a, Concentration of AIP-I during S. aureus growth. Strain LAC (USA300) was grown in TSB, and AIP-I concentrations were measured by RP-HPLC/ESI-MS. Calibration was performed using synthetic AIP-I. The detection limit of this assay is around 0.3 μM. The experiment was performed with n = 3 independent biological samples. Data are mean ± s.d. b, B. subtilis colonization kinetics in the mouse intestinal colonization experiment. Mice (n = 5) received 200 μl of a 108 CFU ml−1 suspension of wild-type B. subtilis or ΔfenA mutant spores by oral gavage; CFU in the faeces were analysed up to five days afterwards. Data are mean ± s.d. c–f, Inhibition mouse model with strains USA300 LAC and ST88 JSNZ. The experimental set-up was as shown in Fig. 5a. In brief, n = 4 or 5 mice per group received 200 μl of 108 CFU ml−1 S. aureus strains USA300 LAC or ST88 JSNZ by oral gavage. On the next day and every following second day, the mice received 200 μl of 108 CFU ml−1 spores of wild-type B. subtilis or its isogenic fenA mutant, also by oral gavage. CFU in the faeces were determined two, four and six days after infection. At the end of the experiment (day seven), CFU in the small and large intestines were determined. The experiment was performed with (c, d) or without (e, f) antibiotic pretreatment. Statistical analysis was performed using Poisson regression versus values obtained with wild-type B. subtilis spore samples. *P < 0.0001. Data are mean ± s.d. Note that no S. aureus were found in the faeces or intestines of any mouse challenged with any S. aureus strain that also received Bacillus wild-type spores. The corresponding zero values are plotted on the x axis of the logarithmic scale.