Even before the bacterial cause was known for the EM and other Lyme-related symptoms, afflicted patients were being treated with antibiotics (primarily penicillin G) with varying degrees of success [ 236 ]. Europeans had been prescribing penicillin for similar tick-acquired rashes as early as the 1950s [ 237 ]. The first clinical trial conducted during the late 1970s for LD in North America compared penicillin G, erythromycin, and tetracycline, which represented the predominantly prescribed treatments for LD symptoms [ 238 ]. From this trial, penicillin and tetracycline were found to reduce the duration of the EM compared to erythromycin which did not show any significant improvement over an absence of treatment [ 238 ]. Oncewas identified as the cause of LD, it was possible to test antibiotic susceptibilities in controlled lab experiments, with isolated (in vitro) bacterial cultures and with animal (in vivo) infection models. Such experiments confirmed the results from early clinical trials, that Bb was sensitive to penicillin and tetracycline but indicated that penicillin was not the most effective antibiotic for clearing an infection [ 239 241 ]. Surprisingly, the latter two studies found culturedisolates to be quite sensitive to erythromycin but, in agreement with clinical trials, an infection was poorly cleared by this antibiotic in animal models. This finding also highlights the risk in relying solely on in vitro antibiotic susceptibilities to inform clinical outcomes for Bb infections.

Even with this high degree of variation in how borrelial in vitro antibiotic sensitivities were measured, some general trends can be observed, as outlined in a 2006 review [ 246 ]. Beta-lactam antibiotics had variable effects on Bb: most penicillin-like beta-lactams and third generation cephalosporins showed good growth inhibition, but first generation cephalosporins were poor. For chemical compounds that target the ribosome and protein synthesis, aminoglycosides did not affect Bb but ketolides, macrolides (including erythromycin), and tetracyclines were good in vitro antibiotics. Further complicating the interpretation of antibiotic susceptibility experiments is that different strains have been shown to have different MIC values [ 247 ], a common observation for other pathogenic bacterial strains. This strain-dependent variability in antibiotic susceptibility places a lot of importance on clinical isolate strain (or even species) identification, and on having the requisite strains tested with appropriate antibiotics. The age and density of the bacterial culture can also affect the response of the bacteria to antibiotic treatment, with older, stationary cultures generating a higher proportion of cells that persist after treatment than early, log-phase cultures [ 248 ]. With many papers devoted to measuring the susceptibilities ofto a wide array of antibiotics, very little has been published on the investigations into possible antibiotic resistance mechanisms, as reviewed in [ 249 ].

The need for high-throughput screening to test thousands of compounds has prompted researchers to develop more efficient assays for measuring antibiotic sensitivities. A fluorescence-based assay that uses a two-dye combination to monitor cellular intactness, and thereby viability, has been optimized for Bb [ 244 ]. Another group has optimized a luciferase-based assay to measure ATP levels to monitor viability as a function of energy metabolism [ 245 ]. It is hoped that as other groups use these assays, a consensus protocol will be achieved, allowing for the direct comparison of reported antibiotic sensitivities of Bb. However, until high-content techniques are developed for Bb, these high-throughput assays cannot completely replace microscopic analysis of antibiotic-treated Bb. As described in more detail below, Bb can undergo multiple morphology changes in response to antibiotic treatment, and these various morphological forms of Bb can display different antibiotic sensitivities which, in turn, could have clinical relevance.

