Ballistic Motion of Spirochete Membrane Proteins

Abstract

Borrelia burgdorferi spirochetes are pathogenic bacteria which can cause Lyme disease. B. burgdorferi use membrane proteins to anchor themselves inside ticks which act as intermediate hosts. Human immune cells that fight bacterial infections bind to bacteria to internalize them. This binding can be mediated by antibodies against bacterial membrane proteins. Therefore these proteins and their mechanical behavior in the membrane play a key role for the disease transmission and the immune response.The mechanical behavior of proteins in bacterial membranes is still poorly understood. We investigated membrane protein dynamics in B. burgdorferi with microparticles coated with monoclonal antibodies against the spirochete outer surface protein A (OspA). We used holographic optical tweezers to attach the microparticles to membranes of live B. burgdorferi and we tracked the subsequent particle motion.Surprisingly the particles were transported ballistically to one end of the bacteria. The average speed of this transport was 3.1 ± 0.2 um/s. To determine if the microparticle motion is driven by the proteins, we are using speckle microscopy with fluorescently labeled OspA. In addition, we measured the stall force of the transport and found that a few piconewtons were sufficient to inhibit the motion. Although B. burgdorferi have a symmetric morphology, the protein motion showed a preferred direction of transport towards one end in individual bacteria. The origin of this symmetry breaking still needs to be unraveled. Mutant B. burgdorferi which lack the flagella that are found in wild-type bacteria did not show directed transport of membrane proteins, but only diffusive motion. Therefore we hypothesize that the protein transport is enabled by the bacterial motility machinery. We will discuss the proposition that this transport of membrane proteins could be a novel type of bacterial defense mechanism against immune cells.[HK and RB contributed equally]