Detection of virulence factors and antifungal susceptibilities of Fusarium strains isolated from keratitis cases

Abstract

Fusarium species have gained importance as a cause of keratitis. The pathogenicity and virulence factors of genus Fusarium remain largely unknown. Several putative virulence factors have been reported for fungal pathogens, including biofilm formation, production of proteinases and other hydrolytic enzymes. It has been emphasized that Fusarium species are generally resistant to antifungals but the resistance may vary depending on the species and even according to the isolate. For this reason, pathogenic features and antifungal susceptibility of the clinical isolates gained importance for the management of keratitis cases. The aim of this study was to identify clinical Fusarium isolates, to evaluate their virulence factors and to show antifungal susceptibility patterns. The identification of Fusarium was made on genus level isolated from 25 keratitis cases. Among them, 13 of the isolates were identified by ITS sequencing on species complex level. The production of hemolytic activity, caseinase, esterase, proteinase and phospholipase activity were investigated in 13 of the isolates. Biofilm production was searched among all 25 isolates. Galleria mellonella larvae was used as in vivo infection model. Antifungal susceptibility for amphotericin B, itraconazole, voriconazole and posaconazole was performed according to the Clinical and Laboratory Standards Institute (CLSI) M38-A2 microdilution assay guidelines. As the subcommittee on antifungal susceptibility tests did not determine the clinical resistance breakpoints (CBP) specific to Fusarium species complex, the epidemiological cut off values (ECV) were used for the interpretation of the minimum inhibitory concentration (MIC) values of the antifungal drugs. Isolates were identified as six F.oxysporum, six F.solani species complex and one F.brachygibbosum. One F.solani, one F.oxysporum were positive for hemolytic activity; all isolates were caseinase positive; three F.oxysporum and two F.solani isolate were esterase positive; one F.solani isolate was proteinase positive; five F.oxysporum and two F.solani isolates were phospholipase positive; biofilm activity was positive in 52% of the 25 isolates. The larvae survived for seven days after Fusarium inoculation in the G.mellonella larvae model. MIC range was 0.5-8 µg/ml for amphotericin B, 2-32 µg/ml for itraconazole, 0.5-8 µg/ml for voriconazole, 0.5-16 µg/ml for posaconazole and according to the ECV values F.solani and F.oxysporum isolates were determined as wild type for four antifungal agents. As a result, it was shown that Fusarium isolates have some virulence factors, there was a concordance between in vitro virulence properties and in vivo virulence characteristics and some of the isolates were classified as antifungal susceptible wild type isolates.