exposed to strips loaded with ketoconazole (KTZ), itraconazole (ITZ), amphotericin B (AMB) and fluconazole (FCZ). Etest minimal inhibitory concentrations were compared with those obtained using a broth microdilution method. All isolates produced clearly detectable growth at 28 degrees C within 72-96 h for reading with the Etest method. The highest interlaboratory agreement between Etest and the microdilution method was shown with FCZ (94%), and the lowest was seen with KTZ (60%). Overall, agreement between the Etest and microdilution method was variable. It was excellent for AMB (97%), good for ITZ (80%) and KTZ (77%), and low for fluconazole (27%).

Evaluation of Etest and macrodilution broth method for antifungal susceptibility testing of Candida sp strains isolated from oral cavities of AIDS patients

A comparison of the Etest and the reference broth macrodilution susceptibility test for fluconazole, ketoconazole, itraconazole and amphotericin B was performed with 59 of Candida species isolated from the oral cavities of AIDS patients. The Etest method was performed according to the manufacturer's instructions, and the reference method was performed according to National Committee for Clinical Laboratory Standards document M27-A guidelines. Our data showed that there was a good correlation between the MICs obtained by the Etest and broth dilution methods. When only the MIC results at +/- 2 dilutions for both methods were considered, the agreement rates were 90.4% for itraconazole, ketoconazole and amphotericin B and 84.6% for fluconazole of the C. albicans tested. In contrast, to the reference method, the Etest method classified as susceptible three fluconazole-resistant isolates and one itraconazole-resistant isolate, representing four very major errors. These results indicate that Etest could be considered useful for antifungal sensitivity evaluation of yeasts in clinical laboratories.

Susceptibility testing of fluconazole by the NCCLS broth macrodilution method, E-test, and disk diffusion for application in the routine laboratory

Antifungal susceptibility testing may be an important aid in the treatment of patients with life-threatening yeast infections. In order to establish the suitability of different susceptibility test methods for fluconazole with yeasts, the Rosco tablet and the E-test were compared with the gold standard NCCLS broth macrodilution method for 106 yeast strains. These included 102 clinical isolates of Candida spp., including Candida glabrata (n = 30), Candida albicans (n = 20), Candida tropicalis (n = 13), Candida parapsilosis (n = 10), Candida krusei (n = 8), plus Cryptococcus neoformans (n = 3), Saccharomyces cerevisiae (n = 2), and 16 strains belonging to other Candida spp. Four American Type Culture Collection strains of Candida were included as quality controls. The NCCLS method was found to be too complex and labor-intensive for routine testing. The E-test is an accurate alternative, but experience in determining MICs and careful attention to procedural details are critically important. The Rosco tablet showed the best agreement with the NCCLS reference method, especially when newly established breakpoints of R < or = 10 mm and S > or = 21 mm were used.

In vitro antifungal susceptibility testing

With the rising frequency of fungal infections, as well as increasing reports of resistance to antifungal agents, it is imperative that clinically applicable antifungal susceptibility testing be available. In 1997 the National Committee for Clinical Laboratory Standards published standard guidelines for antifungal susceptibility testing of Candida sp and Cryptococcus neoformans with amphotericin B, flucytosine, fluconazole, itraconazole, and ketoconazole. Although the methods are standard, they are time consuming, can be difficult to interpret, and are approved only for testing limited organisms and drugs. Modifications to the methods and alternative approaches have been proposed to make these tests more convenient and efficient, applicable to a greater number of species, and appropriate for performing in the clinical laboratory.

Evaluation of Etest for direct antifungal susceptibility testing of yeasts in positive blood cultures

