|Figure 4 from Schwan et al., 1995. B. burgdorferi incubated at 24ºC (lanes 2 and 6), transferred from 24ºC to 37ºC (lanes 3 and 7), incubated at 37ºC (lanes 4 and 8), or transferred from 37ºC to 24ºC (lanes 5 and 9). Panel A, SDS-PAGE gel stained for total proteins with Coomassie brilliant blue. Arrow marks location of OspC. Panel B, Western blot with flagellin antibody (Fla) and OspC antibody.|
The standard culture medium for Borrelia is BSK-II with 6% rabbit serum, a complex nutrient-rich concoction. They made two new formulations of the culture medium to slow the growth rate: (1) quarter strength BSK-II with the rabbit serum concentration remaining at 6%; (2) full-strength BSK-II with the rabbit serum concentration reduced to 1.2%. Medium #1 slowed the doubling time at 34ºC to 40 hours, and medium #2 reduced it to 32 hours. Western blots of the spirochetes harvested from both cultures revealed low levels of the OspC and Erp proteins. When these spirochetes were inoculated into the standard culture medium (BSKII/6% rabbit serum) and incubated at 34ºC, high levels of the proteins were again detected. Therefore, B. burgdorferi is capable of adjusting OspC and Erp expression by monitoring its growth rate, even if the surrounding temperature does not change.
The final experiment from the study demonstrates that not even growth rate is the direct signal. The authors froze B. burgdorferi at -80ºC for at least a month and then inoculated the bacteria into standard culture medium for incubation at 23ºC. As a control, bacteria being maintained at 34ºC were also transferred to standard culture for incubation at 23ºC. Both cultures grew with the same doubling time. Nevertheless, the spirochetes that were revived from the frozen state produced more OspC and Erp proteins that those that were initially maintained at 34ºC.
So what's the real cue? Going back to the natural life cycle of B. burgdorferi, the spirochetes living in the unfed tick's midgut do not really grow or divide. The metabolism of B. burgdorferi is slowed by the nutrient-poor conditions in the tick's midgut. When the tick finally takes a blood meal, the surge of nutrients entering the tick signals B. burgdorferi to rev up its metabolism, triggering production of OspC. This model would explain why the frozen spirochetes, whose metabolism was undoubtedly slowed, were able to produce large amounts of OspC and Erp proteins when inoculated into standard culture medium at 23ºC, the temperature usually associated with diminished production of the proteins. The challenge will be to figure out how B. burgdorferi is sensing its metabolic state at the molecular level.
Jutras, B.L., Chenail, A.M., & Stevenson, B. (2012). Changes in bacterial growth rate govern expression of the Borrelia burgdorferi OspC and Erp infection-associated surface proteins. Journal of Bacteriology, 195 (4), 757-764 DOI: 10.1128/JB.01956-12
Schwan, T.G., Piesman, J., Golde, W.T., Dolan, M.C., & Rosa, P.A. (1995). Induction of an outer surface protein on Borrelia burgdorferi during tick feeding. Proceedings of the National Academy of Sciences, 92 (7), 2909-2913 DOI: 10.1073/pnas.92.7.2909