Genetic engineering of E. coli K-12 for heterologous carbohydrate antigen production.

BACKGROUND

Carbohydrate-based vaccines have made a remarkable impact on public health over the past three decades. Efficient production of carbohydrate antigens is a crucial prerequisite for the development of such vaccines. The enzymes involved in the synthesis of bacterial surface carbohydrate antigens are usually encoded by large, uninterrupted gene clusters. Non-pathogenic E. coli glycoengineering starts with the genetic manipulation of these clusters. Heterologous gene cluster recombination through an expression plasmid has several drawbacks, including continuous antibiotic selection pressure, genetic instability, and metabolic burdens. In contrast, chromosome-level gene cluster expression can minimize the metabolic effects on the host and reduce industrial costs.

RESULTS

In this study, we employed the suicide vector-mediated allelic exchange method to directly replace the native polysaccharide gene clusters in E. coli with heterologous ones. Unlike previously strategies, this method does not rely on I-SceI endonuclease or CRISPR/Cas system to release the linearized DNA insert and λ-red recombinase to promote its homologous recombination. Meanwhile, the vectors could be conveniently constructed by assembling multiple large DNA fragments in order in vitro. The scarless chromosomal insertions were confirmed by whole-genome sequencing and the polysaccharide phenotypes of all glycoengineered E. coli mutants were evaluated through growth curves, silver staining, western blot, and flow cytometry. The data indicated that there was no obvious metabolic burden associated with the insertion of large gene clusters into the E. coli W3110 O-antigen locus, and the glycoengineered E. coli can produce LPS with a recovery rate around 1% of the bacterial dry weight. Moreover, the immunogenicity of the heterologously expressed carbohydrate antigens was analyzed by mice immunization experiments. The ELISA data demonstrated the successful induction of anti-polysaccharide IgM or IgG antibodies.

CONCLUSIONS

We have provided a convenient and reliable genomic glycoengineering method to produce efficacious, durable, and cost-effective carbohydrate antigens in non-pathogenic E. coli. Non-pathogenic E. coli glycoengineering has great potential for the highly efficient synthesis of heterologous polysaccharides and can serve as a versatile platform to produce next-generation biomedical agents, including glycoconjugate vaccines, glycoengineered minicells or outer membrane vesicles (OMVs), polysaccharide-based diagnostic reagents, and more.