The performance of the Etest (AB BIODISK, Solna, Sweden) for direct antifungal susceptibility testing of yeasts in positive blood cultures was compared with that of the macrodilution method for determining the MICs of five antifungal agents. Culture broths with blood from bottles positive for yeasts were inoculated directly onto plates for susceptibility testing with the Etest, and the MICs were read after 24 and 48 h of incubation. A total of 141 positive blood cultures (72 cultures of Candida albicans, 31 of Candida tropicalis, 14 of Candida glabrata, 11 of Candida parapsilosis, 3 of Candida krusei, and 3 of Cryptococcus neoformans, 4 miscellaneous yeast species, and 3 mixed cultures) were tested, and the rates of MIC agreement (+/-1 log(2) dilution) between the direct Etest (at 24 and 48 h, respectively) and macrodilution methods were as follows: amphotericin B, 81.8 and 93.5%; flucytosine, 84.8 and 87.7%; fluconazole, 89.4 and 85.5%; itraconazole, 69.7 and 63.8%; ketoconazole, 87.9 and 79.0%. By a large-sample t test, the difference in log(2) dilution between the direct Etest and the macrodilution method was found to be small (P < 0.05). The lone exceptions were ketoconazole at 48 h of incubation and itraconazole at both 24 and 48 h of incubation (P > 0.05). By Tukey's multiple comparisons, the difference between the direct Etest (48 h) and reference methods among different species was found to be less than 1 log(2) dilution. When the MICs were translated into interpretive susceptibility, the minor errors caused by the direct Etest (at 24 and 48 h, respectively) were as follows: flucytosine, 2.3 and 1.4%; fluconazole, 3.0 and 3.6%; itraconazole, 21.2 and 21.3%. Itraconazole also produced an additional 3.0 and 3.6% major errors as determined by the direct Etest at 24 and 48 h, respectively. It was concluded that, except for itraconazole, the Etest method was feasible for direct susceptibility testing of blood cultures positive for yeasts. The method is simple, and the results could be read between 24 and 48 h after direct inoculation, whenever the inhibition zones were discernible.

Comparison of Etest with the broth microdilution method in susceptibility testing of yeast isolates against four antifungals

A comparative evaluation of the Etest and the broth microdilution methods for antifungal susceptibility testing of 102 clinical yeast isolates against amphotericin B, fluconazole, itraconazole, and ketoconazole was conducted. The agreements between the Etest and the broth microdilution methods were 93.1% for amphotericin B 85.2% for ketoconazole, 82.3% for itraconazole and 79.4% for fluconazole. These results suggest that the Etest approach to antifungal susceptibility testing may be a viable alternative to the NCCLS reference methods for testing yeasts, but that further evaluations are needed.

Susceptibility testing of Candida species: comparison of NCCLS microdilution method with Fungitest

Fungitest is a new commercially available and easy-to-perform breakpoint test system using six antifungal agents. We compared this test with a modified standard method described by the National Committee for Clinical Laboratory Standards (NCCLS). One hundred isolates of Candida species were tested with both methods. Based on the same breakpoints, the correlation of qualitative results between the reference method and Fungitest was high. Best results were obtained after incubation of Fungitest for 48 h. Overall agreement was high, an excellent correlation was given with amphotericin B and flucytosine (100% and 99%, respectively), whereas itraconazole showed only 86% concordance. When Fungitest was read after 24 h the agreement was lower ranging from 100% to 75%. Some of the breakpoints used with Fungitest differ from the breakpoints recommended by NCCLS, whereas others have not been elaborated by the NCCLS. The adaptation of Fungitest breakpoints to NCCLS and determination of further breakpoints have to be discussed before Fungitest can be recommended for routine use.

Comparative evaluation of PASCO and national committee for clinical laboratory standards M27-A broth microdilution methods for antifungal drug susceptibility testing of yeasts

The PASCO antifungal susceptibility test system, developed in collaboration with a commercial company, is a broth microdilution assay which is faster and easier to use than the reference broth microdilution test performed according to the National Committee for Clinical Laboratory Standards (NCCLS) document M27-A guidelines. Advantages of the PASCO system include the system's inclusion of quality-controlled, premade antifungal panels containing 10, twofold serial dilutions of drugs and a one-step inoculation system whereby all wells are simultaneously inoculated in a single step. For the prototype panel, we chose eight antifungal agents for in vitro testing (amphotericin B, flucytosine, fluconazole, ketoconazole, itraconazole, clotrimazole, miconazole, and terconazole) and compared the results with those of the NCCLS method for testing 74 yeast isolates (14 Candida albicans, 10 Candida glabrata, 10 Candida tropicalis, 10 Candida krusei, 10 Candida dubliniensis, 10 Candida parapsilosis, and 10 Cryptococcus neoformans isolates). The overall agreements between the methods were 91% for fluconazole, 89% for amphotericin B and ketoconazole, 85% for itraconazole, 80% for flucytosine, 77% for terconazole, 66% for miconazole, and 53% for clotrimazole. In contrast to the M27-A reference method, the PASCO method classified as resistant seven itraconazole-susceptible isolates (9%), two fluconazole-susceptible isolates (3%), and three flucytosine-susceptible isolates (4%), representing 12 major errors. In addition, it classified two fluconazole-resistant isolates (3%) and one flucytosine-resistant isolate (1%) as susceptible, representing three very major errors. Overall, the agreement between the methods was greater than or equal to 80% for four of the seven species tested (C. dubliniensis, C. glabrata, C. krusei, and C. neoformans). The lowest agreement between methods was observed for miconazole and clotrimazole and for C. krusei isolates tested against terconazole. When the data for miconazole and clotrimazole were removed from the analysis, agreement was >/=80% for all seven species tested. Therefore, the PASCO method is a suitable alternative procedure for the testing of the antifungal susceptibilities of the medically important Candida spp. and C. neoformans against a range of antifungal agents with the exceptions only of miconazole and clotrimazole and of terconazole against C. krusei isolates.

