Membrane fusion between Borrelia spirochetes, a new type of bacterial interaction

It's not unusual for bacteria to collide while swimming around in culture medium.  When this happens, the bacteria simply bounce off each other and swim off in different directions.  However scientists have discovered that the encounter between Borrelia spirochetes, the agents of Lyme disease and relapsing fever, can progress to something more intimate.¹  When they looked at Borrelia cultures under the microscope, the spirochetes that bumped remained stuck to each other side-by-side as they swam.  These encounters were usually brief, lasting for less than 10 seconds, although sometimes they lasted for more than a minute before separating.  If you watch the video below, you'll see two Lyme disease B. afzelii cells near the bottom left corner coming together side-by-side and then separating several seconds later.  The investigators saw similar interactions in cultures of other Lyme disease spirochetes (B. burgdorferi and B. garinii) and a relapsing fever spirochete (B. hermsii).

Movie S2 from Kudryashev et al., 2011

When I first saw the video, I thought that the spirochetes were simply getting tangled up and that it took several seconds for them to get untangled.  However when the investigators examined the cultures by cryoelectron tomography (a type of electron microscopy), they saw that the Borrelia cells weren't merely tangled or stuck to each other.  Their outer membranes were actually fused, sometimes so extensively that both cytoplasmic cylinders ended up in a single outer membrane sheath.  Panel A below shows a cross-section of fused B. garinii cells.  Panel B  shows a 3-dimensional rendering of the the fused spirochetes from panel A.  The two cytoplasmic cylinders (bright and dark magenta) are surrounded by a single outer membrane sheath.  The flagellar filaments from both cells form a single bundle and are shown in yellow.

Figure 4A and 4B from Kudryashev et al., 2011

Artifacts are minimized in specimens observed by cryoelectron tomography because the specimen does not have to be fixed with harsh chemicals.  Instead, the live specimen is placed on an electron microscope grid and plunge-frozen to preserve the structure of the biological sample.  Still, as the authors point out, fusion of Borrelia cells could be an artifact of preparing the spirochetes for cryoelectron tomography.  The outer membrane of Borrelia cells could have fused while collecting the spirochetes by centrifugation or when blotting excess liquid from the electron microscopy grid before freezing the specimen.

Assuming that this was not a preparation artifact, what could be the role of outer membrane fusion in the biology of Borrelia?  The authors present two possibilities.  First, Borrelia cells can share their outer membrane contents by fusing their outer membranes together.  Sharing may be advantageous to Lyme Borrelia during transmission from the tick to the victim's skin, when the spirochetes are turning on genes encoding protective proteins such as OspC.  One can imagine Borrelia cells sharing its protective surface proteins with others that have yet to express them so that a larger number of spirochetes can fend off host defenses and establish an infection.  Another intriguing possibility is that DNA is exchanged between the two spirochetes.  Out of the 110 pairs of fused Borrelia cells observed by cryoelectron tomography, the investigators found one pair whose inner membranes were also fused, providing a conduit (at least theoretically) for transfer of DNA.  Cells in culture may not remain fused long enough to transfer DNA, but Lyme disease Borrelia lie dormant in the tick midgut for months, giving Borrelia cells lying next to each other plenty of time to exchange DNA, assuming that membrane fusion and DNA transfer can even occur in this setting.

The outer membrane of spirochetes is unique among diderm (double-membrane) bacteria because of its loose association with the underlying peptidoglycan layer.  For this reason the outer membrane of all spirochetes, not just those of Borrelia, may be especially prone to fusing.  This raises the possibility that the outer membranes of other spirochetes such as Leptospira and Treponema could also fuse.

Note: This work has also been described in the blog Small Things Considered.

Reference

1. Kudryashev M., Cyrklaff M., Alex B., Lemgruber L, Baumeister W, Wallich R, and Frischknecht F (May 2011).  Evidence of direct cell-cell fusion in Borrelia by cryogenic electron tomography.  Cellular Microbiology 13(5):731-741.  DOI: 10.1111/j.1462-5822.2011.01571.x