How does the host discriminate between pathogenic and commensal microbes ?

In response to the previous session on defining the molecular and cellular identity of pathogens, the first lecture focused on how the host distinguishes between commensal and pathogenic bacteria. It showed that Janeway's classic paradigm of recognition of specific prokaryotic molecular patterns (PAMPs) by dedicated receptors (PRRs), particularly Toll-like receptors (TLRs), was not sufficient and needed to be enriched by the integration of the notion of host perception of danger. This brings pathogenicity's own factors to the fore, with discrimination based on a mixed paradigm associating PAMP-PRR complexes and pathogenicity-induced signals: adhesion, invasion, membrane alteration, intracellular presence and growth, etc. This lecture was completed by Gérard Eberl (Institut Pasteur, Paris), whose seminar highlighted the major role played by the intestinal microbiota in the development of the mucosal immune system, in particular the maturation of secondary lymphoid organs and the appearance of tertiary lymphoid organs.