Gut bacteria influence Blastocystis sp. phenotypes and may trigger pathogenicity

Whilst the influence of intestinal microbiota has been shown in many diseases such as irritable bowel syndrome, colorectal cancer, and aging, investigations are still scarce on its role in altering the nature of other infective organisms. Here we studied the association and interaction of Blastocystis sp. and human intestinal microbiota. In this study, we investigated the gut microbiome of Blastocystis sp.-free and Blastocystis sp. ST3-infected individuals who are symptomatic and asymptomatic. We tested if the expression of phenotype and pathogenic characteristics of Blastocystis sp. ST3 was influenced by the alteration of its accompanying microbiota. Blastocystis sp. ST3 infection alters bacterial composition. Its presence in asymptomatic individuals showed a significant effect on microbial richness compared to symptomatic ones. Inferred metagenomic findings suggest that colonization of Blastocystis sp. ST3 could contribute to the alteration of microbial functions. For the first time, we demonstrate the influence of bacteria on Blastocystis sp. pathogenicity. When Blastocystis sp. isolated from a symptomatic individual was co-cultured with bacterial suspension of Blastocystis sp. from an asymptomatic individual, the parasite demonstrated increased growth and reduced potential pathogenic expressions. This study also reveals that Blastocystis sp. infection could influence microbial functions without much effect on the microbiota diversity itself. Our results also demonstrate evidence on the influential role of gut microbiota in altering the characteristics of the parasite, which becomes the basis for the contradictory findings on the parasite's pathogenic role seen across different studies. Our study provides evidence that asymptomatic Blastocystis sp. in a human gut can be triggered to show pathogenic characteristics when influenced by the intestinal microbiota. Author summarySingle-cell eukaryotes in the intestinal microbiota are increasingly gaining attention for their ability to influence microbiota composition. Blastocystis sp. is no exception. This study for the first time demonstrates gut microbiota alteration due to the colonization of Blastocystis sp. ST 3 in symptomatic and asymptomatic conditions. Colonization of Blastocystis sp. ST3, regardless of symptoms, significantly alters the diversity and microbiota composition. The abundance of Prevotella sp. was significantly elevated in symptomatic Blastocystis sp. carriage. Inferred metagenomic findings revealed that predicted metabolic functions were altered in Blastocystis sp. carriage in symptomatic and asymptomatic conditions. The current study demonstrates a bidirectional influence that seems to be crucial in Blastocystis sp.-microbiota interaction. Altering the accompanying microbiota of a symptomatic Blastocystis sp. with bacterial suspension from an asymptomatic condition resulted in the protozoan exhibiting asymptomatic characteristics. This implies, for the first time the effect of accompanying microbiota on Blastocystis sp. phenotypic characteristics. The findings lead to a postulation that a harmless protozoan parasite can be turned harmful by its accompanying microbiota in the gut.