Pentamidine Resistance in the Parasitic Flagellate Crithidia (Strigomonas) oncopelti

SYNOPSIS. A strain of Crithidia (Strigomonas) oncopelti has been developed which shows increased resistance to pentamidine. Growth of organisms from the sensitive strain was inhibited by 5.07 × 10−5 M pentamidine while the resistant strain was capable of growth in 5.07 × 10−5 M drug. Resistance developed gradually during 26 subcultures in the presence of sub‐inhibitory concentrations of the drug and was markedly reduced after a further 84 subcultures in the absence of pentamidine. The pentamidine‐resistant strain was also resistant to stilbamidine and propamidine and showed some cross resistance to an aminoquinaldine drug (antrycide, quinapyramine) and a phenanthridine drug (ethidium, homidium). The binding of pentamidine and stilbamidine by pentamidine‐resistant and sensitive strains of the organism was compared. The pentamidine‐resistant strain bound very low concentrations of both drugs compared with the sensitive strain. The distribution of pentamidine among those subcellular fractions of disrupted preparations of C. oncopelti, which could be separated by differential centrifugation, was compared in the two strains. The majority of the drug was bound to material sedimenting in 10 min at 700 ×g in both strains.