Ubiquitin is highly conserved across eukaryotes and is essential for normal eukaryotic cell function. The bacterium Bacteroides fragilis is a member of the normal human gut microbiota, and the only bacterium known to encode a homologue of eukaryotic ubiquitin. The B. fragilis gene sequence indicates a past horizontal gene transfer event from a eukaryotic source. It encodes a protein (BfUbb) with 63% identity to human ubiquitin which is exported from the bacterial cell. The aim of this study was (i) to determine if there was antigenic cross‐reactivity between B. fragilis ubiquitin and human ubiquitin and (ii) to determine if humans produced antibodies to BfUbb. Molecular model comparisons of BfUbb and human ubiquitin predicted a high level (99·8% confidence) of structural similarity. Linear epitope mapping identified epitopes in BfUbb and human ubiquitin that cross‐react. BfUbb also has epitope(s) that do not cross‐react with human ubiquitin. The reaction of human serum (n = 474) to BfUbb and human ubiquitin from the following four groups of subjects was compared by enzyme‐linked immunosorbent assay (ELISA): (1) newly autoantibody‐positive patients, (2) allergen‐specific immunoglobulin (Ig)E‐negative patients, (3) ulcerative colitis patients and (4) healthy volunteers. We show that the immune system of some individuals has been exposed to BfUbb which has resulted in the generation of IgG antibodies. Serum from patients referred for first‐time testing to an immunology laboratory for autoimmune disease are more likely to have a high level of antibodies to BfUbb than healthy volunteers. Molecular mimicry of human ubiquitin by BfUbb could be a trigger for autoimmune disease.

Immunomagnetic separation (IMS) can selectively isolate and concentrate Mycobacterium bovis cells from lymph node tissue to facilitate subsequent detection by PCR (IMS-PCR) or culture (IMS-MGIT). This study describes application of these novel IMS-based methods to test for M. bovis in a survey of 280 bovine lymph nodes (206 visibly lesioned (VL), 74 non-visibly lesioned (NVL)) collected at slaughter as part of the Northern Ireland bovine TB eradication programme. Their performance was evaluated relative to culture. Overall, 174 (62.1%) lymph node samples tested positive by culture, 162 (57.8%) by IMS-PCR (targeting IS6110), and 191 (68.2%) by IMS-MGIT culture. Twelve (6.9%) of the 174 culture positive lymph node samples were not detected by either of the IMS-based methods. However, an additional 79 M. bovis positive lymph node samples (27 (13.1%) VL and 52 (70.3%) NVL) were detected by the IMS-based methods and not by culture. When low numbers of viable M. bovis are present in lymph nodes (e.g. in NVLs of skin test reactor cattle) decontamination prior to culture may adversely affect viability, leading to false negative culture results. In contrast, IMS specifically captures whole M. bovis cells (live, dead or potentially dormant) which are not subject to any deleterious treatment before detection by PCR or MGIT culture. During this study only 2.7% of NVL lymph nodes tested culture positive, whereas 70.3% of the same samples tested M. bovis positive by the IMS-based tests. Results clearly demonstrate that not only are the IMS-based methods more rapid but they have greater detection sensitivity than the culture approach currently used for the detection of M. bovis infection in cattle. Adoption of the IMS-based methods for lymph node testing would have the potential to improve M. bovis detection in clinical samples.