Detection of resistance to amphotericin B in Candida isolates by using Iso-Sensitest broth

Antifungal susceptibility testing following the CLSI M27 document, along with the measurement of MFC/MIC ratio, could be the optimal approach to detect amphotericin B resistance in Clavispora (Candida) lusitaniae. Susceptibility patterns of contemporary isolates of this species

Antifungal susceptibility testing (AST) is crucial in clinical settings to guide appropriate therapy. Nevertheless, discrepancies between treatment response and some results still persist, particularly in detecting resistance to amphotericin B (AMB) in Clavispora (Candida) lusitaniae. This study aimed to assess the susceptibility patterns of 48 recent isolates of C. lusitaniae to 9 antifungal agents and explore the feasibility of using a CLSI reference-based method to identify AMB resistance. Microdilution techniques revealed a wide range of minimal inhibitory concentration (MIC) values for azole antifungals, while echinocandins and AMB exhibited a narrow range of MIC values, with all strains considered wild-type for the tested polyene and echinocandins. However, when agar diffusion (ellipsometry) was employed for AST, certain strains displayed colonies within the inhibition ellipse, indicating potential resistance. Interestingly, these strains did not respond to AMB treatment and were isolated during AMB treatment (breakthrough). Moreover, the evaluation of AMB minimum fungicidal concentrations (MFCs) indicated that only the strains with colonies inside the ellipse had MFC/MIC ratios ≥ 4, suggesting reduced fungicidal activity. In conclusion, this study confirms the effectiveness of ellipsometry with RPMI-1640 2% glucose agar for detecting AMB resistance in C. lusitaniae. Additionally, the proposed approach of culturing "clear" wells in the microdilution method can aid in uncovering resistant strains. The findings highlight the importance of appropriate AST methods to guide effective treatment strategies for deep-seated candidiasis caused by C. lusitaniae. Further collaborative studies are warranted to validate these findings and improve the detection of AMB clinical resistance.

Modifications of antifungal sensibility testing as suggested by CLSI document M27-A4: proposal for using different culture medium and buffer

A common strategy in antifungal susceptibility testing is the utilization of the standardized protocol based on the microbroth dilution assay approach as described by the Clinical Laboratory Standards Institute (CLSI) (M27-A4). One major problem for laboratories in resource-limited countries with this protocol arises from the use of expensive culture media like RPMI-1640 and 3-N-morpholinopropanesulfonic acid (MOPS) buffer. One approach of circumventing this problem in cases of economic need is the evaluation of alternative culture media and buffers. The overall goal of this work was to investigate the influence of modifications in the protocol M27-A4 on diagnostic reliability. We performed univariate analyses evaluating (1) 2 different culture media (YNB and modified SAB); (2) three different buffers (sodium bicarbonate, Tris-HCL, and phosphate), as well as the influence of inoculum concentration (10², 10³, 10⁴, 10⁵ cells/mL), the influence of incubation time, and the influence of the assessment mode (visual, biological dye, and spectrophotometer). Our results suggested that (1) RPMI-1640 may be substituted by modified SAB and (2) MOPS buffer may be substituted by Tris-HCl buffer for defined analyses. By comparing the CLSI protocol and the alternative protocol proposed in the present study (modified SAB and Tris-HCl buffer) for the assessment of fluconazole susceptibility of eighteen yeasts (clinical isolates), similar results with both methodologies were recorded. We feel that this study should stimulate a discussion on the feasibility and evolution of the M27-A4 protocol in order to include pragmatic alternatives for resource-limited settings.

