L-form bacteria cohabitants in human blood: significance for health and diseases.

From a historical perspective, intriguing assumptions about unknown "live units" in human blood have attracted the attention of researchers, reflecting their desire to define a new class of microorganisms. Thus, the concept of "blood microbiota" brings about many questions about the nature, origin, and biological significance of the "unusual microbial cohabitants" in human blood. In contrast to current views that bloodstream in healthy humans is sterile, the hypothesis about the existence of microbes as L-forms (cell wall deficient bacteria) in human blood has evolved on the basis of known facts about their unique biology, as observed in our studies and those of other authors. Recently, we reported that bacterial L-forms persist in the human blood and that filterable, self-replicating bodies with a virus-like size of 100 nm are able to cross the maternal-fetal barrier by vertically transmitted pathway, then enter fetus blood circulation and colonize newborns. Subjects discussed here include the following: Is the existence of L-form bacteria in human blood a natural phenomenon? Are L-form bacteria commensal cohabitants in the human body? Since blood is an unfavorable compartment for the classical bacteria and their propagation, how do L-forms survive in blood circulation? How does L-form microbiota in blood influence the host immune system and contribute to systemic inflammatory, autoimmune, and tumor diseases? The current commentary presents the topic of "human microbiota and L-form bacteria" in its microcosm. It contains details of the hypothesis, supporting evidence and important implications.