Background: The potential aetiological relevance of Blastocystis spp. and Dientamoeba fragilis in the human intestine, and their possible associations with Campylobacter spp. and Giardia duodenalis, remain unclear. By incorporating Bayesian priors to account for diagnostic test accuracy, we statistically analysed the interactions among these microorganisms. Methods: Diagnostic test accuracy data were derived from multiple PCR assays and incorporated as priors to adjust for non-differential misclassification. Bayesian odds ratios and relationships based on DNA quantity were assessed for a dataset of 1,065 stool samples containing at least one of the four target microorganisms. Results: Accounting for diagnostic test accuracy resulted in wide credibility intervals. Blastocystis spp. were negatively associated with G. duodenalis. G. duodenalis was most often detected in the absence of Blastocystis spp. and D. fragilis, whereas detection of Blastocystis spp. was associated with lower Campylobacter spp. DNA abundance. A negative association between Blastocystis spp. and Campylobacter spp. was observed only in the absence of D. fragilis. Conclusion: The assumed variation in detection rates of Campylobacter spp. and G. duodenalis based on the presence of Blastocystis spp. and/or D. fragilis was confirmed. Future epidemiological studies should explore interactions among multiple microorganisms using robust statistical approaches.  Kulcsszavak: cycle threshold value, Bayesian analysis, diagnostic test accuracy, odds ratio, opportunistic bacterial interaction