Factors influencing susceptibility testing of antifungal drugs: a critical review of document M27-A4 from the Clinical and Laboratory Standards Institute (CLSI)

Due to the increasing numbers of fungal infections and the emergence of drug-resistant fungi, optimization and standardization of diagnostic methods for the measurement of antifungal susceptibility are ongoing. The M27-A4 document by the US Clinical and Laboratory Standards Institute (CLSI) is presently used for the interpretation of minimum inhibitory concentrations of major opportunistic yeast species as measured by broth microdilution testing in many countries. Although microdilution is considered a benchmark for reproducible and accurate results, increased testing capacity, and limited human bias, the method is often inaccessible to routine clinical laboratories and researchers, especially in low-income countries. Furthermore, several studies suggest that there are still a considerable number of factors that make the estimation of in vitro activity of antifungal agents challenging. This review article summarizes the limitations of the M27-A4 standard which, despite the advances and improvements obtained by the standardization of antimicrobial resistance testing methods by CLSI, still persist.

Antifungal drug resistance mechanisms in pathogenic fungi: from bench to bedside

The phenotypic methods for identification of antifungal resistance are reliable procedures, and MIC determination by reference techniques is the gold standard to detect resistant clinical isolates. In recent years, progress has been made towards the description of resistance mechanisms at molecular level. There are methods of detection that can be useful for clinical laboratories, but lack of standardization precludes their full and effective integration in the routine daily practice. The molecular detection of Candida resistance to azoles and to echinocandins and of Aspergillus resistance to triazoles can be clinically relevant and could help to design more efficient prevention and control strategies. This text reviews the present state of the detection of mechanisms of resistance at the molecular level in Candida spp. and Aspergillus spp. and its relevance to clinical practice.

EUCAST technical note on the EUCAST definitive document EDef 7.2: method for the determination of broth dilution minimum inhibitory concentrations of antifungal agents for yeasts EDef 7.2 (EUCAST-AFST)

The European Committee on Antimicrobial Susceptibility Testing-Subcommittee on Antifungal Susceptibility Testing (EUCAST-AFST) has revised the EDef 7.1 document on the method for the determination of broth dilution minimum inhibitory concentrations of antifungal agents for fermentative yeasts. Changes are: dimethylsulphoxide is now the recommended solvent for caspofungin, micafungin and fluconazole; the shelf-life of plates containing the echinocandins prepared from stock solutions in dimethylsulphoxide is extended to 6 months at -80°C; testing of amphotericin and Cryptococcus has been incorporated; and minimum inhibitory concentration ranges for quality control strains and anidulafungin are included.

Echinocandin susceptibility testing of Candida species: comparison of EUCAST EDef 7.1, CLSI M27-A3, Etest, disk diffusion, and agar dilution methods with RPMI and isosensitest media

This study compared nine susceptibility testing methods and 12 endpoints for anidulafungin, caspofungin, and micafungin with the same collection of blinded FKS hot spot mutant (n = 29) and wild-type isolates (n = 94). The susceptibility tests included EUCAST Edef 7.1, agar dilution, Etest, and disk diffusion with RPMI-1640 plus 2% glucose (2G) and IsoSensitest-2G media and CLSI M27A-3. Microdilution plates were read after 24 and 48 h. The following test parameters were evaluated: fks hot spot mutants overlapping the wild-type distribution, distance between the two populations, number of very major errors (VMEs; fks mutants misclassified as susceptible), and major errors (MEs; wild-type isolates classified as resistant) using a wild-type-upper-limit value (WT-UL) (two twofold-dilutions higher than the MIC(50)) as the susceptibility breakpoint. The methods with the lowest number of errors (given as VMEs/MEs) across the three echinocandins were CLSI (12%/1%), agar dilution with RPMI-2G medium (14%/0%), and Etest with RPMI-2G medium (8%/3%). The fewest errors overall were observed for anidulafungin (4%/1% for EUCAST, 4%/3% for CLSI, and 3%/9% for Etest with RPMI-2G). For micafungin, VME rates of 10 to 71% were observed. For caspofungin, agar dilution with either medium was superior (VMEs/MEs of 0%/1%), while CLSI, EUCAST with IsoSensitest-2G medium, and Etest were less optimal (VMEs of 7%, 10%, and 10%, respectively). Applying the CLSI breakpoint (S Collapse