Comparison of E-test and broth microdilution methods for antifungal drug susceptibility testing of molds

We compared the E test with a broth microdilution method, performed according to National Committee for Clinical Laboratory Standards document M27-A guidelines, for determining the in vitro susceptibilities of 90 isolates of pathogenic molds (10 Absidia corymbifera, 10 Aspergillus flavus, 10 Aspergillus fumigatus, 10 Aspergillus niger, 10 Aspergillus terreus, 10 Exophiala dermatitidis, 10 Fusarium solani, 10 Scedosporium apiospermum, 5 Scedosporium prolificans, and 5 Scopulariopsis brevicaulis). Overall, there was 71% agreement between the results of the two methods for amphotericin B (E-test MICs within +/-2 log2 dilutions of broth microdilution MICs) and 88% agreement with the results for itraconazole. The overall levels of agreement (within +/-2 log2 dilutions) were >/=80% for 5 of the 10 species tested against amphotericin B and 8 of the 10 species tested against itraconazole. The best agreement between the results was seen with A. fumigatus and A. terreus (100% of results for both agents within +/-2 log2 dilutions). The poorest agreement was seen with S. apiospermum, S. prolificans, and S. brevicaulis tested against amphotericin B (20% of results within +/-2 log2 dilutions). In every instance, this low level of agreement was due to isolates for which the broth microdilution MICs were low but for which the E-test MICs were much higher. The E test appears to be a suitable alternative procedure for testing the susceptibility of Aspergillus spp. and some other molds to amphotericin B or itraconazole.

A comparative evaluation of Etest and broth microdilution methods for fluconazole and itraconazole susceptibility testing of Candida spp

The Etest strip is a promising tool of broad application in clinical microbiology. The method provides MIC readings and is easier to perform than broth microdilution. We carried out a study to compare the MICs of fluconazole and itraconazole obtained by the Etest with those obtained by broth microdilution, performed according to the guidelines of the NCCLS document M27-A, with 402 clinical isolates (360 Candida albicans, 17 Candida tropicalis, nine Candida krusei, nine Candida glabrata and seven Candida parapsilosis) and seven control isolates. The agreement between MICs by the two methods (at +/- 2 dilutions) was 74.5% for fluconazole and 61.4% for itraconazole. These results suggest that further development is necessary to standardize the medium and incubation conditions before introduction of the Etest as a routine method in the clinical microbiology laboratory for fluconazole and itraconazole susceptibility testing.

Multicenter comparison of the sensititre YeastOne Colorimetric Antifungal Panel with the National Committee for Clinical Laboratory standards M27-A reference method for testing clinical isolates of common and emerging Candida spp., Cryptococcus spp., and other yeasts and yeast-like organisms

National Committee for Clinical Laboratory Standards (NCCLS) standard guidelines are available for the antifungal susceptibility testing of common Candida spp. and Cryptococcus neoformans, but NCCLS methods may not be the most efficient and convenient procedures for use in the clinical laboratory. MICs of amphotericin B, fluconazole, flucytosine, itraconazole, and ketoconazole were determined by the commercially prepared Sensititre YeastOne Colorimetric Antifungal Panel and by the NCCLS M27-A broth microdilution method for 1,176 clinical isolates of yeasts and yeast-like organisms, including Blastoschizomyces capitatus, Cryptococcus spp., 14 common and emerging species of Candida, Hansenula anomala, Rhodotorula spp., Saccharomyces cerevisiae, Sporobolomyces salmonicolor, and Trichosporon beigelii. Colorimetric MICs of amphotericin B corresponded to the first blue well (no growth), and MICs of the other agents corresponded to the first purple or blue well. Three comparisons of MIC pairs by the two methods were evaluated to obtain percentages of agreement: 24- and 48-h MICs and 24-h colorimetric versus 48-h reference MICs. The best performance of the YeastOne panel was with 24-h MICs (92 to 100%) with the azoles and flucytosine for all the species tested, with the exception of C. albicans (87 to 90%). For amphotericin B, the best agreement between the methods was with 48-h MIC pairs (92 to 99%) for most of the species tested. The exception was for isolates of C. neoformans (76%). These data suggest the potential value of the YeastOne panel for use in the clinical laboratory.

