Using a sequence-based approach we previously identified an IncI1 CTX-M-1 plasmid, pIFM3791, on a single pig farm in the UK that was harboured by Klebsiella pneumoniae, Escherichia coli and Salmonella enterica serotype 4,5,12:i:-. To test the hypothesis that the plasmid had spread rapidly into these differing host bacteria we wished to assess whether the plasmid conferred a fitness advantage. To do this an IncI1 curing vector was constructed and used to displace the IncI1 CTX-M-1 plasmids from K. pneumoniae strain B3791 and several other unrelated IncI1-harbouring strains indicating the potential wider application of the curing vector. The IncI1 CTX-M-1 plasmid was reintroduced by conjugation into the cured K. pneumoniae strain and also a naturally IncI1 plasmid free S. enterica serotype 4,5,12:i:-, S348/11. Original, cured and complemented strains were tested for metabolic competence using Biolog technology and in competitive growth, association to mammalian cells and biofilm formation experiments. The plasmid-cured K. pneumoniae strain grew more rapidly than either the original plasmid-carrying strain or plasmid-complemented strains in competition experiments. Additionally, the plasmid-cured strain was significantly better at respiring with L-sorbose as a carbon source and putrescine, γ-amino-n-butyric acid, L-alanine and L-proline as nitrogen sources. By contrast, no differences in phenotype were found when comparing plasmid-harbouring and plasmid-free S. enterica S348/11. In conclusion, the IncI1 curing vector successfully displaced multiple IncI plasmids. The IncI1 CTX-M1 plasmid conferred a growth disadvantage upon K. pneumoniae, possibly by imposing a metabolic burden, the mechanism of which remains to be determined.