Development of in vitro functional antibody assays to identify and assess vaccine targets against E. coli bacteraemia

Extra-intestinal pathogenic E. coli (ExPEC) is one of the leading causes of bacteraemia and urinary tract infections (UTI) worldwide. At present, there is no licensed vaccine available and with the increasing incidence of multidrug resistance (MDR) the demand for developing alternatives to antibiotics is paramount. We have developed in vitro functional antibody assays to fast track screening of patient convalescent sera from ExPEC infections to identify potential vaccine antigens. A functional immunological assay will also facilitate the assessment of vaccine candidates against the many disease-causing serotypes circulating in the population. Outer membrane vesicles (OMVs) purified from a mutant (ΔlpxM) MDR O25 (ST131) strain were used to immunise mice to raise antiserum against the bacterial strain. Immunised mice were protected in a mouse model of E. coli bacteraemia. The antiserum from protected animals was used to develop an antibody-mediated bacterial killing assay and a complement deposition assay (CDA). The antiserum mediated killing with human IgG and IgM-depleted plasma as the complement source, and also showed deposition of C3b and C5b-9 onto the bacterial surface determined by flow cytometry. This may be a useful screening tool for mining for new antigens and in assessing the antisera raised against the candidates for cross serotype protection.