LAUSR

Graphical abstract Figure. No caption available. Abstract Poorly formulated probiotic supplements intended for oral administration often fail to protect bacteria from the challenges of human digestion, meaning bacteria do not reach the small intestine in a viable state. As a result, the ability of probiotics to influence the human gut microbiota has not been proven. Here we show how (i) considered formulation of an aqueous probiotic suspension can facilitate delivery of viable probiotic bacteria to the gut and (ii) quantitate the effect of colonisation and proliferation of specific probiotic species on the human gut microbiota, using an in‐vitro gut model. Our data revealed immediate colonisation and growth of three probiotic species in the luminal and mucosal compartments of the proximal and distal colon, and growth of a fourth species in the luminal proximal colon, leading to higher proximal and distal colonic lactate concentrations. The lactate stimulated growth of lactate‐consuming bacteria, altering the bacterial diversity of the microbiota and resulting in increased short‐chain fatty acid production, especially butyrate. Additionally, an immunomodulatory effect of the probiotics was seen; production of anti‐inflammatory cytokines (IL‐6 and IL‐10) was increased and production of inflammatory chemokines (MCP‐1, CXCL 10 and IL‐8.) was reduced. The results indicate that the probiotic species alone do not result in a clinical effect; rather, they facilitate modulation of the gut microbiota composition and metabolic activity thereby influencing the immune response.