Crithidia fasciculata Shows Non-Pathogenic Behavior in Leishmania Co-Infection Related to Temperature Stress, In Vitro and In Vivo Infections, and Amphotericin B Susceptibility

There is increasing evidence on the occurrence of Crithidia spp. in patients presenting either cutaneous or visceral leishmaniasis, solely or associated with Leishmania. We analyzed growth, morphology, and temperature tolerance of two C. fasciculata strains, the reference strain COLPROT048 and patient isolate COLPROT606. We also evaluated their co-cultivation with L. braziliensis, macrophage infectivity, and infections in hamsters, BALB/c mice, and sandflies. In culture, both Crithidia strains survived at 32 °C for 96 h, showing major morphological alterations and decreased mitochondrial membrane potential, with ΔΨm reducing to 52% in COLPROT606. At 34 °C, the patient isolate showed an 80% reduction in cell number. Mixed cultivation of Crithidia-Leishmania led to recovery of only Crithidia. In macrophages, C. fasciculata alone was virtually eliminated, and in co-infection only Leishmania was detected. No Crithidia lesion or RNA were found in infected mice or hamsters, while L. braziliensis reached 1145–1625 parasites/mg of tissue. In sandflies, C. fasciculata successfully established infection for up to 7 days, both alone and in coinfections. Amphotericin B IC50 values at 72 h were 4- to 5-fold higher in C. fasciculata strains compared to L. braziliensis. Our results indicate that both C. fasciculata strains are unable to reproduce the pathogenic effect in vitro and in vivo models.