The relatively unusual biology of the spirochetal pathogen has made it difficult to use internationally-accepted protocols and guidelines that have been established for more common bacterial pathogens. Poor light absorption byliquid cultures precludes quantifying growth by absorbance, a preferred method for monitoring ongoing culture growth. Inconsistent growth on solid media necessitates alternative methods to measure treated culture viability. Also, the growth media commonly used for Bb is relatively complex and may reduce the antibiotic efficacy (e.g., penicillin incubated ingrowth media was reduced to 17% of its initial concentration after 3 days [ 242 ]). This lack of easily transferrable guidelines has led to a wide array of experimental conditions to monitor antibiotic susceptibility, including varying definitions used to calculate antibiotic minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values in the borrelial literature. A 2002 review contained a non-exhaustive list of 12 studies with their different definitions and conditions for determining MIC/MBC values [ 243 ]. The earliest (and still widely used) method was to grow Bb cultures in liquid growth media with varying concentrations of antibiotics, and then culture growth was quantified by the inefficient method of visual cell counting using dark-field or phase-contrast microscopy. Multiple variations exist in the literature at each stage for this basic protocol, including (but not limited to): the age of Bb culture before antibiotic addition; the duration of antibiotic treatment; the volume of the liquid culture (i.e., ‘macro’ grown in culture tubes versus ‘micro’ grown in microtiter plates); the temperature and atmosphere (oxygen and carbon dioxide levels) of growth; and, the criteria for cell enumeration (e.g., total number of cells, motile versus non-motile cells, intact spirochetes, etc.). There have been several attempts made in the literature to standardize these types of experiments, but there is little indication of a consensus or an adoption of any particular set of guidelines/conditions.

The generation of antibiotic-resistantin laboratory cultures demonstrates that the spontaneous acquisition of resistance can occur. If resistance to treatment were generated in the clinic in a particular patient, it is unlikely that the mechanism could be perpetuated beyond the initial patient [ 255 ] in traditional vector-mediated infection wherein the human is considered a dead-end host, although transplacental transmission from mother to fetus could be an exception [ 256 ]. Moreover, if resistance was generated in a reservoir species in the wild, the propagation of antibiotic-resistance is much more feasible. The exposure of reservoir species to clinically-relevant antibiotics in nature is improbable; however, that could change. Rodent bait boxes containing doxycycline (and a tick acaricide) have been tested as a means to reduce Bb infections in reservoir animals [ 257 ]. In principle, the reduction ofburden in reservoir animals would in turn reduce the probability of spreading this infection to humans. All-in-all though, the potential of selecting resistant Bb strains in reservoir animals makes the use of front-line antibiotics seem ill-conceived and potentially devastating.

Strains showing increased resistance to coumermycin A1 were generated by spontaneous mutation of Bb grown in the presence of the aminocoumarin antibiotic [ 252 ]. The resistance was mapped to a single mutation in, the DNA gyrase B gene and known target of coumermycin A1. In a follow-up study, this coumermycin A1 mutation was transferrable to a sensitive strain via electroporation [ 253 ]. Another group selected for erythromycin-resistance in a lab strain of Bb [ 254 ], although they were unable to identify any genetic element responsible for the change in resistance. This increased erythromycin-resistance could also be transferred to both Gram+ and Gram− recipients via conjugation, demonstrating the potential for natural dissemination.

Another, more general, resistance mechanism initially identified by bioinformatics was an efflux pump system (BesABC) with sequence homology to the TolC-tripartite multi-drug efflux pumps found in many bacteria [ 251 ]. BesC forms an outer membrane pore, and deletion of thegene has been shown to abolish virulence in a mouse infection model. The deletion strain had higher susceptibility to antibiotics, although the improvements in MIC were modest, in the range of 2–8-fold changes for the antibiotics tested. Naturally occurring antibiotic resistance mechanisms in Bb are poorly understood and under-studied, although some laboratory experiments have demonstrated the potential for generating resistance after antibiotic exposure.

Common antibiotic resistance mechanisms that are either acquired by horizontal gene transfer or by selective pressure in the clinic have not been described to date for. Also, few studies investigate the molecular basis of resistance to those antibiotics that show poor activity. Nevertheless, one mechanism that has been studied is the resistance to fosfomycin (or phosphomycin). Fosfomycin normally alkylates an active site Cysteine (Cys) residue in a peptidoglycan biosynthetic enzyme, MurA, which has a naturally occurring Aspartate (Asp) in the homologous position in Bb. When the Bb MurA Asp was substituted with the more common Cys residue, the isolated enzyme was now inhibited by fosfomycin [ 250 ]. This modified MurA also had 20-fold higher activity than wild-type Bb MurA, suggesting that Bb exists in nature with peptidoglycan biosynthetic machinery that is not capable of activity levels achieved by most other bacteria. It is likely that the Cys to Asp substitution provides a selective advantage that overcomes the reduction in activity.

