These appendages are very difficult to see using the available electron microscopy techniques, and were originally classified as non-fibre or 'afimbrial'.
The new key piece of evidence emerged from some nifty protein engineering of the protein subunit, known as AfaE.
These subunits make up the fibre and cannot fold properly as proteins as they are synthesised with a piece of sequence information missing. In nature, the adjacent subunit possesses this bit of missing information, and 'lends' it to complete the subunit and build the fibre, in a process known as donor strand complementation.
In the lab, the Imperial team artificially reintroduced the missing protein sequence so that it could complement itself and fold correctly, demonstrating that the fibres assemble in an similar fashion to fimbrial proteins.