The Lyme disease spirochete has flagella but doesn't use them to penetrate the gut of the feeding tick

The Lyme disease agent Borrelia burgdorferi possesses flagella, which are the thin motility structures owned by many members of the bacteria world.  Flagella propel bacteria towards their destination by spinning (read this post to see how flagella function in Borrelia).  It has been assumed B. burgdorferi spin their flagella whenever they need to move from one location to another.  A recent paper in The Journal of Clinical Investigation has demonstrated otherwise, at least for B. burgdorferi in the midgut of a feeding Ixodes (blacklegged) tick.

Borrelia burgdorferi spends much of its life cycle lying dormant in the midgut of Ixodes ticks.  The spirochetes lightly pepper the inner surface of the midgut cell lining, with a few spirochetes also hiding between cells.  None live at the base of the cells at the basement membrane surrounding the midgut.  The spirochetes wake up and multiply only when the tick attaches to an animal or human and imbibes blood.  A few days into the blood meal, some spirochetes eventually breech the basement membrane and enter the hemocoel, the fluid-filled space between the tick organs where they must avoid the phagocytes patrolling the area.  From there the spirochetes invade the salivary glands, which can then release B. burgdorferi-tainted saliva into the skin of the victim.  After completing its satisfying meal of blood, the tick detaches from the skin of the victim, who may end up suffering from Lyme disease.

Dunham-Ems and colleagues wanted to follow the spirochetes in the midgut as ticks took their meal of blood.  They engineered a strain of B. burgdorferi expressing green fluorescent protein so that they could watch the spirochetes in the gut by fluorescence microscopy.  They allowed ticks with the green B. burgdorferi strain in their midguts to feed on laboratory mice.  24, 48, and 72 hours after the ticks were placed on the mice, the investigators removed the midguts and examined the organ by fluorescence microscopy to see what the spirochetes were doing.  Surprisingly, they never saw motile spirochetes in the midgut even though the spirochetes eventually found their way at 72 hours into the hemocoel, where they were highly motile.

If the spirochetes in the midgut remained nonmotile during tick feeding, how did they reach the basement membrane? The few spirochetes that initially populated the midgut multiplied exponentially and formed growing networks of spirochetes on the cell surfaces as the tick drank blood from the mice.  By 72 hours the networks eventually coalesced, encasing many gut cells in spirochetes (see the figures below).  Spirochetes at the base of the encased cells were poised to penetrate the basement membrane and invade the hemocoel.  All of this happened without B. burgdorferi ever spinning its flagella.  Only when they broke through into the hemocoel did the flagella start spinning.

Figure 4F-H from Dunham-Ems 2009.  Confocal fluorescence microscopy of a midgut from a nymph that fed on a mouse for 72 hours.  Panel F shows a network of spirochetes (green) attached to the inner surface of the midgut.  An optical section taken 24-26 µm into the lining of the midgut (panel G) reveals aggregates of spirochetes surrounding the cells. Panel H shows that some spirochetes have made it to the basement membrane, which is found 50 µm below the surface.  The midgut cell membrane is stained in red.  Scale bars = 25 µm.  Some of the gut cells are extremely large because they are differentiating as part of the digestion process.

Figure 5 A and B from Dunham-Ems 2009.  Silver stain of sections from ticks that fed for 48 hours (panel A) and 72 hours (panel B).  The edges of the epithelial cells are easier to see than in the previous figure.  Arrows point to aggregates of spirochetes (hairy bodies).  At 72 hours at least one cell is encased in spirochetes.  Scale bars = 25 µm.  Some of the cells are extremely large because they are differentiating as part of the normal digestion process of the tick (dc, differentiated cells; uc, undifferentiated cells). 

The investigators also found that something in the tick midgut inhibited the motility of B. burgdorferi.  They placed a bit of minced midgut from a tick that had been feeding on a mouse for 72 hours at the edge of a gelatin matrix containing motile fluorescent B. burgdorferi.  (Because of their helical shape, spirochetes love to move about in viscous substances such as gelatin.)  Most of the spirochetes near the tissue ceased moving and remained motionless throughout the 15 minute viewing period.  In contrast, the spirochetes continued moving when mouse blood was placed at the edge of the gelatin matrix.

Why does B. burgdorferi employ a nonmotile mode of penetration of the cell lining of the tick midgut?  Is there some advantage for the spirochete to avoid using their flagella?  As blood is known to be a powerful chemoattractant for B. burgdorferi, the authors offered the following explanation:

These results, although counterintuitive at first blush, make sense; if blood in the midgut acted as a chemoattractant, spirochetes would never disseminate during feeding.

Hence the "inhibitor" of motility released by the tick gut serves as a signal to the spirochete to not spin their flagella.

To me, this explanation isn't satisfying.  It would seem simple for B. burgdorferi to have evolved a regulatory scheme that would allow the spirochete to temporarily uncouple blood chemotaxis from flagellar motility so that they could bore through the gut lining in minutes rather than days. There must be a reason why B. burgdorferi chooses to take its time to penetrate the gut lining.

Perhaps B. burgdorferi delays its journey to the salivary glands to allow the feeding tick to properly prepare the skin, which is an inhospitible environment for both tick and spirochete.  As the tick feeds, it releases a brew of anti-immune factors into the skin to protect itself from attack by the immune system.  Early arrival of B. burgdorferi to the salivary gland would release the spirochetes into the skin before the anti-immune factors have taken full effect, potentially allowing the host immune system to eliminate the spirochetes before they could establish an infection.

Reference

Dunham-Ems, S.M., Caimano, M.J., Pal, U., Wolgemuth, C.W., Eggers, C.H., Balic, A., & Radolf, J.D. (2009). Live imaging reveals a biphasic mode of dissemination of Borrelia burgdorferi within ticks. Journal of Clinical Investigation. 119(12):3652-3665. DOI: 10.1172/JCI39401