Regardless of the underlying biochemical mechanism(s) of persister formation, focus has recently shifted to examining pharmacological strategies for eradication. One such study screened an FDA drug library of 1524 compounds and identified 27 drugs that had better activity on stationary Bb than doxycycline or amoxicillin [ 273 ]. The same group later demonstrated that multi-drug cocktails containing daptomycin were more effective than any single agent alone [ 269 ]. Another study tested several antibiotics on late-stage Bb cultures but did not find any that could overcome persistence unless the antibiotic was applied with pulse-dosing, a method that allows cells to recover in fresh growth media between multiple exposures to antibiotic [ 262 ]. Pulse-dosing was used based on the model that persistent Bb reverts to actively-growing cells during the recovery phase, making them antibiotic-susceptible again. A subsequent study found that pulse-dosing did not eradicate all morphological forms present in a late stage Bb culture, and was inferior to the cocktail approach [ 274 ]. Thus, despite intriguing advances, the discipline has yet to identify a consensus strategy, and test its relevance to mammalian disease. The impetus to translate in vitro findings comes from several prongs of investigation that have recovered Bb from mice [ 275 ], non-human primates [ 79 ], and human Lyme patients [ 48 ] after antibiotic treatment. Therapeutic regimens for borreliosis should require additional considerations to ensure the complete eradication of Bb, including this persistent phenotype.

These observations have prompted speculation about the involvement of the borrelial stringent response and quorum sensing in the antibiotic-tolerant phenotype of stationary phase cells. In late stage cultures, ~20% of the transcriptome has been found to be altered under the control of the stringent alarmones, and the corresponding changes associated with this pathway are generally consistent with dormancy-based drug avoidance mechanisms [ 196 226 ]. Although deletion of the key stringent response enzyme,, failed to alterantibiotic MICs [ 227 ], this assay does not directly evaluate the formation of persister cells. Moreover, drug responses of stationary phase ΔBb have not yet been tested, so it remains unclear whether the relationship between the stringent response andantibiotic tolerance is causal or correlational. Meanwhile, there is some indication that eliminating LuxS, the enzyme responsible for biosynthesis of the quorum sensing autoinducer, impacts both borrelial viability and aggregate structures of stationary phase cultures treated with sub-MBC concentrations of doxycycline [ 271 ]. Ultimately, clarity is required around the contributions of these and other environmental response networks topathophysiology and persistence. Although the stringent response and biofilm formation may seem like complementary stress avoidance mechanisms that converge to generate persisters, this may not be the case in other microorganisms. In, for example, induction of the stringent response triggers biofilm dispersal [ 272 ], essentially rendering the two strategies mutually exclusive.

As is the case with other bacteria, the persistence phenotype inappears to be heavily influenced by the physicochemical environment prior to the addition of antimicrobials, arguing against a model in which the drug itself is an exclusive trigger. This is evidenced by drastic differences in the viability profile of a young (exponential) and old (stationary) culture treated with antimicrobials. Several groups have independently demonstrated that borrelial cultures are largely susceptible to doxycycline and other frontline drugs during exponential growth, but become increasingly tolerant as they mature [ 248 269 ]. Indeed, stationary or late stage cultures are considered persister-enriched, and have become a common laboratory model of treatment-refractory. This implies that there are priming events or contextual cues prior to drug exposure that renderless vulnerable to a pharmaceutical even before encountering it, which is consistent with the contemporary concept of “triggered persistence” [ 258 ]. Mature cultures are presumed to differ from their young counterparts in several ways, including increased cell density, decreased growth rate, accumulation of secreted factors, exhaustion of growth medium, and a build-up of metabolic biproducts. Persister-enriched stationary phase cultures also demonstrate an increased proportion of non-spirochetal morphologies (pleomorphic forms, as reviewed in [ 270 ]), which may represent the persistent phenotype, and will be discussed in Section 5.2 , below.

