a–c, Faecal bacterial-population sizes and inflammatory status of mice in Fig. 1c, d (comparison of invasive versus non-invasive donors in the oral model). Faecal loads of donors (blue, SmR and CmR), recipients (green, KanR), and transconjugants (red, CmR and KanR) were determined by selective plating on MacConkey agar. Black dotted line indicates detection limits for donors and transconjugants. Green dotted line indicates detection limit for recipients. The detection limit is higher for recipients once transconjugants reach a density of >108 CFU per gram of faeces. Before this happens, recipients can be found below the detection limit; the black dotted line should then be considered as the detection limit. Blue lines connect medians of donor populations; red lines connect medians of transconjugant populations. a, Mice infected with invasive S. Typhimurium donors (solid circles; n = 15 singly housed mice from 5 independent experiments). b, Mice infected with non-invasive S. Typhimurium donors (open circles; n = 6 singly housed mice from 2 independent experiments). c, Inflammatory status was determined by lipocalin-2 ELISA. Statistics were performed using a two-tailed Mann–Whitney U-test. NS, not significant (P ≥ 0.05), ****P < 0.0001, comparing mice infected with an invasive donor (solid black circles; n = 15 singly housed mice from 5 independent experiments) to mice infected with a non-invasive donor (open black circles; n = 6 singly housed mice from 2 independent experiments) at each time point. Medians are shown (solid red line for invasive donors; dotted red line for non-invasive donors). Dotted line indicates the detection limit. d, e, Carrying P2cat does not lead to a measurable fitness cost or benefit. d, A ‘locked’ transconjugant (14028S P2aphT ΔoriT (KanR and AmpR), in which conjugation is blocked by removing the origin of transfer) was competed against a recipient (14028S cat (CmR and AmpR)). e, To ensure that removing the origin of transfer did not affect fitness, the locked transconjugant was competed against a transconjugant with a normal P2cat plasmid (that is, mobile transconjugant) (14028S P2cat (CmR and AmpR)). In d, e, both strains were introduced at a 1:1 ratio (total inoculum size about 5 × 107 CFU per os) and faeces were monitored daily by selective plating (n = 6 singly housed mice from 2 independent experiments for both experiments). The competitive index is calculated by dividing the population size of one competitor by the other. Lines indicate medians. The dotted line indicates no competitive advantage for either strain. This is consistent with previously published data20. These data indicate that it is the plasmid-encoded conjugation efficiency (not the effects of the plasmid on host bacterial fitness) that drives the rise of transconjugants (Fig. 1c). f, g, Plasmid transfer in the oral model does not require an invasive recipient. Invasive donors (SL1344 P2cat (SmR and CmR)) were orally infected into pre-treated mice. After ciprofloxacin treatment, a non-invasive mutant of S. Typhimurium 14028S was used as a recipient (non-invasive 14028S aphT (KanR and AmpR)). n = 5 mice. f, Selective plating determined faecal loads of donors (blue, SmR and CmR), recipients (green, KanR), and transconjugants (red, CmR and KanR). Black dotted line indicates the detection limit for donors and transconjugants. Green dotted line indicates the detection limit for recipients. The detection limit is higher for recipients once transconjugants reach a density of >108 CFU per gram of faeces. Before this happens, recipients can be found below the detection limit; the black dotted line should then be considered as the detection limit. Blue lines connect medians of donor populations; red lines connect medians of transconjugant populations. g, Donor populations enumerated after a gentamicin protection assay on caecal tissue of mice shown in f. Median indicated by solid line. Dotted line indicates the detection limit. h, i, Conjugation is required for plasmid transfer after antibiotic treatment. Mice were infected with invasive S. Typhimurium that lacks the origin of transfer in P2cat (SL1344 P2cat ΔoriT (SmR and CmR)) as a donor, and 14028S aphT (KanR and AmpR) as a recipient, after antibiotic treatment. n = 5 mice. h, Selective plating determined the faecal loads of donors (blue, SmR and CmR), recipients (green, KanR) and transconjugants (red, CmR and KanR). Black dotted line indicates the detection limit for donors and transconjugants. Green dotted line indicates detection limit for recipients. The detection limit is higher for recipients once transconjugants reach a density of >108 CFU per gram of faeces. Before this happens, recipients can be found below the detection limit; the black dotted line should then be considered as the detection limit. Blue lines connect medians of donor populations; red lines connect medians of transconjugant populations. i, Donor populations enumerated after a gentamicin protection assay on caecal tissue of mice shown in h. Median indicated by solid line. Dotted line indicates the detection limit. Source data