## 10 Summary and Conclusions  
Over the last two decades, relatively little progress has been made in identifying possible mechanisms of persistence and antibiotic resistance for _B. burgdorferi_ s.l., as has been the case for other bacterial pathogens. We still have many lessons to learn about the interactions between antimicrobial agents and _B. burgdorferi_ s.l., as well as the potential mechanisms of spirochetes may have devised to overcome the presence of antimicrobial agents and host immunity in vivo. Recent studies provide evidence for the development of acquired in vitro resistance of _B. burgdorferi_ s.l. against selected aminoglycos, hygromycin A, and quinolones after prolonged exposure to subinhibitory concentrations of these drugs during in vitro culture. To date, however, there is no scientific evidence for clinically relevant acquired antimicrobial resistance against drugs that are commonly used for the therapy of _B. burgdorferi_ s.l. infection as a cause of culture-confirmed persistence of borreliae in LB patients. The public health impact on patients with the persistence of borreliae after antimicrobial therapy is difficult to assess. Extrapolating the above-mentioned numbers from clinical investigations, a persistence rate of 1.5% in EM patients [7] and an annual number of >200,000 cases of LB in Germany alone [11] would add up to ~3000 patients with possible persistence. Such a spirochetal persistence, however, would go unnoted as regular follow-up cultures are not performed in these individuals under routine clinical conditions. In order to address the issues of treatment resistance and persistence in the presence of antimicrobial agents, further investigations need to be conducted on more clinical isolates obtained from LB patients after chemotherapy. This task, however, remains challenging, as the overall culture detection rate of the pathogen in clinical specimens obtained from cutaneous lesions usually does not exceed 40 to 70% [105; 165; 166; 167; 105]. Moreover, culture positivity even falls to <1% in cases with Lyme arthritis and 20% in cases with neuroborreliosis [105], and cultivation is rarely successful after antimicrobial therapy [7; 152]. During the last decade, novel antimicrobial agents have been introduced and may become therapeutic alternatives for a variety of human infections. In addition to ongoing clinical trials on the optimum treatment regimen for LB, further basic research is urgently needed in order to better understand possible genetic or phenotypic mechanisms of persistence in _Borrelia_ spp. In light of the excess morbidity and financial burden resulting from the healthcare challenges of LB patients, these issues contribute to critically important ongoing major public health issues [17; 61; 114]. Therefore, additional investigations involving clinical first isolates alongside in vitro and in vivo experiments are clearly necessary, not only to determine more tailored treatment strategies for patients with LB [17; 52] but also to achieve in a better understanding of the critical interactions between spirochetes and antimicrobial agents [60].  
Conceptualization, K.-P.H., D.E.N. and B.L.; Software, B.L.; Formal Analysis, K.-P.H., B.L. and D.E.N.; Investigation, K.-P.H., P.K. and B.L.; Writing--Original Draft Preparation, K.-P.H. and B.L.; Writing, Review & Editing, K.-P.H., D.E.N. and B.L.; Visualization, K.-P.H.; Supervision, P.K. and D.E.N.; Project Administration, K.-P.H. All authors have read and agreed to the published version of the manuscript.  
The paper was funded by a grant (KPHBO23) from INSTAND, ev. Dusseldorf, Germany.  
This paper is dedicated to [PERSON] on the occasion of his 80 th birthday.  
The authors declare no conflict of interest.