Nevertheless, in vitro regrowth experiments that examined cellular dynamics following antimicrobial treatment suggest that the persistence trait is not selected—the population of survivors from the first antibiotic exposure does not fare better in subsequent exposures to the same drug [ 248 262 ]. This has also been documented in humans for whom Bb was recovered from the site of an EM both before and after antimicrobial therapy [ 268 ].isolated from this small cohort of Slovenian patients following treatment did not show an altered susceptibility profile compared to pre-treatment response, despite withstanding drug exposure in the host [ 268 ]. This is characteristic of persisters, and not observed with resistant mutants. However, isolates from different patients did demonstrate substantial interstrain variability in MIC and MBC, as mentioned in Section 5.1.1 above.

In the case of, the existence of antibiotic persister cells has been demonstrated by numerous groups, both in vitro and in vivo with animal models and human patients [ 262 266 ]. Notably, this persister fraction has been found to tolerate high concentrations of antimicrobials that exceed levels considered clinically achievable [ 262 ]. Although the mechanisms of this phenomenon are still largely unknown, there is evidence of both direct (active) and indirect mechanisms of drug avoidance. For example, examining the transcriptome of antibiotic treated cells revealed a shift in gene expression in response to the drugs, including induction of some ORFs associated with stress response. One comparison of doxycycline- and amoxicillin-treated Bb identified targets that were common to both frontline therapeutics, and a number of genes that were differently regulated between the two treatments when dosed at the MBC [ 248 263 ]. Although the extended viability, recovery, and regrowth of these remaining cells were not assessed, the distinct gene expression profiles suggest that drug exposure may be a primary trigger of a physiological response, rather than simply enriching for stochastically pre-formed persisters. A subsequent study profiled the transcriptome of cells under doxycycline MBC and during drug-free rebound, and found a pattern speculated to represent onset and resolution of dormancy [ 267 ]. Moreover, treated cells were found to retain some infectious capacity in an immunocompetent murine host [ 267 ].

The ability of Bb to persist after antibiotic treatment can further complicate the choice of treatment for LD in the clinic. In contrast to antimicrobial resistance, the concept of persistence implies the survival, but not replication, of bacteria exposed to normally lethal concentrations of antibiotic [ 258 ]. It is a well-documented phenomenon for most pathogens, and has been associated with chronic infection (reviewed in [ 259 ]). In laboratory practice, persistence is characterized by a biphasic killing curve in which the bulk of the population is rapidly eliminated, and succeeded by a subpopulation of more tolerant cells with different kill kinetics [ 260 ]. The conventional view of bacterial persisters holds that they represent natural phenotypic heterogeneity in a culture, such that at any given time, a subgroup of cells is in a transient state that avoids the mechanism of many antibiotics that rely on metabolically active, dividing cells for their antibacterial function. Persistence thereby renders cells temporarily impervious to an antimicrobial agent [ 260 ]. Resistance mechanisms usually arise from genetic alterations that are subject to selective pressures that enrich for the population of resistant cells. Conversely, antibiotic-treated persister cells subcultured in antibiotic-free growth media produce roughly the same proportion of persistent cells when retreated with the antibiotic compared to the first drug exposure [ 261 ]. Therefore, persistence is considered a temporary phenotypic exemption from the effects of antimicrobials via dormancy or similar metabolic perturbation, rather than an active countermeasure strategy in response to a drug.

