Abstract: The present invention relates to a method for the preparation of a strain-adapted vaccine specific for a bacterial strain, comprising the steps of: (a) genetically engineering a bacterial strain obtained from a subject, wherein said genetic engineering comprises introducing a nucleic acid molecule encoding a fusion protein, wherein the fusion protein comprises a bacterial membrane protein fused to at least one affinity tag, (b) growing the genetically engineered bacterial strain obtained in step (a) in solution, (c) isolating membrane vesicles from the growth culture of step (b) by affinity purification using the affinity tag, and (d) formulating the membrane vesicles isolated in step (c) into a strain-adapted vaccine. The present invention further relates to a nucleic acid molecule encoding a fusion protein comprising a bacterial membrane protein fused to at least one affinity tag and a kit comprising said fusion protein.