No increase in frequency of antifungal resistance among yeasts isolated from normally sterile sites in patients at Foothills Hospital from 1993 to 1996

OBJECTIVE

To determine the prevalence of resistance to antifungal drugs among yeasts isolated from sterile sites from patients in one hospital and the relationship of resistance to antifungal use, and to assess whether resistance was increasing.

METHOD

Susceptibility testing performed by National Committee for Clinical Laboratory Standards (NCCLS) (Villanova, Pennsylvania) microdilution method and by E test. Antifungal use was determined by selected chart review and from pharmacy data. SPECIMENS AND SETTING: Tertiary care adult hospital with neonatal intensive care.

POPULATION STUDIED

Distinct yeast isolates from sterile site specimens collected during the years 1993 to 1996.

RESULTS

A total of 132 yeast isolates were studied, of which 78 (59%) were Candida albicans. The proportion of C albicans remained steady over the four-year period, and there was no trend to increased resistance among C albicans. The number of isolates of all species with fluconazole microdilution minimum inhibitory concentration (MIC) greater than 8 mg/L in each of the four years were one of 32 in 1996, three of 26 in 1994, six of 33 in 1995, and one of 41 in 1996. A single isolate had an itraconazole microdilution MIC greater than 0.5 mg/L in each year. Prior use of antifungal therapy was rare in this patient population.

CONCLUSIONS

The increase in resistance to antifungal drugs reported by some centres did not occur in this institution over the course of the study. This experience may reflect differences in infection control practices and in patterns of use of antifungal agents. The NCCLS method was found to be superior to the E test as a routine method for testing susceptibility of yeasts.

Current and emerging azole antifungal agents

Major developments in research into the azole class of antifungal agents during the 1990s have provided expanded options for the treatment of many opportunistic and endemic fungal infections. Fluconazole and itraconazole have proved to be safer than both amphotericin B and ketoconazole. Despite these advances, serious fungal infections remain difficult to treat, and resistance to the available drugs is emerging. This review describes present and future uses of the currently available azole antifungal agents in the treatment of systemic and superficial fungal infections and provides a brief overview of the current status of in vitro susceptibility testing and the growing problem of clinical resistance to the azoles. Use of the currently available azoles in combination with other antifungal agents with different mechanisms of action is likely to provide enhanced efficacy. Detailed information on some of the second-generation triazoles being developed to provide extended coverage of opportunistic, endemic, and emerging fungal pathogens, as well as those in which resistance to older agents is becoming problematic, is provided.

Antifungal susceptibility of Candida isolates at the American University of Beirut Medical Center

For the first time from Lebanon, the antifungal susceptibility patterns of 70 consecutive clinical candida isolates (each from one patient) representing 48 C. albicans, 12 C. tropicalis, 6 C. parapsilosis, 2 C. kruseii, and 2 C. (Torulopsis) glabrata were studied against amphotericin B (AP), 5-fluorocytosine (FC), ketoconazole (KE), fluconazole (FL), and itraconazole (IT) using the Epsilometer test (E-test; AB Biodisk, Solna, Sweden). The MIC90 (and MIC range, mg/l) determined, at 24 h incubation, for each antifungal agent against C. albicans were: AP 0.032 (< or = 0.002-0.064), FC 0.75 (0.023-2), KE 0.064 (0.002- > 32), FL 2 (0.064- > 256), and IT 0.19 (0.012-2), against C. tropicalis were: AP 0.016 (< 0.002-0.047), FC 0.125 (0.023-0.19), KE 0.094 (0.012-0.19), FL 2 (0.5-2), and IT 0.5 (0.047-1) and against C. parapsilosis were: AP < 0.002 (< 0.002-0.002), FC 0.047 (0.003-0.5), KE 0.004 (0.002-0.004), FL 0.125 (0.032-0.19), and IT 0.004 (< 0.002-0.004). Based on the NCCLS established MICs breakpoints, resistance was found among C. albicans to FL (MIC > or = 6 mg/l) and IT (MIC > or = 1 mg/l) in 6 and 4%, respectively, and among C. tropicalis to IT in 17% of the isolates. The susceptibility dependent upon dose (S-DD) was noted only to IT (MIC 0.25-0.5 mg/l) among C. albicans (8%) and C. tropicalis (58%). MICs determination at 48 h incubation were higher, showed more resistance rates and more endpoint trailing particularly with the azoles drugs. The small numbers of C. kruseii and C. glabrata preclude providing meaningful results. Thus, this study indicates that the antifungal susceptibility by E-test can be conveniently incorporated and performed in a hospital-based clinical laboratory. Despite the uniform susceptibility to AP and FC, resistance to azoles drugs is encountered in a range of 4-17% among candida isolates in this country.