5.1.4. Clinical Antibiotic Treatment

Borrelia infection, or in reducing the duration of symptoms. Current clinical treatment guidelines for many Lyme symptoms recommend doxycycline (from the tetracycline family of antibiotics), amoxicillin (a penicillin-like beta-lactam), or cefuroxime axetil (a beta-lactam-containing second generation cephalosporin), and these recommendations are generally accepted worldwide [ The first cases of Lyme disease (and EMs prior to the emergence of LD) were originally treated with penicillin G based on success with syphilis, another infection caused by a spirochetal pathogen. As described above, early clinical trials and in vivo animal models demonstrated that penicillin G was not the most efficient antibiotic in either clearing ainfection, or in reducing the duration of symptoms. Current clinical treatment guidelines for many Lyme symptoms recommend doxycycline (from the tetracycline family of antibiotics), amoxicillin (a penicillin-like beta-lactam), or cefuroxime axetil (a beta-lactam-containing second generation cephalosporin), and these recommendations are generally accepted worldwide [ 57 66 ]. Doxycycline was first introduced as a treatment in the 1980s [ 276 ] based on in vitro susceptibility experiments [ 239 ]. Amoxicillin was introduced as a treatment for LD around the same time based on syphilis treatment success [ 277 ]. Cefuroxime axetil was tested as a therapeutic agent in the 1990s [ 278 ] because of good in vitro Bb susceptibility [ 279 ] and fewer complications such as age contraindications and photosensitivity. These recommended treatments have been in use for early Lyme for close to 40 years, but a recent randomized trial in Norway demonstrated their continued efficacy in the treatment of EMs and in preventing the progression of LD symptoms [ 280 ]. More serious Lyme symptoms, such as disseminated EMs, neuroborreliosis and Lyme carditis, can also be treated with intravenous ceftriaxone (a third generation cephalosporin), but side effects may be more prevalent [ 57 66 ]. Ceftriaxone was originally tested as a Lyme treatment in the late 1980s because it was known to penetrate a variety of tissues well, including the central nervous system, in addition to having a good MIC against Bb.

Despite the general consensus around the use of doxycycline, amoxicillin, or cefuroxime axetil as frontline agents for treatment of early infection, guidelines differ in the suggested dose and duration of chemotherapy. The IDSA advises ~14–21 days (drug dependent) of oral therapy [ 66 ], while this range constitutes the minimum recommendation of the International Lyme and Associated Diseases Society (ILADS), which also emphasizes clinical judgment in determining the optimal treatment protocol [ 64 ]. The latter recommendation and provision for extending therapy in early LD is based on “unacceptably high” treatment failure rates, parsed from acute and re-treatment clinical trials, prospective investigations, retrospective studies, and meta-analyses thereof [ 64 ]. Estimates of treatment failure, incomplete resolution, sequalae and chronic complications vary widely in the literature, with anywhere from zero [ 280 ] to ~40% of participants reporting prolonged symptoms and/or functional impairment [ 25 ]. Meanwhile, a recent meta-analysis of randomized antibiotic trials of early localized LD in adults determined that treatment failures were “rare”, although the certainty of evidence for response was considered low [ 281 ].

How might these incongruent outcomes be reconciled? Although a detailed critique of clinical trials is beyond the scope of this review, findings from the past decades suggest study design elements that could substantially influence conclusions. These considerations include the geography of participant recruitment, which is associated with borrelial serotype distribution and prevalence, duration of longitudinal tracking, definitions of treatment response, failure, relapse, and syndrome, and the approach to capturing and quantifying subjective symptoms (e.g., fatigue, musculoskeletal pain) in addition to the objective manifestations that are often reported as primary outcomes (e.g., EM resolution). Fundamentally, all trials are constrained by the lack of a definitive indication of “cure”, or microbial eradication, and thus rely on other indications of infection and morbidity that may be subject to interpretation.

As a case in point, three studies that were intentionally developed to evaluate post-treatment LD sequelae arrived at vastly different estimates of protracted disease burden, despite enrolling participants with EM lesions and following their response to the same antibiotic. When Cerar and colleagues recruited 285 patients with localized LD from Slovenia, treated with 15 days of doxycycline or cefuroxime axetil, and evaluated subjective symptoms experienced by the patient cohorts and a non-LD control group at 6 months (exam, interview) and 12 months (exam, interview, questionnaire), they concluded that the frequency of new or increased symptoms in the LD groups did not differ significantly from that of the control cohort [ 282 ]. Meanwhile, Aucott’s team enrolled 63 American patients exhibiting EM and symptoms of illness, and evaluated the cohort over a 6-month period, including a 3-week intervention with doxycycline during acute disease. Exams, interviews, and validated questionnaires assessing functional impact (SF-36) and mental health (Beck Depression Inventory) were administered at each of 5 visits, leading to the discovery that 35% of participants fulfilled general criteria for PTLDS at 6 months [ 25 ]. When the same group applied an operationalized definition of PTLDS (described in Section 2.2 ) to a patient cohort (n = 74) and controls in a subsequent study, only 11% of participants met the case definition, while 28% experienced either ongoing symptoms or functional impact but not both, thereby disqualifying them from the diagnosis [ 54 ]. The latter comparison illustrates the pivotal role of definitions in determining study outcomes and ultimately influencing recommendations, which in turn emphasizes the need for consensus language that also represents biologically- and clinically-relevant phenomena.

