Fig. 2: Lactose metabolism facilitates E. coli 8178 growth in the mouse intestine.
a, Detection of LacI expression in E. coli 8178. Soluble extracts of E. coli 8178 strains (WT and lacI mutant) were subjected to a 12.5% acrylamide SDS-PAGE analysis followed by an immunodetection step using an anti-LacI antibody. Detected LacI is indicated on the right. The molecular weight marker (in kDa) is indicated on the left. The western blot was performed independently twice, and a representative experiment is shown. b, β-galactosidase activity assays. WT and lacI-deficient E. coli 8178 strains were grown for 8 h on a minimal medium supplemented with glucose (20 mM) or lactose (10 mM or 100 mM). Bacterial cells were collected and subjected to a β-galactosidase activity assay. The β-galactosidase activity is expressed in Miller units (MU). Activities are the mean of six biological replicates, and the error bars indicate standard deviation. c, Experimental scheme. Streptomycin-pretreated SPF 129S6/SvEvTac mice were inoculated with an equal mixture of the E. coli 8178 WT and lacZ-deficient strains. The drinking water was supplemented (3% w/v) or not (0%) with lactose. d, Competitive experiment. The WT and lacZ-deficient strains were used to inoculate streptomycin-pretreated 129S6/SvEvTac mice supplemented (3%) or not (0%) with lactose in the drinking water. The WT and lacZ counts were determined by selective plating. The normalized C.I is calculated as the ratio between the mutant (lacZ) and WT divided by the ratio of both strains in the inoculum. e, Complementation experiments. The lacZ mutant was transformed with a plasmid constitutively expressing the lacZ gene. The fitness of the resulting strain (lacZ + p-lacZ) was analysed through a competitive experiment in streptomycin-pretreated 129S6/SvEvTac mice supplemented with lactose (3%) in the drinking water. d,e, The x axis represents the time post-inoculation (in days). The bars represent the median, and the dotted lines represent the C.I expected for a fitness-neutral mutation. The results from at least two independent replicates are shown. b,d,e, Two-tailed Mann–Whitney U tests were used to compare two groups in each panel. NS, not significant (P ≥ 0.05); **P < 0.01; ***P < 0.001.
