## 9 Evidence from Isolates from Patients and Primates with Spirochetal Persistence
In a primate model of LB [156], in which antimicrobial agents were administered 4 to 6 months after infection, antimicrobial-tolerant persister cells could be identified/isolated following treatment, demonstrating that _B. burgdorferi_ s.s. can withstand antimicrobial treatment administered post dissemination. In addition, spirochetes were recovered by xenodiagnosis in two of the three treated animals and a few slow-growing organisms were recovered by culture from each animal. Furthermore, _ospA_ transcripts were detected in culture pellets from animal tissues [156]. In a follow-up study of _Rhesus macaques_, morphologically intact spirochetes could be observed in several organs of treated animals and adjacent to a peripheral nerve of an untreated animal. Antigen staining of probable spirochete cross-sections was also observed in the heart, skeletal muscle, and near peripheral nerves of treated and untreated animals. These findings support the notion that ongoing symptoms in some LB patients following treatment may be attributable to residual inflammation in and around tissues that harbor a low burden of persistent host-adapted spirochetes and/or residual antigens [157]. The exact mechanisms of such spirochetal persistence have not yet been identified, but eradication of persister cells has been achieved at least in vitro by pulse dosing with third-generation cephalosporors or daptomycin combinations with doxycycline and/or cephalosporors [127]. In terms of antimicrobial resistance, there is also circumstantial evidence for the development of acquired antimicrobial resistance in _B. burgdorferi_ s.l. against a few antimicrobial agents at least under laboratory conditions in vitro. It has been speculated that erythromycincin resistance may develop in _Borrelia_ isolates from LB patients who have been pre-exposed to erythromycincin. This is based on the occurrence of resistant subpopulations in vitro, even though the authors of that study were unable to provide a scientifically sound explanation of the underlying mechanism [114]. The possible occurrence of acquired resistance in borreliae is also supported by the generation of mutants resistant to quinolones and aminoglycosides after the continuous culture in the presence of subinhibitory concentrations of these drugs in vitro [96; 158]. Similarly, mutations in the encoding 23S-rRNA gene of _Treponema pallidum_ have been found to confer functional resistance to azithromycin in patients with clinical treatment failure after taking this drug to treat a syphilis infection [159]. The findings in a clinical _B. garini_ isolate cultured from an EM patient after azithromycin treatment, however, did not support increasing resistance after exposure to this agent [7]. Instead, the findings are similar to those of [PERSON] et al. [20] and [PERSON] et al. [61] in relapsed patients with early LB. This shows that isolates cultured after the conclusion of roxithromycin and ceftaxone therapy remain fully susceptible to these agents in vitro. The absence of confirmed acquired resistance in organisms isolated from antibiotically treated patients, however, does not rule out phenotypic resistance mechanisms similar to those assumed to cause relapse in syphilis and leptospirosis patients [146; 149]. Likewise, for another slow-growing organism, _Mycobacterium tuberculosis_, the Cornell model of latent tuberculosis, in which mice infected with _M. tuberculosis_ are treated with antibiotics [isoni-azid and pyrazinamide], initially produces no detectable bacilli by organ culture. However, reactivation of infection during this culture-negative state obviously occurs spontaneously or following immune suppression after the conclusion of treatment. Such reactivation is similarly documented for LB and syphilis in the literature [18; 20; 75; 146; 160]. In addition, _B. burgdorferi_ s.l. has been observed in cardiac myocytes of experimentally infected mice and in dermal macrophages in vivo [161; 162]. Some authors have therefore speculated that _B. burgdorferi_ s.l.