An anticoagulant heparin-chondroitin chimeric saccharide molecule as well as a preparation method and application thereof are disclosed. The anticoagulant heparin-chondroitin chimeric saccharide molecule has a structure as shown in formula I. The heparin-chondroitin chimeric saccharide molecule of the present disclosure has potent activities against an Xa factor and IIa, and the activity of the heparin-chondroitin chimeric saccharide molecule can be effectively neutralized by protamine, with a neutralization rate of greater than or equal to 70%. The risk of causing adverse reactions such as fatal HIT is obviously lower than that of enoxaparin and other low-molecular-weight heparins. The heparin-chondroitin chimeric saccharide molecule disclosed by the present disclosure is suitable for the preparation of a safer potent anticoagulant and antithrombotic new drug.

Provided are methods for producing polysaccharides in bacteria by expressing in a bacterium one or more coding sequences selected from the group consisting of a pglF dehydrogenase coding sequence, a wbpP UDP-N-acetyl-d-glucosamine C4 epimerase coding sequence, a wcfR aminotransferase coding sequence, and a wcfS phospho-glycosyltransferase coding sequence, a wcfQ glycosyltransferase coding sequence, a wcfO pyruvyltransferase coding sequence, a wcfP glycosyltransferase coding sequence, a wcfM UDP-galactopyranose mutase coding sequence, a wcfN glycosyltransferase coding sequence, a wza polysaccharide export protein coding sequence, a wzx fippase coding sequence, a wzy polymerase coding sequence, and a wzz coding sequence, wherein at least one of the coding sequences is heterologous to the bacterium. Also provided are expression cassettes with one or more of the disclosed coding sequences, recombinant bacteria that harbor one or more of the expression cassettes, and methods for producing immunogenic compositions using the polysaccharides produced by the recombinant bacteria.