Antifungal susceptibility testing of yeast isolates from blood cultures by microbroth dilution and the E test

The results of microbroth dilution were compared with those of the E test for 169 yeast isolates tested for their susceptibility to antifungal agents. All isolates were tested by both methods against amphotericin B, ketoconazole, fluconazole, and itraconazole. The E test results generally correlated well with those obtained by the reference method. There was at least 80% agreement of minimum inhibitory concentration results within two dilutions for all yeast species and agents tested, except for Cryptococcus neoformans tested with fluconazole (8% agreement). The E test appears to be a suitable alternative antifungal susceptibility test method for yeasts, although improvements are required for testing Cryptococcus neoformans against fluconazole.

Azole resistance in Candida

Resistance of Candida to azoles is an increasing problem. Susceptibility testing of Candida against fluconazole and ketoconazole is now feasible and desirable. Good correlation of resistance in vitro with clinical failure of fluconazole therapy has now been shown in mucosal candidiasis. The relationship, if any, between resistance and clinical failure in the context of invasive candidiasis is not clear at present and additional correlative work needs to be done. Monitoring of resistance trends in Candida is clearly important now.

Evaluation of the E test for fluconazole susceptibility testing of Candida albicans isolates from oropharyngeal candidiasis

The aim of the present study was to evaluate the utility of the E test in determining the antifungal susceptibility of Candida albicans. Reproducibility of the E test was determined for amphotericin B, fluconazole, and itraconazole using three different solid media: RPMI 1640, Casitone, and yeast nitrogen base agar. Minimum inhibitory concentrations (MICs) were comparable (results at +/- 2 dilutions) in 92% of the tests for amphotericin B and in 100% for fluconazole and itraconazole. Determination of MIC endpoints was easiest on Casitone agar. Candida albicans isolates from 23 patients undergoing fluconazole therapy for oropharyngeal candidiasis were tested for fluconazole susceptibility. Good correlation was obtained between the MICs of fluconazole and clinical outcome. Clinical failure was associated with strains for which MICs were > or = 48 micrograms/ml. These results suggest that the E test has potential utility for fluconazole susceptibility testing of clinical yeast isolates.

Comparative evaluation of three antifungal susceptibility test methods for Candida albicans isolates and correlation with response to fluconazole therapy

In vitro susceptibilities were determined for 56 Candida albicans isolates obtained from the oral cavities of 41 patients with human immunodeficiency virus infection. The agents tested included fluconazole, itraconazole, ketoconazole, flucytosine, and amphotericin B. MICs were determined by the broth microdilution technique following National Committee for Clinical Laboratory Standards document M27-P (M27-P micro), a broth microdilution technique using high-resolution medium (HR micro), and the Etest with solidified yeast-nitrogen base agar. The in vitro findings were correlated with in vivo response to fluconazole therapy for oropharyngeal candidiasis. For all C. albicans isolates from patients with oropharyngeal candidiasis not responding to fluconazole MICs were found to be > or = 6.25 micrograms/ml by the M27-P micro method and > or = 25 micrograms/ml by the HR micro method as well as the Etest. However, for several C. albicans isolates from patients who responded to fluconazole therapy MICs found to be above the suggested breakpoints of resistance. The appropriate rank order of best agreement between the M27-P micro method and HR micro method was amphotericin B > fluconazole > flucytosine > ketoconazole > itraconazole. The appropriate rank order with best agreement between the M27-P micro method and the Etest was flucytosine > amphotericin B > fluconazole > ketoconazole > or = itraconazole. It could be concluded that a good correlation between in vitro resistance and clinical failure was found with all methods. However, the test methods used in this study did not necessarily predict clinical response to therapy with fluconazole.

