Administration of harmless bacteria induces rapid tolerance to non-pathogenic and pathogenic gut bacteria.

<p><b>A</b>) Neutrophil infiltration in murine Peyer’s patches 3 h after the last of one, two, or three daily doses of probiotic <i>EcN</i> to BALB/c mice. Neutrophils numbers in control mice that received only PBS are also shown. Similar data were obtained in two independent experiments. <b>B</b>) Neutrophil accumulation in Peyer’s patches two days after oral infection with pathogenic <i>Salmonella</i> in wildtype C57BL/6 mice (B6) and mutant mice deficient in TLR2. One day before <i>Salmonella</i> infection, mice received PBS (open circles), or a single dose of live (filled circles) or formalin-fixed (filled triangles) <i>EcN</i>. Similar data were obtained in one independent experiment. <b>C</b>) Neutrophil accumulation in Peyer’s patches two days after oral infection with pathogenic <i>Salmonella</i> in BALB/c mice. One day before <i>Salmonella</i> infection, mice received PBS (open circles), or a single dose of avirulent live <i>Salmonella aroA asd</i> (filled circles). Pooled data from two independent experiments are shown. Significance of differences between groups was determined using t-test (***, <i>P</i><0.001; **, <i>P</i><0.01; *, <i>P</i><0.05).