Complementation of a <i>Borrelia afzelii</i> OspC mutant highlights the crucial role of OspC for dissemination of <i>Borrelia afzelii</i> in <i>Ixodes ricinus</i>

Alteration of the outer surface protein (Osp) composition – especially that of OspA and OspC – seems to be important for the adaptation of <i>Borrelia burgdorferi</i> sensu lato to its endothermic hosts (mammals) and poikilothermic vectors (ticks). OspA possibly mediates adherence to tick midgut cells thus enabling the borreliae to survive in the vector, while OspC is associated with borrelial invasion of the tick salivary glands and infection of the mammalian hosts. Here we describe the first successful transformation and complementation of a <i>Borrelia afzelii ospC</i> mutant with the wild-type <i>ospC</i> in trans. To test the influence of OspC on the dissemination behavior in ticks, unfed Ixodes ricinus nymphs were artificially infected by capillary feeding either with <i>B. afzelii</i> wild type, the <i>B. afzelii</i> ospC mutant or the <i>ospC</i>-complemented clone. Tick midguts and salivary glands were investigated after different time intervals for the presence of borreliae and for OspA and OspC by immunfluorescence staining with monoclonal antibodies. While the <i>B. afzelii</i> wild-type strain exhibiting abundant OspC on its surface disseminated to the salivary glands, the OspC-negative mutant was only present in the tick midguts. The <i>ospC</i>-complemented clone which constitutively expresses the wild-type <i>ospC</i> was again able to colonize the salivary glands. This finding demonstrates that OspC is crucial for dissemination of <i>B. afzelii</i> from the tick midgut to the salivary glands, a prerequisite for infection of the warm-blooded host. A summary of the detailed data presented here has already been given in Goettner et al. [2006. OspC of <i>B. afzelii</i> is crucial for dissemination in the vector as shown by transformation and complementation of a European OspC-deficient <i>B. afzelii</i> strain. Int. J. Med. Microbiol. 296S1(Suppl. 40), 122–124].