Multisite reproducibility of the Etest MIC method for antifungal susceptibility testing of yeast isolates

A multicenter study was performed to establish the interlaboratory reproducibility of Etest, to provide an additional comparison of Etest MICs with reference broth macrodilution MICs, and to develop some tentative quality control (QC) guidelines for using Etest for antifungal susceptibility testing of Candida spp. Two QC strains, Candida parapsilosis ATCC 22019 and Candida krusei ATCC 6258, were tested by Etest against amphotericin B, fluconazole, flucytosine, itraconazole, and ketoconazole in each of four laboratories. The QC strains were tested 20 times each against the five antifungal agents by using a common lot of RPMI agar. A total of 80 MICs per drug per strain were generated during the study. Overall, 98 to 100% of the MICs fell within a 3 log2 dilution range for the respective yeast-antifungal agent combinations. The level of agreement of Etest MICs with broth macrodilution MICs was 86 to 100% with amphotericin B (C. krusei and C. parapsilosis), itraconazole (C. krusei and C. parapsilosis), flucytosine (C. parapsilosis), and fluconazole (C. parapsilosis). A lower level of agreement was observed with ketoconazole (C. krusei and C. parapsilosis). Although all participants reported identical Etest MICs, the MICs of flucytosine and fluconazole when tested against C. krusei fell well above the upper limits of the reference range for this strain. The tentative QC limits for the two QC strains and five antifungal agents when tested by the Etest methodology are the same as the QC limits when tested by the reference broth macrodilution method for amphotericin B and C. krusei, itraconazole and C. krusei, flucytosine and C. parapsilosis, fluconazole and C. parapsilosis, and itraconazole and C. parapsilosis. The Etest QC ranges are 1 dilution broader (4-dilution range) than the reference macrodilution method QC ranges for ketoconazole and C. krusei, amphotericin B and C. parapsilosis, and ketoconazole and C. parapsilosis.

Interlaboratory evaluation of Etest method for testing antifungal susceptibilities of pathogenic yeasts to five antifungal agents by using Casitone agar and solidified RPMI 1640 medium with 2% glucose

An interlaboratory evaluation (two centers) of the Etest method was conducted for testing the antifungal susceptibilities of yeasts. The MICs of amphotericin B, fluconazole, flucytosine, itraconazole, and ketoconazole were determined for 83 isolates of Candida spp., Cryptococcus neoformans, and Torulopsis glabrata. Two buffered (phosphate buffer) culture media were evaluated: solidified RPMI 1640 medium with 2% glucose and Casitone agar. MIC endpoints were determined after both 24 and 48 h of incubation at 35 degrees C. Analysis of 3,420 MICs demonstrated higher interlaboratory agreement (percentage of MIC pairs within a 2-dilution range) with Casitone medium than with RPMI 1640 medium when testing amphotericin B (84 to 90% versus 1 to 4%), itraconazole (87% versus 63 to 74%), and ketoconazole (94 to 96% versus 88 to 90%). In contrast, better interlaboratory reproducibility was determined between fluconazole MIC pairs when RPMI 1640 medium rather than Casitone medium was used (96 to 98% versus 77 to 90%). Comparison of the flucytosine MICs obtained with RPMI 1640 medium revealed greater than 80% reproducibility. The study suggests the potential value of the Etest as a convenient alternative method for testing the susceptibilities of yeasts. It also indicates the need for further optimization of medium formulations and MIC endpoint criteria to improve interlaboratory agreement.

Susceptibilities of Candida spp. to antifungal agents visualized by two-dimensional scatterplots of relative growth

The growth of 811 clinical yeast isolates in the presence of single concentrations of antifungal agents was measured spectrophotometrically and expressed as a percentage of growth in inhibitor-free control cultures. Two-dimensional scatterplots of the relative growth data allowed for the simple visual determination of some susceptibility trends, including correlations in relative growth between different agents and in relative susceptibilities between different yeast species. A positive susceptibility correlation was found for relative growth results with the azole antifungal agents fluconazole, itraconazole, and ketoconazole for 504 Candida albicans isolates. The relative growth scatterplots for fluconazole versus itraconazole showed that 50 (9.9%) of 504 C. albicans isolates were outliers with respect to the 95% confidence limits for a line of correlated relative growth established with an initial test panel of 59 isolates of this species. The outlying isolates were relatively less susceptible to fluconazole than to itraconazole under the conditions of the test. Most of the outliers were received in 1993 and 1994; only 3.9% of the isolates received in 1991 and 1992 and 1.7% of the isolates received before 1991 showed this differential susceptibility. In addition, most of the outliers came from patients with human immunodeficiency virus infections. The relative growth scatterplots confirmed the known high susceptibility of most Candida parapsilosis isolates to both fluconazole and itraconazole and the specifically low susceptibility of Candida krusei isolates to fluconazole. The scatterplots also illustrated a tendency towards lower (and correlative) relative growth among oral isolates obtained from AIDS patients who responded to azole antifungal treatment than among isolates from clinical nonresponders.

