## 5 In Vitro Antimicrobial Susceptibility Pattern of the _B. burgdorferii_ Complex
### Macrolides, Azalides and Ketolides  
Macrolides are important second-line agents offering treatment options for LB in cases where traditional \(\upbeta\)-lactams cannot be administered due to detrimental side effects such as allergies. Current data suggest a rank order of activity for traditional macrolides and azalides against borreliae that corresponds to the efficacy of these agents as revealed by current in vitro susceptibility studies and clinical trials [17; 36; 50; 52; 55; 61; 65; 65; 82]. These studies demonstrate higher in vitro efficacy for azithromycin (MIC\({}_{90}\), 0.0156 \(\upmu\)g/mL) than for erythromycinycin, roxithromycin, or clarithromycin. Median MIC values of the different substances, however, tended to vary over a 10-fold range for individual strains, with the _B. garriii_ isolate PSth and the _B. afzelii_ isolate EB1 showing the highest MIC values for both the traditional macrolides and the ketolides. In contrast to the findings of [PERSON] et al. [83], other studies found no significant differences in MBC values for the different genospecies tested against macrolides or ketolides, possibly owing to differences in test methodology and inoculum. However, most findings point to inter-strain variability of the in vitro susceptibilities of _B. burgdorferi_ s.l. to macrolides rather than to inter-genospecies-specific variations as observed for other antimicrobial agents [53; 55; 59]. As noted above, testing of _Streptococcus pneumoniae_ ATCC 49619 clearly demonstrated increased activity of some macrolides in BSK medium [36; 59; 84], which might be of consequence for in vitro susceptibility testing of these agents against _B. burgdorferi_ s.l. This is critically relevant, for example roxithromycin is highly active in vitro, but is also highly prone to clinical failure when treating patients with LB [61]. This is usually not the case with clarithromycin and azithromycin. Ketolides are 3-des-cladinosyl-3-oxo 11, 12-cyclic carbamate clarithromycin derivatives and, in contrast to traditional macrolides, exhibit enhanced physiochemical properties and increased antibacterial potency owing to their higher affinity to bacterial ribosomes [85; 86; 87]. Compounds such as erythromycin and tellithromycin belong to this relatively new class of antimicrobial agents that show modifications in the sugar moiety of the lactone ring structure [85; 86]. Ketolides have been shown to be highly active against a broad range of aerobic and anaerobic Gram-positive and Gram-negative bacteria, including macrolide-resistant strains [85; 87], with an up to 100-fold higher binding affinity to ribosomes than erythromycin [85]. Both erythromycincin and tellithromycin reduced the number of intact motile borreliae for a more than three log\({}_{10}\)-unit dilutions at 48 to 120 h after incubation at concentrations that were eight times the MIC [59]. Thus, in time-kill experiments, they exhibited a superior in vitro efficacy against borreliae on a \(\upmu\)g/mL basis compared to the traditional macrolide derivative erythromycincin [59]. However, strain variability was observed and the number of spirochetes tended to decrease more slowly with both substances in the _B. burgdorferi_ s.s. isolate PKA-1 [59]. The excellent in vitro efficacy of erythromycincin against borreliae was further substantiated by electron microscope analysis at 4 \(\log_{2}\) dilutions above the MIC\({}_{90}\) [0.0312 \(\upmu\)g/mL] and at 2 \(\upmu\)g/mL, the drug's tentative breakpoint concentration for fastidious organisms (Figure 4) [87; 88]. In terms of the plasma levels of both ketolide agents tested, recent studies have found that achievable maximum plasma concentrations after regular oral dosage are known to be 90 to 270 times higher than the MIC\({}_{90}\) against borreliae [86; 87; 89]. Moreover, tissue concentrations exceed the maximum plasma concentrations of both drugs in healthy individuals by tenfold [86; 87; 89]. According to the results obtained for _S.