Blastocystis is an emerging protistan parasite of controversial pathogenesis. Although metronidazole (Mz) is standard therapy for Blastocystis infections, there have been accumulating reports of treatment failure, suggesting the existence of drug-resistant isolates. Furthermore, very little is known about Blastocystis susceptibility to standard antimicrobials. In the present study, we established resazurin and XTT viability microassays for Blastocystis spp. belonging to subtypes 4 and 7, both of which have been suggested to represent pathogenic zoonotic subtypes. The optimized resazurin assay was used to screen a total of 19 compounds against both subtypes. Interestingly, subtype 7 parasites were resistant to Mz, a 1-position-substituted 5-nitroimidazole (5-NI), while subtype 4 parasites were sensitive. Some cross-resistance was observed to tinidazole, another 1-position 5-NI. Conversely, subtype 4 parasites were resistant to emetine, while subtype 7 parasites were sensitive. Position 2 5-NIs were effective against both subtypes, as were ornidazole, nitazoxanide, furazolidone, mefloquine, quinicrine, quinine, cotrimoxazole (trimethoprim-sulfamethoxazole), and iodoacetamide. Both subtypes were resistant to chloroquine, doxycycline, paromomycin, ampicillin, and pyrimethamine. This is the first study to report extensive variations in drug sensitivities among two clinically important subtypes.
Our study highlights the need to reevaluate established treatment regimens for Blastocystis infections and offers clear new treatment options for Mz treatment failures.Blastocystis is an emerging enteric protistan parasite with zoonotic potential (39,57,58). It is one of the most common parasites colonizing the human gut, with prevalences ranging between 10% of the population in developed countries and 50% in developing countries (58). It frequently infects immunocompromised individuals (27,40,59) and has a high prevalence in impoverished children (35) and HIV/AIDS (27) and cancer (59) patients. Individuals infected with Blastocystis present with common intestinal symptoms, such as abdominal pain, vomiting, and bloating, as well as mucous and watery diarrhea (58). Blastocystis infections are commonly associated with dermatological disorders (25, 67) and irritable bowel syndrome (54).Although metronidazole (Mz) treatment is considered firstline therapy for Blastocystis infections, therapeutic intervention is equivocal because of the large number of asymptomatic carriers and frequent reports of treatment failure (3,23,37,53,55). The confusion concerning the status of Blastocystis as a pathogen is primarily due to limitations of diagnostic techniques, purported subtype-dependent variations in parasite virulence, and variable host responses (55). The variation in treatment response suggests the presence of metronidazoleresistant (Mz r ) subtypes of the parasite, but there are currently no in vitro or in vivo data to support this hypothesis. Despite these controversies, interest in the parasite has increased in recent years, as sig...