In the hunt for targets for a new class of antibacterials to overcome antibiotic resistance, we stumbled across an extraordinary shape-shifting protein. Proteus mirabilis ScsC – which supports bacterial swarming motility under copper stress – is a disulfide isomerase. Previously characterised bacterial disulfide isomerases are dimeric quality control agents that shuffle incorrect disulfide bonds in misfolded proteins. Unexpectedly, we show that PmScsC is not a dimer – but a trimer. We solved three crystal structures of PmScsC, revealing that it undergoes extraordinary twisting and extending motions. These motions – that underpin the function of PmScsC – are a consequence of a short, highly flexible shape-shifting motif resulting in major changes to the overall dimensions of the proteins, almost doubling its length in one dimension.