Invasive Infection Associated With Probiotics in Adults: Does the Pathogen Originate From the Product or the Host Intestine?

Probiotics are live microorganisms administered on an outpatient basis and in hospital settings, including intensive care units, and the genera they contain have been associated with bacteremia, fungemia, and endocarditis.The question of how frequently this occurs is often raised.This review argues that this question cannot be answered until a prior question is resolved: whether the isolated microorganism originates from the administered product.Establishing this attribution requires comparing the isolate with the strain listed in the formulation because the same genera inhabit healthy intestines, and routine identification does not distinguish one strain from another.Therefore, this review addresses whether the relationship between probiotic administration and invasive infection by the microorganism it contains is causal and to what extent it is.The outcome is operationally defined as the isolation of one of these microorganisms in the blood or another normally sterile site in a patient with compatible symptoms, excluding colonization.We organized the evidence according to the nine Bradford Hill criteria, applied at two distinct levels: whether administering the product increases the risk among those exposed and whether a specific episode stems from the administered strain.Four databases were consulted, with coverage until August 2026 and a status check in September.Neonatal and pediatric populations were excluded from the study.Among the identified evidence, the association is strongly supported by biological plausibility, coherence, and analogy and weakly supported by specificity and experimental evidence.These two weak criteria are precisely what would determine causality; therefore, the published frequencies represent a maximum, rather than an estimate.The risk is demonstrated in individual cases; however, its population-level magnitude cannot be established based on the evidence we identified.