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Abstract
The spirochaete bacterium Borrelia burgdorferi sensu lato is the causative agent of Lyme disease, the most common tick-borne infection in the northern hemisphere. There is a long-standing debate regarding the role of pleomorphic forms in Lyme disease pathogenesis, while very little is known about the characteristics of these morphological variants. Here, we present a comprehensive analysis of B. burgdorferi pleomorphic formation in different culturing conditions at physiological temperature. Interestingly, human serum induced the bacterium to change its morphology to round bodies (RBs). In addition, biofilm-like colonies in suspension were found to be part of B. burgdorferi’s normal in vitro growth. Further studies provided evidence that spherical RBs had an intact and flexible cell envelope, demonstrating that they are not cell wall deficient, or degenerative as previously implied. However, the RBs displayed lower metabolic activity compared with spirochaetes. Furthermore, our results indicated that the different pleomorphic variants were distinguishable by having unique biochemical signatures. Consequently, pleomorphic B. burgdorferi should be taken into consideration as being clinically relevant and influence the development of novel diagnostics and treatment protocols.
Highlights
Lyme disease is the most commonly reported tick-borne infection in Europe and North America, and is endemic in many areas in Asia (Mead, 2011; Radolf et al, 2012)
We compiled the descriptions of different morphological variants based on our findings at a physiologically relevant culturing temperature of 37 uC (Table 1)
It is notable that the mean size of round body (RB) (2.8 mm) was greater when compared with the blebs (1.3 mm) on spirochaetes (Table 1)
Summary
Lyme disease is the most commonly reported tick-borne infection in Europe and North America, and is endemic in many areas in Asia (Mead, 2011; Radolf et al, 2012). The disease is caused by the different genospecies of the spirochaete bacterium Borrelia burgdorferi sensu lato group (Radolf et al, 2012). The cell envelope of B. burgdorferi consists of a protoplasmic cylinder covered by two lipid membranes (Barbour & Hayes, 1986). The general structure of B. burgdorferi’s cell envelope is exceptional and differs significantly from the typical Gram-negative bacteria. LPSs are usually outer membrane components of Gram-negative bacteria; B. burgdorferi lacks LPS (Takayama et al, 1987), and has immunoreactive glycolipids instead
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