Table 1.

Non-pathogenic bacterial members of the intestinal microbiota with immune effects

	Concept	Examples*	Association with Host	Immune Effects	Mechanisms*
Probiotics	confer health benefit to the host when administered in adequate amount not necessarily part of the “normal microbiota” may affect beneficial microbiota (indirect effects)	Bifidobacterium spp; Lactobacillus spp	Transient	Innocuous Immunostimulatory	Cytokine induction, TLR activation, Pathobiont and pathogen suppression, Lactic acid Short-chain fatty acids
Autobionts	direct influence on host immune cell homeostasis or function part of the “normal microbiota.”	Bacteroides fragilis Clostridia XIVa and IV SFB Faecalibacterium prauznitsii	Permanent Host-dependent Symbiotic	Immunomodulatory	Largely Unknown (TLR2, metabolites (?), antigens (?), effects on IEC function (?))
Pathobionts	do not cause disease in the presence of normal microbiota in healthy host cause disease when microbiota or host immunity is perturbed	Helicobacter hepaticus Clostridium difficile Prevotela spp. Klebsiella spp Bilophila wadsworthia	Permanent Pa rasitic/I nfectious	Innocuous Detrimenta	Invasive mechanisms, Spore formation, Toxins

^*Examples and mechanistic studies have been performed in mice (with the exception of F. prauznitsii and C. difficile). See text for details and references. (?) Speculative mechanisms