Evolution of the Sterol Biosynthetic Pathway of Pythium insidiosum and Related Oomycetes Contributes to Antifungal Drug Resistance

Pythiosis is a life-threatening infectious disease caused by the oomycetePythium insidiosum . Direct exposure toPy. insidiosum zoospores can initiate infections of the eye, limb, gastrointestinal tract, or skin/subcutaneous tissue. Treatments for pythiosis have mostly relied on surgery. Antifungal drugs are generally ineffective againstPy. insidiosum . However, one patient with an invasivePy. insidiosum infection recovered completely following treatment with terbinafine and itraconazole. Additionally, the drug target sterol biosynthetic enzymes have been identified in the oomyceteAphanomyces euteiches . It remains an open question whetherPy. insidiosum is susceptible to the antifungal drugs and harbors any of the known drug target enzymes. Here, we determined thein vitro susceptibilities of terbinafine and itraconazole against 30 isolates ofPy. insidiosum . We also analyzed endogenous sterols and searched for genes encoding the sterol biosynthetic enzymes in the genomes ofPy. insidiosum and related oomycetes. The susceptibility assay showed that the growth of each of thePy. insidiosum isolates was inhibited by the antifungal agents, but only at difficult-to-achieve concentrations, which explains the clinical resistance of the drugs in the treatment of pythiosis patients. Genome searches ofPy. insidiosum and related oomycetes demonstrated that these organisms contained an incomplete set of sterol biosynthetic enzymes. Gas chromatographic mass spectrometry did not detect any sterol end products inPy. insidiosum . In conclusion,Py. insidiosum possesses an incomplete sterol biosynthetic pathway. Resistance to antifungal drugs targeting enzymes in the ergosterol biosynthetic pathway inPy. insidiosum was due to modifications or losses of some of the genes encoding the drug target enzymes.