Lyme Disease Microbiology - Structure and Growth

Structure and Growth

B. burgdorferi is a highly specialized, motile, two-membrane, flat-waved spirochete, ranging from about 9 to 32 micrometers in length. Because of its double-membrane envelope, it is often mistakenly described as Gram negative, though it stains weakly in Gram stain. The bacterial membranes in at least the B31, NL303 and N40 strains of B. burgdorferi do not contain lipopolysaccharide, which is extremely atypical for Gram negative bacteria; instead, the membranes contain glycolipids. However, the membranes in the B31 strain have been found to contain a lipopolysaccharide-like component. B. burgdorferi is a microaerophilic organism, requiring little oxygen to survive. Unlike most bacteria, B. burgdorferi does not use iron, hence avoiding the difficulty of acquiring iron during infection. It lives primarily as an extracellular pathogen, although in vitro it can also hide intracellularly (see Mechanisms of persistence section).

Like other spirochetes, such as Treponema pallidum (the agent of syphilis), B. burgdorferi has an axial filament composed of flagella which run lengthways between its cell wall and outer membrane. This structure allows the spirochete to move efficiently in corkscrew fashion through viscous media, such as connective tissue.

B. burgdorferi is very slow growing, with a doubling time of 12–18 hours (in contrast to pathogens such as Streptococcus and Staphylococcus, which have a doubling time of 20–30 minutes). Since most antibiotics kill bacteria only when they are dividing, this longer doubling time necessitates the use of relatively longer treatment courses for Lyme disease.