Screening of azole resistance in Aspergillus fumigatus using the EUCAST E.Def 10.2 azole-containing agar method: a single study suggests that filtration of conidial suspensions prior to inoculum preparation may not be needed

Background: Azole resistance screening in A. fumigatus isolates can be routinely carried out by using azole-containing plates (E.Def 10.2 method), that requires filtering conidial suspensions prior inoculum adjustment.

Objectives: We evaluated whether skipping the filtration step of conidial suspensions negatively influences the performance of the E.Def 10.2. Patients/Methods A. fumigatus sensu stricto isolates (n=92), classified as azole-susceptible or azole-resistant according to the EUCAST microdilution E.Def 9.4 method, were studied. Azole-resistant isolates had either wild type cyp51A gene sequence (n = 3) or the TR₃₄ -L98H (n = 26), G54R (n = 5), TR₄₆ -Y121F-T289A (n = 1), F46Y-M172V-N248T-D255E-E427K (n = 1), F165L (n=1), or G448S (n=1) cyp51A gene substitutions. In-house azole-containing agar plates were prepared according to the EUCAST E.Def 10.2 procedure. Conidial suspensions were obtained by adding distilled water (Tween 20 0.1%). Subsequently, the suspensions were either filtered or left unfiltered prior to inoculum adjustment to 0.5 McFarland. Using microdilution as the gold standard, agreement, sensitivity, and specificity of the agar plates inoculated with two inoculums were assessed.

Results: Agreements for the agar screening method with either unfiltered or filtered conidial suspensions were high for itraconazole (100%), voriconazole (100%), and posaconazole (97.8%). Sensitivity (100%) and specificity (98.2%) of the procedure to rule in or out resistance when unfiltered suspensions were used were also high. Isolates harbouring the TR₃₄ -L98H, G54R, and TR₄₆ -Y121F-T289A substitutions were detected with the modified method.

Conclusions: Unfiltered conidial suspensions does not negatively influence the performance of the E.Def 10.2 method when screening for A. fumigatus sensu stricto.