Bioconjugate Vaccines Engineered in Bacteria Promise Cheaper, Broader Pathogen Protection

Glycoconjugate vaccines represent some of the safest and most effective vaccines ever developed, dramatically reducing global rates of meningitis and pneumonia. By covalently linking a bacterial surface glycan to a carrier protein, they elicit T cell-dependent B cell responses, long-lived IgG antibody production, and immunological memory — advantages that polysaccharide-only vaccines cannot provide. Despite decades of success against Haemophilus influenzae type B, Neisseria meningitidis, Streptococcus pneumoniae, and Salmonella Typhi, the traditional chemical conjugation method remains costly, technically demanding, and slow to adapt, limiting application to a handful of pathogens.

Bioconjugation, or PGCT, bypasses these limitations by co-expressing a glycan biosynthesis pathway, a carrier protein, and a coupling enzyme — typically the oligosaccharyltransferase PglB from Campylobacter jejuni — within engineered E. coli. The glycan is assembled on an undecaprenyl-pyrophosphate lipid carrier and transferred directly onto asparagine residues within a D/E-X-N-Y-S/T sequon on the acceptor protein in the bacterial periplasm. The entire conjugation occurs in one cellular compartment, eliminating separate purification and chemical coupling steps.

On the glycan side, synthetic biology tools such as start-stop assembly enable 'pathway refactoring': each gene in a biosynthesis cluster is decoupled from its native regulation and recombined with standardized promoter, ribosome-binding site, and terminator parts, generating optimized pathway libraries. This approach improved C. jejuni heptasaccharide and Group B Streptococcus capsular polysaccharide yields substantially. Scaffold glycan engineering, demonstrated for Shigella flexneri, allows a conserved polysaccharide backbone to be decorated with serotype-specific modifications, rapidly generating panels of native and novel glycan structures.

On the protein side, pathogen-specific carrier proteins identified through reverse vaccinology are replacing generic toxoid carriers, potentially providing dual protection by combining the glycan antigen with a protein antigen from the same or co-infecting pathogen. Novel oligosaccharyltransferases from organisms such as Desulfovibrio desulfuricans, Campylobacter lari, and Neisseria species are being explored to expand substrate compatibility, including transfer of eukaryotic-type glycans and capsular polysaccharides from Gram-positive bacteria. Directed evolution and machine-learning-guided OST engineering further promise enzymes tailored to specific glycan-protein combinations.

Several bioconjugate vaccines are now in clinical trials, including a quadrivalent Shigella bioconjugate vaccine in Phase II and candidates for ExPEC, Campylobacter, and Klebsiella. The authors conclude that continued advances in glycan expression, OST engineering, and carrier protein design collectively position bioconjugation as a transformative platform for delivering affordable glycoconjugate vaccines against antimicrobial-resistant bacteria and neglected pathogens in low- and middle-income settings.