The caspofungin paradoxical effect is a tolerant "Eagle effect" in the filamentous fungal pathogen Aspergillus fumigatus

Cell responses against antifungals different from resistance have been rarely studied in filamentous fungi, while terms such as tolerance and persistence are well-described for bacteria and increasingly examined in yeast-like organisms. <i>Aspergillus fumigatus </i>is a filamentous fungal pathogen that causes a disease named aspergillosis and caspofungin (CAS), a fungistatic drug, used as second-line therapy. Some <i>A. fumigatus </i>clinical isolates can survive and grow in CAS concentrations above the minimum effective concentration (MEC), a phenomenon known as “caspofungin paradoxical effect” (CPE). Here we evaluated the CPE in 67 <i>A. fumigatus </i>clinical isolates by calculating the Recovery Rate (RR) value, where isolates with RR ≥ 0.1 are considered CPE⁺ while isolates with RR &lt; 0.1 are classified as CPE^-. Conidia produced by three CPE⁺ clinical isolates, CEA17 (RR=0.42), Af293 (RR=0.59), CM7555 (RR=0.38) all showed the ability to grow in high levels of CAS while all conidia produced by the CPE^- isolate IFM61407 (RR=0.00) showed no evidence of paradoxical growth. Given the importance of calcium/calcineurin/transcription factor CrzA pathway in CPE regulation, we also demonstrated that all Δ<i>crzA</i>^CEA17 (CPE⁺) conidia showed CPE while 100% of Δ<i>crzA</i>^Af293(CPE^-) were CPE^-.As all spores derived from an individual strain were phenotypically indistinct with respect to CPE it is likely that CPE is a genetically encoded adaptive trait that should be considered an antifungal tolerant phenotype. As the RR parameter shows that the strength of the CPE is not uniform between strains, we propose that the mechanisms that govern this phenomenon are multi-factorial. <b> </b>