[Fluconazole and itraconazole susceptibility testing with clinical yeast isolates and algae of the genus Prototheca by means of the Etest]

Preliminary own results suggest, that the Etest (produced by AB BIODISK, Solna, Sweden) performed on casitone medium meets the requirements of a routine test of yeast susceptibility to fluconazole and itraconazole. Testing of 46 clinical yeast isolates, of 5 strains of Exophiala dermatitidis and 4 strains of algae of the genus Prototheca revealed species-, genus- and strain-specific variations of the susceptibility to fluconazole and itraconazole. Candida glabrata was less susceptible to both triazoles than the other Candida species with exception of Candida krusei. Exophiala dermatitidis was highly susceptible to itraconazole. Prototheca wickerhamii and P. zopfii were resistant to both triazoles. Casitone medium is most appropriate for the determination of susceptibility to fluconazole and itraconazole by the Etest. The results of the Etest were comparable with those of a breakpoint test (microdilution method).

[Comparison of three antifungal susceptibility testing methods for the in vitro testing of Candida isolates from patients with HIV infection]

The MIC values of fluconazole were determined for 96 Candida isolates (56 C. albicans, 15 C. glabrata, 9 C. krusei and 10 C. tropicalis). The methods employed for antifungal susceptibility testing were: microdilution according to the protocol M27-P of the NCCLS (M 27-Pmicro) using RPMI 1640 medium or HR medium following Troke & Pye (HRmicro) as well as the agar diffusion method by means of the Etest (YNB agar). The in vitro results were compared with the clinical outcome of patients. All C. albicans isolates received from AIDS patients with fluconazole-refractory candidosis showed MICs of > or = 6.25 mg/ml (M27-Pmicro) or > or = 25 mg/ml (MRmicro) and Etest). On the other hand some MICs of C. albicans isolates from AIDS patients with fluconazole-sensitive candidosis were also beyond these breakpoints. Therefore, a possible success of a fluconazole therapy cannot unequivocally be predicted from the MIC value determined in vitro.

Comparison of two alternative microdilution procedures with the National Committee for Clinical Laboratory Standards reference macrodilution method M27-P for in vitro testing of fluconazole-resistant and -susceptible isolates of Candida albicans

The National Committee for Clinical Laboratory Standards has proposed a reference broth macrodilution method for in vitro antifungal susceptibility testing of yeasts (the M27-P method). This method is cumbersome and time-consuming and includes MIC endpoint determination by the visual and subjective inspection of growth inhibition after 48 h of incubation. Two alternative microdilution procedures for MIC endpoint determination, a spectrophotometric MIC endpoint test that evaluates 80% growth inhibition by the drug and a colorimetric method with an oxidation-reduction indicator (Alamar Blue), were compared with the M27-P method for fluconazole susceptibility testing of 45 susceptible and resistant isolates of Candida albicans. The spectrophotometric method was performed with RPMI 1640 medium with 2% glucose, and the other two tests were performed with plain RPMI 1640 medium. All tests were incubated at 35 degrees C. Excellent agreement was demonstrated between the M27-P method and both 24-h microdilution tests (97.7%) as well as between the two microdilution tests (95.5%). Also, there was agreement in the detection in vivo of fluconazole resistance by the three methods. These preliminary data indicate that both microdilution methods may serve as less subjective alternatives to the M27-P method for the determination of fluconazole MIC endpoints.

Comparison of Etest and National Committee for Clinical Laboratory Standards broth macrodilution method for antifungal susceptibility testing: enhanced ability to detect amphotericin B-resistant Candida isolates

The National Committee for Clinical Laboratory Standards (NCCLS) proposed macrobroth reference method (M27P) for susceptibility testing of yeasts is technically difficult. We evaluated Etest, a simple agar-based MIC methodology, as a possible alternative. In studies of six yeast quality control strains, Etest yielded results identical to those obtained by the NCCLS reference method for both amphotericin B and fluconazole. In studies of 91 clinical Candida isolates, agreement +/- 2 dilutions between the two methods was 95% for fluconazole with phosphate-buffered RPMI 1640 agar and 96 to 97% for amphotericin B with either MOPS (morpholinepropanesulfonic acid)-buffered RPMI 1640 agar or antibiotic medium 3 agar. While the two methods had excellent general agreement, testing of a collection of amphotericin B-resistant isolates demonstrated that, unlike the NCCLS reference method, Etest readily identified the resistant isolates and could do so with a defined medium. Etest is equivalent to the NCCLS proposed method for susceptibility testing of yeasts and superior in its ability to detect amphotericin B resistance.

