Brorson's observation in this paper, that it takes old cysts 4 weeks to convert to as many as 5 mobile spirochetes per cyst, may thus explain the clinically observed 4- week CYCLICITY in Lyme borreliosis (LB) - as well as the observed RELATIVE RESISTANCE to beta-laktam antibiotics, and SERONEGATIVITY !!   - and it also explains the clinical observation, that in early LB of only 1-3 weeks duration (only young cysts), shorter antibiotic courses of 10-14 days duration may just about be effective in preventing relapses, while antibiotic treatment of late LB (old cysts) teoretically must be of at least 4 weeks duration, because one must treat long enough for all the oldest cysts to convert back to spirochetes and build their cell walls and thus be susceptible to a beta-laktam-antibiotic!!  

/Marie Kroun

APMIS. 1998 Dec;106(12):1131-41.

A rapid method for generating cystic forms of Borrelia burgdorferi, and their reversal to mobile spirochetes.
Brorson O, Brorson SH.

Department of Microbiology, Vestfold Sentralsykehus, Tønsberg, Norway.

Mobile Borrelia burgdorferi were transferred to distilled water (10(6) per ml). The cultures were observed by dark field microscopy (DFM), interference contrast microscopy (ICM) and transmission electron microscopy (TEM). 95% of the spirochetes were converted to cysts after 1 min, and after 4 h no normal mobile borreliae were observed. When transferred to growth medium (BSK-H), the cysts became smaller and more irregular, and were filled with organic substances. After 1 day, 1-5 thin structures sprouted from the cysts. They continued to grow in both length and thickness until they attained a normal spirochetal structure. Finally, these new-born spirochetes detached from the cysts, by which time their mobility had become normal. The present method for producing large amounts of cystic forms of B. burgdorferi is well suited for further studies of this unique microbe.

PMID: 10052721 [PubMed - indexed for MEDLINE]

Excerpt from discussion section:

The cysts observed in our study seem to resemble the spheroplast-L-forms observed by other researchers (11, 35), which seem to have defects in their cell wall manifested by resistance towards beta-lactam antibiotics (16). Since the conversion from normal mobile spirochetes to cystic forms occurred very rapidly, complete absence of cell wall as in L-forms is dubious, but the similarity with L-forms is shown by the inability to retain their original shape.
The biological activity of the cystic forms was confirmed by the step by step development to normal mobile spirochetes in BSK-H medium, and also indicated by the presence of RNA in 5-week-old cysts due to red-orange staining with acridine orange (pH 6.4) (Fig. 4b). This seems to be a new observation in the development of B. burgdorferi (20). Bruck et al. (35) also illustrated biological activity by incorporation or S-methionine in the cysts (spheroplast). The creation of as many as five spirochetes from each cyst may explain why the generation time was shorter for production of mobile spirochetes from cysts compared to that for normal mobile spirochetes cultivated conventionally. The generation of the normal mobile spirochetes which were converted from cysts was somewhat variable in the sense that they sometimes seemed to need a long stationary period before exponential growth occurred. Whether cysts are converted to normal mobile spirochetes or not depends strongly on the growth medium used, and possibly also the generation time.
It seems as though normal mobile spirochetes are developed from the dense core structures or the cyst by being "fed" with core substances as the "infant-spirochete" protrudes from the cyst. The development of vegetative bacteria from dense L-forms has been suggested as a method for Enterococcus faecalis to convert from L-forms to vegetative forms (40). The observation by TEM that blebs transformed into thin filaments leads us to speculate that these filaments develop to normal mobile spirochetes. If so, the blebs have to contain enough genetic material to synthesize a new bacterium (22).
Old cystic forms of B. burgdorferi require prolonged cultivation to convert to normal mobile spirochetes (4 weeks as opposed to 9 days for young cysts).
Similar cystic forms may occur in the human organism (11, 14, 15), and they may explain the long periods or latency, resistance to antibiotics, negative serological results (3-7, 10, 12, 13, 25), and low PCR sensitivity (5, 8, 10). For these reasons it is important to examine the antigens of the envelope of the cysts. DNA sequences for PCR analysis, and the cysts sensitivity to antibiotics and other chemicals. It may be hypothesized that antigenic variation in B. burgdorferi (19, 41) occurs inside the cyst while the microbe is protected against external stress, and therefore antigens detected on the cyst envelope in vitro differ from the ones present in vivo. In conclusion, we believe that the present method for rapid and easy generation of stable cysts will be a useful tool in further research on B. burgdorferi."