B. burgdorferi s.s., whereas B. afzelii was by far the most prevalent isolate identified in the Slovenian patients (86.9% of cultivable organisms) [ B. afzelii are less severe than North American LD [ B. burgdorferi in Slovenia was found to be less aggressive [ Borrelia and LD that vary by virulence and geography. It is important to note that despite the disability they documented in their PTLDS cohort, Aucott’s group did not evaluate patients for microbiological evidence of ongoing infection. Beyond terminology, the aforementioned European and American studies differ in important ways, including the disease status of the patients at the time of enrollment, and the identity of the pathogen. Whereas Cerar et al. excluded participants with multiple EM and followed a cohort with fewer than two symptoms on average at baseline [ 282 ], Aucott et al. recruited patients with evidence of early systemic disease (mean ~10 symptoms) in addition to EM, who then received a longer course of doxycycline [ 25 ]. Although the pathogens were not recovered or genotyped from American patients, geography predicts that they were most likelys.s., whereaswas by far the most prevalent isolate identified in the Slovenian patients (86.9% of cultivable organisms) [ 282 ]. This is a highly relevant distinction considering the discussion in Section 4.1 that borrelial genospecies and serotype influence host invasion and virulence. Notably, early manifestations of Europeanare less severe than North American LD [ 125 ], and indeed evenin Slovenia was found to be less aggressive [ 126 ], possibly due to the scarcity of the more virulent OspC type A [ 150 ]. Incongruous estimates of post-treatment disease burden arising from these studies could reasonably reflect genetically-distinct populations ofand LD that vary by virulence and geography. It is important to note that despite the disability they documented in their PTLDS cohort, Aucott’s group did not evaluate patients for microbiological evidence of ongoing infection.

Borrelia in response to conventional antibiotic treatment [ With these considerations and findings in mind, it appears that first-line antimicrobials may not be universally effective at preventing protracted complications, although it remains unclear whether individual drugs differ in their influence on long-term quality of life. The findings also suggest that there may be an unmet need for novel therapeutics early in disease, particularly in regions with a high prevalence of invasive serotypes where patients may be at increased risk of disseminated, treatment-recalcitrant disease. This speculation is likewise spurred by in vitro work, discussed in Section 5.1.3 , that suggests that Bb strains can differ in their MICs, and that antibiotics may instigate persistent bacterial phenotypes. Ideally a font-line therapeutic agent should not push surviving cells toward a treatment-intractable state, and as described above, some research is now focusing on the relevance of the transcriptomic changes that occur inin response to conventional antibiotic treatment [ 267 ].

284,285, For LD patients who do experience prolonged, relapsing, or new-onset symptoms despite receiving recommended treatment, the question of appropriate intervention strategy is the subject of controversy and uncertainty. In the United States, several randomized, placebo-controlled clinical re-treatment trials were conducted to determine whether IV ceftriaxone with or without adjunctive doxycycline could resolve persistent manifestations, particularly fatigue and neurocognitive impairment [ 283 286 ]. Two of the four studies identified improvements in some domains of interest, but ultimately recommended against the broad adoption of the protocol due to IV-related complications [ 284 286 ].

