The development of direct genetic analysis techniques for Borrelia burgdorferi is now beginning to give researchers an insight into the specific functions of this spirochete's outer membrane proteins, as three recent publications have shown.

The life cycle of B. burgdorferi, the aetiological agent of Lyme disease in the United States, is divided between mammals and Ixodes scapularis ticks. The ticks ingest spirochetes from an infected mammalian host (commonly the white-footed mouse). B. burgdorferi colonizes the tick mid-gut and remains in a latent state until the tick matures and begins feeding. B. burgdorferi then migrates from the tick mid-gut to the salivary glands, from where it can be transmitted into a new host.

To survive in such different environments, B. burgdorferi has an extensive repertoire of cell-surface lipoproteins. Until recently, the paucity of direct genetic analysis techniques has hampered the investigation of the precise functions of these proteins. However, three groups have now used direct genetic analysis in B. burgdorferi and have recently reported their results.

Yang et al. targeted the ospA/B gene. Analysis of the infectivity of the resulting deletion mutant in mice and ticks showed that, although OspA/B is not essential for B. burgdorferi to colonize mice, it is essential for colonization of the tick mid-gut.

In a separate study, also a collaboration between Michael Norgard and Erol Fikrig's groups, the gene encoding another outer surface protein, OspC, was disrupted. In ticks, immunoelectron microscopy showed that an OspC deletion mutant was unable to bind to tick salivary glands, supporting earlier thinking that OspC is involved in the migration of B. burgdorferi from the mid-gut to the salivary glands. However, in a third study, this time from Patricia Rosa's group, in which ospC was also disrupted, although OspC was found to be essential for B. burgdorferi to infect mice, the opposite result was obtained for the analysis in ticks, with the OspC deletion mutant able to migrate to the salivary glands.

The different experimental designs could account for these different results. One major difference that is apparent between the two studies is the method used to artificially infect ticks with B. burgdorferi — Pal et al. used direct microinjection into the mid-gut, whereas Grimm et al. infected naive ticks by immersion in exponential-phase cultures of B. burgdorferi. Additionally, Pal et al. deleted the entire ospC gene, whereas in the work of Grimm et al., ospC was disrupted rather than deleted. Finally, migration to the salivary glands and transmisson to mice were assessed at different times post-tick infection.

The availability of techniques for the effective genetic manipulation of B. burgdorferi is an exciting development for the field. However, it is clear that, even once these techniques are available, teasing out the roles of specific proteins in the complex life cycle of this spirochete will be far from easy.