B. burgdorferi s.l. is capable of producing extracellular vesicles (blebs) when grown in laboratory culture media (Barbour and Hayes, 1986). From earlier studies on the aetiologic agent of relapsing fever, it is evident that a number of different chemical and physical agents can disturb the bacteria, i.e. when the borrelial cells are treated with penicillin, freeze-thawing, prolonged cultivation, or the addition of a specific antibody together with a complement source, the spirochaetes form outer membrane blebs which detach from the parent organism (Barbour and Hayes, 1986). In the case of B. burgdorferi, the exposure to stress from the mammalian milieu, even in the absence of anti-B. burgdorferi antibodies, is enough to trigger the production of membrane-bound and possibly secreted blebs (Shoberg and Thomas, 1995). All borrelial blebs appear to contain an outer membrane, while some also contain cytoplasmic membranes (Radolf et al., 1994). However, all vesicle preparations made to date have no detectable quantities of the periplasmic 41 kDa flagellin, investigated by Western blotting (Dorward et al., 1991; Shoberg and Thomas, 1995). These blebs contain plasmid DNA (Garon et al., 1989), including osp DNA (Persing et al., 1994), and in their membranes the OspA and OspB proteins and another low molecular weight lipoprotein, Lp6.6, have been found (Dorward et al., 1991; Katona et al., 1992; Lahdenne et al., 1997). The bleb membranes are also thought to contain the borrelial adhesin for human endothelial cells since they are capable of binding to these cells (Shoberg and Thomas, 1993). As an anecdote, in the first half of the 20th century an alternative viewpoint was that these large blebs or 'gemmae' represented another stage of the life cycle of spirochaetes and this encouraged the early classification of borreliae as protozoans (Barbour and Hayes, 1986).
The recently described phenomenon of cyst formation is another sign of disturbance of cell architecture. Cyst formation involves changes in membrane integrity and cell shape (Alban et al., 2000). It is apparently a starvation response, which can be induced by cultivation in serum-free BSKII medium. It is a slow but active process requiring protein synthesis and is reversible in up to 53% of the cells. It is not known whether cyst formation is important for persistence of borreliae in the host and the explanation for survival during antibiotic therapy.
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