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MESH Headings

• Anidulafungin
• Antifungal Agents/pharmacology
• Candida/drug effects
• Candida/genetics
• Candidiasis/microbiology
• Caspofungin
• Culture Media
• Echinocandins/pharmacology
• Indicator Dilution Techniques
• Lipopeptides/pharmacology
• Micafungin
• Microbial Sensitivity Tests/methods
• Microbial Sensitivity Tests/standards
• Reference Standards

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Grants

• R01 AI069397 NIAID NIH HHS
• AI069397 NIAID NIH HHS

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Affiliation(s)

Maiken Cavling Arendrup Unit of Mycology and Parasitology (43/117), Statens Serum Institut, Artillerivej 5, DK-2300 Copenhagen S, Denmark.
Guillermo Garcia-Effron
Cornelia Lass-Flörl
Alicia Gomez Lopez
Juan-Luis Rodriguez-Tudela
Manuel Cuenca-Estrella
David S Perlin

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Relationship between susceptibility of Candida spp. isolates to amphotericin B and death or survival of patients with candidemia episodes

In this study, the susceptibility to amphotericin B of Candida spp. isolates obtained from patients with candidemia was related to their respective clinical outcomes. The susceptibility tests were carried out in three culture media: RPMI 1640, Antibiotic medium 3 and Yeast Nitrogen Base dextrose. We have found that minimal inhibitory concentrations and minimal fungicidal concentrations obtained using AM3 and YNBd media were significantly higher for Candida spp. from patients who died than for those from patients who survived the candidemia (P < 0.05). The assays with RPMI 1640 medium did not show these differences.

Effects of antifungal interventions on the outcome of experimental infections with phenotypic switch variants of Cryptococcus neoformans

In cryptococcal infection, phenotypic switching from a smooth to a mucoid variant can occur in vivo, producing variants with enhanced virulence that are subsequently selected and affect the outcome of infection. Here, we demonstrate that antifungal treatment of the chronically infected host can promote this phenomenon. Amphotericin B treatment reduces fungal burden less effectively in mucoid variant-infected than in smooth variant-infected mice. Consequently, amphotericin B treatment resulted in a more pronounced prolongation of survival in smooth variant-infected than in mucoid variant-infected mice (20 versus 42 days; P < 0.05). Administration of anticapsular monoclonal antibody mediated better protection in smooth variant-infected than in mucoid variant-infected mice, although a protective effect was not consistently observed at all doses. Most interestingly, both antifungal drug therapy and administration of anticapsular monoclonal antibody promoted the selection of mucoid variants in smooth variant-infected mice, a phenomenon manifested by a statistically higher percentage of mucoid colonies in smooth variant-infected mice than in nontreated control mice. This finding suggests that both chemotherapeutic and immunological antifungal interventions may promote the selection of the more virulent mucoid variant, which could affect the outcome of infection in chronically infected hosts.

Collaborative study of antibiotic medium 3 and flow cytometry for identification of amphotericin B-resistant Candida isolates

Center 1 used the National Committee for Clinical Laboratory Standards M27-A2 method and antibiotic medium 3 (AM3) test to determine amphotericin B resistance in 5 of 30 Candida isolates. These isolates were tested at center 2 by AM3 test and flow cytometry (FC). The agreements (C1-C2) were 90% for AM3 test and FC and 73% for the AM3 tests.

Transient fungemia caused by an amphotericin B-resistant isolate of Candida haemulonii

A bloodstream infection due to Candida haemulonii afflicting a patient with fever and a medical history of megaloblastic anemia is reported. The clinical isolate was misidentified by the API 20C and VITEK identification systems. The results of susceptibility tests showed that the MIC of amphotericin B for C. haemulonii was 4 microg/ml. Additional susceptibility testing procedures based on the use of antibiotic medium 3 and Iso-Sensitest broth were performed, and killing curves were determined. Two collection strains of C. haemulonii were employed as controls. The three isolates exhibited resistance to amphotericin B in vitro regardless of the antifungal susceptibility testing method employed. In addition, the MICs of fluconazole for the three isolates were high. Further studies are needed in order to ascertain whether this species exhibits innate or acquired resistance to amphotericin B and other antifungal agents.