Genetic transformation of Borrelia spp. is limited in development and has found application in only one species. For a non‐genetic approach for manipulating the phenotype of these spirochaetes, we determined whether exogenous recombinant lipoproteins would incorporate in the cell's outer membrane. Using unlabelled or 125I‐labelled Osp proteins, Osp‐specific monoclonal antibodies, proteinase K and formaldehyde as reagents, we found that decoration of spirochaetes had the following characteristics. (i) Purified recombinant OspA or OspD lipoproteins associated with Borrelia burgdorferi and B. hermsii cells that lacked abundant lipoproteins of their own. (ii) This decoration of the cells with exogenous OspA did not affect cell's viability. (iii) The decoration was concentration and temperature dependent and stable for at least 24 h. (iv) Like native OspA, the recombinant OspA decorating the cells was accessible to antibodies and proteases and could be cross‐linked to the integral outer membrane protein, P66. (v) Decoration of viable B. burgdorferi and B. hermsii with OspA rendered the cells susceptible to killing by OspA‐specific antiserum. Such non‐genetic alteration of the surface of a bacterium may be used to study functions and properties of lipoproteins in situ.

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