Feasibility of crude F4 fimbriae extract as a vaccine candidate for preventing Escherichia coli-induced diarrhea in piglets

Background and Aim: Enterotoxigenic Escherichia coli (ETEC) poses a substantial risk of neonatal diarrhea and postweaning diarrhea among piglets, with F4⁺ ETEC strains emerging as a particularly challenging issue within the pig farming industry. This study aimed to introduce a straightforward approach for generating a crude extract of F4 fimbriae that shows promise as an antigenic determinant for potential vaccination strategies. Materials and Methods: A crude F4 fimbriae extract was obtained from F4⁺ ETEC using a combination of heat shock and homogenization techniques. Subsequently, three 4-week-old piglets were immunized with a primary dose of 150 µg and a booster dose 2 weeks later. Blood samples were collected to evaluate the level of serum F4-specific antibodies using an enzyme-linked immunosorbent assay. Results: Analysis using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and liquid chromatography tandem-mass spectrometry techniques unveiled crucial insights into the composition of the crude F4 fimbriae extract. Notably, a distinct prominent band (~24 kDa) was identified, corresponding to the size of FaeG, the major subunit of F4 fimbriae. Regarding antibody response, there was a remarkable disparity between the levels of serum immunoglobulin (Ig)G and IgA antibodies targeting F4 compared with other E. coli strains (F18⁺ ETEC, F41⁺ ETEC, and F4⁻F18⁻F41⁻ EC), as well as with the unvaccinated control group (p < 0.01). Specifically, the levels of IgG antibodies against other E. coli strains were also significantly higher than those observed in the unvaccinated control group (p < 0.05). Conclusion: Our findings suggest that the crude F4 fimbriae extracts obtained using our simple extraction method induce specific immune responses against F4⁺ E. coli and stimulate cross-immunity against other E. coli strains. Therefore, our method shows potential for use in future vaccine development against diarrhea in pigs caused by E. coli.