Monday, December 31, 2012

Biofilms of the Lyme disease spirochete

Thanks to a recent study published in PLoS One, we now know that free-swimming Borrelia burgdorferi are able to organize themselves into a sedentary community called a biofilm.  This is not too surprising since most other bacteria are capable of the same feat when provided the opportunity.  In fact, outside of the laboratory many bacteria, including those that live on and within us, spend much of their time within biofilms.

Prior to the 1990s biofilms were thought to be blobs of goo containing bacteria randomly distributed throughout their sticky matrix.  In reality, the bacteria and matrix are carefully organized into a complex three-dimensional structure. B. burgdorferi biofilms are no exception.  The organization of B. burgdorferi is apparent even at the earliest stages of biofilm development.  The images below show B. burgdorferi developing into a biofilm on a solid surface.  Instead of randomly associating with each other, the spirochetes organize themselves into "nets" of the type you see hanging from basketball hoops.  The spirochetes come together lengthwise to form the "strands" of the net.  With time, the biofilm thickens as the bacteria form additional layers.  Most of the spaces in the net close up with the rest probably ending up forming a network of channels.  The remaining holes can be seen as pits along the surface of the mature biofilm.  The pits appear to be entry points for the channels, which are thought to circulate nutrients to the members of the community and remove waste products.

Figure 2 from Sapi et alAtomic force microscopy of a developing B. burgdorferi biofilm.
Stalks can also rise up from the surface of microbial biofilms.  One stalk can be seen in this "flyover" along the surface of a mature B. burgdorferi biofilm.

video
Video S2 from Sapi et al.  Composte image from atomic force microscopy.

The matrix of B. burgdorferi biofilms includes DNA and an alginate-like substance bound to calcium.  The images below capture what appears to be matrix being laid down at an early stage of biofilm formation.

Figure 4 from Sapi et al.  Atomic force microscopy of an aggregate of B. burgdorferi in an early stage of biofilm development.  The matrix is colored blue in panel B.

What is the biological significance of B. burgdorferi biofilms?  To answer this question, the authors will need to determine whether B. burgdorferi assembles into biofilms at some point during its life cycle, which involves stages in the tick and vertebrate host.

Reference

Sapi, E., Bastian, S.L., Mpoy, C.M., Scott, S., Rattelle, A., Pabbati, N., Poruri, A., Burugu, D., Theophilus, P.A.S., Pham, T.V., Datar, A., Dhaliwal, N.K., MacDonald, A., Rossi, M.J., Sinha, S.K., & Luecke, D.F. (2012). Characterization of biofilm formation by Borrelia burgdorferi in vitro. PLoS ONE, 7 (10) DOI: 10.1371/journal.pone.0048277