## 8 Possible Mechanisms of Resistance of _Borrelia burgdorferi_ s.l. In Vitro  
### Currently Known Phenotypic and Genotypic Mechanisms of Resistance  
_Borrelia burgdorferi_ s.l. is currently not known to have traditional antimicrobial resistance mechanisms and is generally susceptible to many antimicrobial agents in vitro [53, 81]. In addition, humans represent a dead-end host and the further spread of whatever resistance to other strains is rather unlikely in this vector-borne organism, at least under natural real-life conditions outside the laboratory. However, a small number of studies have demonstrated the development of acquired resistance mechanisms in both laboratory and clinical settings. A development of resistance to erythromycincin (100 to 500 \(\upmu\)g/mL) has been reported in clinical isolates of _B. burgdorferi_ s.l. [114]. A low-passage clinical isolate with an unusually high level of resistance to macrolides and incosamides was phenotypically shown to have a modified ribosome structure when radio-labelled erythromycincin was used. However, the genes encoding this resistance could not be clearly identified. The resistance determinant appeared to be encoded by mobile genetic elements because the phenotype was transferable to other bacterial species [114]. Based on conjugation frequency, the elements were thought to be constins or integrating conjugative elements. Furthermore, a ToIC-like efflux system, in which the outer membrane porin BesC plays an integral role, has been identified in _B. burgdorferi_ s.l. This mechanism of antibiotic resistance results from the efflux of antibiotics from the spirochete through the use of a resistance-modulation-division (RND) efflux pump called BesABC [115]. A _besC_ knockout in a genetically modified _B. burgdorferi_ s.s. strain was shown to result in an increased susceptibility to multiple classes of antibiotics used to treat LB. For tetracycline, an 8-fold decrease in the MIC\({}_{90}\) value (0.31 \(\upmu\)g/mL in the wild-type strain vs. 0.04 \(\upmu\)g/mL in the _BesC_ knockout strain) and a 15-fold decrease in the MBC value (2.5 \(\upmu\)g/mL in the wild-type strain vs. 0.16 \(\upmu\)g/mL in the _besC_ knockout strain) was observed using standardized colorimetric susceptibility testing [115]. Resistance to coumermycin (due to mutations in the _gyrA_ gene), to aminoglycosodes (due to homologous mutations in the small ribosomal RNA subunit), and to fluoroquinolones (due to mutation in the _parC_ gene) have been demonstrated following mutant selection in the laboratory [116; 117]. Nonetheless, relatively little is known to date about the in vitro pharmacodynamic interactions between new fluoroquinolones and _Borrelia_ spp. The observed increase in in vitro efficacy of some of the 4-quinolones against borreliae, as outlined above, is consistent with those of other investigators who demonstrated the activity of ciprofloxacin, mozifloxacin, sparfloxacin and the DNA-gyrase inhibitor coumermycin A1 against _B. burgdorferi_ s.l. [40; 96; 97]. Interestingly, investigations into _S. pneumoniae_ revealed that, in addition to gemifloxacin's activity against the pneumococcal DNA-gyrase, the improved efficacy of the drug is probably related to its remarkably high affinity to topoisomerase IV [118]. Similarly, the activity of gemifloxacin, sitafloxacinin and grepafloxacin against borreliae is approximately 10 to 100 times higher than that of the older 4-quinolones, but clearly lower than that of ceftaxone, which served as a control substance as it is known to display a high rate of activity [58]. Furthermore, time-kill experiments indicate that gemifloxacin has a stronger cytotoxic effect on borreliae than ciprofloxacin [58]. Thus, it can be speculated that enhanced in vitro activity of gemifloxacin and sitafloxacin against borreliae may be due to a higher affinity to one of the _Borrelia_ topoisomerases.