Comparative evaluation of macrodilution and alamar colorimetric microdilution broth methods for antifungal susceptibility testing of yeast isolates

A comparative evaluation of the macrodilution method and the Alamar colorimetric method for the susceptibility testing of amphotericin B, fluconazole, and flucytosine was conducted with 134 pathogenic yeasts. The clinical isolates included 28 Candida albicans, 17 Candida tropicalis, 15 Candida parapsilosis, 12 Candida krusei, 10 Candida lusitaniae, 9 Candida guilliermondii, 18 Torulopsis glabrata, and 25 Cryptococcus neoformans isolates. The macrodilution method was performed and interpreted according to the recommendations of the National Committee for Clinical Laboratory Standards (document M27-P), and the Alamar colorimetric method was performed according to the manufacturer's instructions. For the Alamar colorimetric method, MICs were determined at 24 and 48 h of incubation for Candida species and T. glabrata and at 48 and 72 h of incubation for C. neoformans. The overall agreement within +/- 1 dilution for Candida species and T. glabrata against the three antifungal agents was generally good, with the values for amphotericin B, fluconazole, and flucytosine being 85.3, 77.9, and 86.2%, respectively, at the 24-h readings and 69.3, 65.2, and 97.2%, respectively, at the 48-h readings. Most disagreement was noted with fluconazole against C. tropicalis and T. glabrata. Our studies indicate that determination of MICs at 24 h by the Alamar colorimetric method is a valid alternate method for testing amphotericin B, fluconazole, and flucytosine against Candida species but not for testing fluconazole against C. tropicalis and T. glabrata. For flucytosine, much better agreement can be demonstrated against Candida species and T. glabrata at the 48-h readings by the Alamar method. Excellent agreement within +/- dilution can also be observed for amphotericin B, fluconazole, and flucytosine (80, 96, and 96%, respectively) against c. neoformans when the MICs were determined at 72 h by the Alamar method.

Antifungal susceptibility testing of yeasts: evaluation of technical variables for test automation

The technical parameters for antifungal susceptibility testing with Candida species were reexamined to determine the optimal conditions for testing with semiautomated preparations of broth microdilution cultures, automated spectrophotometric readings of the cultures, and dose-response and endpoint determinations by means of a computer spreadsheet. Tests were based on proposed standard method M27P of the National Committee for Clinical Laboratory Standards for antifungal agents. RPMI 1640 broth with extra glucose to a final concentration of 2% gave higher and more reproducible drug-free control readings without affecting susceptibility endpoint readings. An inoculum of 8 x 10(4) yeasts per ml prepared from a carbon-limiting broth culture without further standardization was found to give optimal control readings after 48 h of incubation at 37 degrees C. For flucytosine, fluconazole, itraconazole, and ketoconazole, endpoints based on 50% growth inhibition (50% inhibitory concentration) gave the minimum variation with inoculum size and the fewest endpoint differences with RPMI 1640 medium obtained from two different suppliers. The 50% inhibitory concentration was also the optimal endpoint for fluconazole and ketoconazole susceptibilities in comparison with broth macrodilution MICs determined by the method of the National Committee for Clinical Laboratory Standards. Intralaboratory reproducibility was determined by retrospective analysis of replicate results for isolates retested at random over a 2-year period. This approach showed less favorable reproducibility than has been reported from purpose-designed, prospective antifungal susceptibility studies, but it may better reflect real-life test reproducibility. Susceptibility data for 616 clinical isolates of yeasts, representing 16 Candida and Saccharomyces spp., confirmed the tendency of Candida lusitaniae isolates to show relatively low susceptibilities to amphotericin B, the tendency of Candida krusei isolates to show low flucytosine and fluconazole susceptibilities, and the presence of some isolates in the species Candida albicans, Candida glabrata, and Candida tropicalis with low susceptibilities to azole derivative antifungal agents. The study demonstrates the value of automation and standardization in all stages of yeast susceptibility testing, from plate preparation to data analysis.