Nevertheless, findings have been misrepresented in some subsequent literature [ 287 ], and the lack of robust recovery and protocol endorsement has in some cases been erroneously interpreted as evidence that persistent symptoms do not have an infectious etiology. Such studies can only draw conclusions about the dose and duration of the drug under investigation, as it relates to the specific outcome measures established for the particular population being evaluated. Indeed, trial success would depend on additional conditions being met: (i) if present, the residual pathogens must be tractable to the antimicrobial of choice, for which (ii) dose and duration have been established in an evidence-based manner, and (iii) inclusion criteria should distinguish participants who are good candidates for the particular therapy. As discussed in Section 2 , laboratory tests of active infection have yet to be validated and broadly adopted, and there are several hypotheses of treatment-refractory illness including immune dysfunction, antigenic debris, and host tissue damage, for which antibiotics would not be appropriate. The possibility that the treatment-refractory population is heterogeneous and may not respond uniformly to intervention was further suggested by a recent registry-mining study that evaluated responses collected from American patients by an NGO. The analysis of MyLymeData identified 34% of nearly 4000 registrants as “high responders” to antibiotic therapy [ 288 ]. The distinguishing features of the cohort are yet unknown, and although there are many variables inherent to this study design, response diversity emphasizes questions of treatment tractability.

Borrelia and evaluation of its drug susceptibilities is still emerging, as described in To that end, the aforementioned American RCTs for longstanding symptoms were published between 2001 and 2008, whereas laboratory modelling of persistentand evaluation of its drug susceptibilities is still emerging, as described in Section 5.1.3 . Notably, recent in vitro work suggest that go-to LD drugs, including ceftriaxone, have markedly reduced efficacy against persistent forms of the bacterium [ 269 274 ]. Consequently, novel treatments specifically targeting persistent Bb infections are beginning to be used in the clinic. Dapsone, a drug used for dermatological pathologies such as acne and leprosy, has shown some efficacy for treating persistent Lyme symptoms in a preliminary clinical trial [ 289 ]. An anti-alcoholism drug, disulfiram, demonstrated positive outcomes for three patients with relapsing neuroborreliosis symptoms who were monitored on the drug after conventional treatment failed to adequately control their disease [ 290 ], and a clinical trial has been initiated to study the safety of disulfiram treatment for post-treatment LD symptoms (clinicaltrials.gov, NCT 03891667).

Borrelia recurrentis , the JHR symptoms that occur after penicillin treatment are preceded by an increase in blood serum levels of cytokine TNF-alpha and followed by increases in IL-6 then IL-8 [ One aspect of clinical treatments specific to Bb infections that is under-studied is the pharmacokinetic and pharmacodynamic properties of the various antibiotics. These properties have been tested in model organisms (e.g., non-human primates [ 79 ]) but very rarely in Lyme patients. Another poorly understood aspect of Bb treatments is the observation that, for a subset of patients, symptoms can worsen after antibiotic treatment. This apparent paradox has been termed the Jarisch–Herxheimer Reaction (JHR) and has been observed in other spirochetal infections, such as syphilis and leptospirosis (as reviewed in [ 291 ]). For relapsing fever, a tick-borne illness caused by the spirochete, the JHR symptoms that occur after penicillin treatment are preceded by an increase in blood serum levels of cytokine TNF-alpha and followed by increases in IL-6 then IL-8 [ 292 ]. A small controlled trial of 40 relapsing fever patients was conducted in which one half of the patient group received antibody fragments specific to TNF-alpha in order to prevent TNF-alpha activity [ 293 ]. Patients who received the TNF-alpha antibodies showed a suppression of JHR symptoms, but not a complete elimination of JHR, indicating that the treatment was not fully effective. Others have suggested that there may be a treatment-inaccessible sub-population of TNF-alpha that causes the JHR symptoms, or that other mediators exist [ 294 ]. The actual molecular component of Bb that initiates JHR in Lyme patients is currently unknown, and the reaction in Lyme patients is usually reported in the literature as being mild relative to other infections. One conceivable cause of increased symptoms and serum cytokine levels post-treatment is the release of immunologically reactive fragments generated by antibiotic-induced cellular destruction [ 295 ], including peptidoglycan [ 296 297 ]. Understanding the root cause of the JHR would lead to treatments that reduce or eliminate these episodes.