In vitro activity of tigecycline against multiple strains of Borrelia burgdorferi

Superior efficacy of combination antibiotic therapy versus monotherapy in a mouse model of Lyme disease

Lyme disease (LD) results from the most prevalent tick-borne infection in North America, with over 476,000 estimated cases annually. The disease is caused by Borrelia burgdorferi (Bb) sensu lato which transmits through the bite of Ixodid ticks. Most cases treated soon after infection are resolved by a short course of oral antibiotics. However, 10-20% of patients experience chronic symptoms because of delayed or incomplete treatment, a condition called Post-Treatment Lyme Disease (PTLD). Some Bb persists in PTLD patients after the initial course of antibiotics and an effective treatment to eradicate the persistent Bb is needed. Other organisms that cause persistent infections, such as M. tuberculosis, are cleared using a combination of therapies rather than monotherapy. A group of Food and Drug Administration (FDA)-approved drugs previously shown to be efficacious against Bb in vitro were used in monotherapy or in combination in mice infected with Bb. Different methods of detection were used to assess the efficacy of the treatments in the infected mice including culture, xenodiagnosis, and molecular techniques. None of the monotherapies eradicated persistent Bb. However, 4 dual combinations (doxycycline + ceftriaxone, dapsone + rifampicin, dapsone + clofazimine, doxycycline + cefotaxime) and 3 triple combinations (doxycycline + ceftriaxone+ carbomycin, doxycycline + cefotaxime+ loratadine, dapsone+ rifampicin+ clofazimine) eradicated persistent Bb infections. These results suggest that combination therapy should be investigated in preclinical studies for treating human Lyme disease.

In vitro efficacy of antibiotics against different Borrelia isolates

In the present study, the effectiveness of six antimicrobial agents have been tested against 24 borrelia strains isolated from Ixodes ricinus ticks (11 Borrelia lusitaniae, eight Borrelia afzelii, three Borrelia garinii and two Borrelia valaisiana) and one B. lusitaniae strain isolated from human skin. The minimum inhibitory concentration range of antimicrobial agents was as follows: amoxicillin, 0.125-2 mg/L; doxycycline, 0.125-1 mg/L, ceftriaxone, 0.016-0.063 mg/L; cefuroxime, 0.063-1 mg/L; azithromycin, 0.0017-0.11 mg/L; amikacin 32-512 mg/L. Potentially pathogenic B. lusitaniae and B. valaisiana species were more susceptible to amoxicillin and azithromycin than pathogenic B. afzelii and B. garinii (P < 0.05); B. garinii, B. lusitaniae and B. valaisiana were more susceptible to doxycycline than B. afzelii (P < 0.05) while all species showed same susceptibility to ceftriaxone and cefuroxime (P > 0.05). This study is the first report on in vitro susceptibility of isolates from Serbia to antimicrobial agents and the first report on susceptibility of larger number of isolates of potentially pathogenic species B. lusitaniae. We showed that antimicrobial agents in vitro inhibit growth of borrelia strains very effectively, indicating the potential of their equally beneficial use in the treatment of Lyme borreliosis.

Bacteriostatic and Bactericidal Effect of Tigecycline on Leptospira spp

Tigecycline is a relatively new antimicrobial, belonging to glycylcyclines with antimicrobial activity against a large spectrum of bacteria. Very few data are available on its effect on Leptospira spp., which consist in a bacteriostatic mechanism. The aim of this investigation was to evaluate the bacteriostatic and bactericidal effect of tigecycline on reference Leptospira strains belonging to 16 serovars. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined through the microdilutions method, and tetracycline was used as the control. Results showed that tigecycline had higher MIC and MBC values than tetracycline. Obtained MIC values were between 4 and 32 µg/mL, while MBC values between 16 and >128 µg/mL. Patoc (MIC: 4 µg/mL; MBC: 16 µg/mL) resulted in the most susceptible serovar, while the most resistant were Bataviae (MIC: 32 µg/mL; MBC: 64 µg/mL), Bratislava (MIC: 8 µg/mL; MBC 128 µg/mL), and Tarassovi (MIC: 8 µg/mL; MBC: >128 µg/mL). This is the first investigation focused on the effect of tigecycline against Leptospira spp. reference strains. Since tigecycline is used as a treatment for bacteremia and urinary tract disease, and these symptoms could be linked to Leptospira infection, the possibility of using this antibiotic as a treatment for leptospirosis should be evaluated. Further studies are needed to explore the possibility to use tigecycline for in vivo application against Leptospira.

Borreliosis in Sport Horse Practice

Given the variable clinical signs attributed to Borrelia burgdorferi, including infectious arthritis, neurologic disease, and behavioral changes, B burgdorferi is an important differential for decreased performance in sport horses. The primary vectors (Ixodes tick species) are expanding their range and thus Borrelia species are located in a wider area, making exposure more likely. Due to regionally high seroprevalence and vague clinical signs, diagnosis of Lyme disease in the horse is believed overestimated. Antibiotics are first-line treatment of confirmed Lyme disease. A single positive serologic test, by itself, is not conformation of Lyme disease but is evidence of current or past infection.

In vitro susceptibility of Borrelia burgdorferi isolates to three antibiotics commonly used for treating equine Lyme disease

Background

Lyme disease in humans is predominantly treated with tetracycline, macrolides or beta lactam antibiotics that have low minimum inhibitory concentrations (MIC) against Borrelia burgdorferi. Horses with Lyme disease may require long-term treatment making frequent intravenous or intramuscular treatment difficult and when administered orally those drugs may have either a high incidence of side effects or have poor bioavailability. The aim of the present study was to determine the in vitro susceptibility of three B. burgdorferi isolates to three antibiotics of different classes that are commonly used in practice for treating Borrelia infections in horses.

Results

Broth microdilution assays were used to determine minimum inhibitory concentration of three antibiotics (ceftiofur sodium, minocycline and metronidazole), for three Borrelia burgdorferi isolates. Barbour-Stoner-Kelly (BSK K + R) medium with a final inoculum of 10⁶Borrelia cells/mL and incubation periods of 72 h were used in the determination of MICs. Observed MICs indicated that all isolates had similar susceptibility to each drug but susceptibility to the tested antimicrobial agents varied; ceftiofur sodium (MIC = 0.08 μg/ml), minocycline hydrochloride (MIC = 0.8 μg/ml) and metronidazole (MIC = 50 μg/ml).

Conclusions

The MIC against B. burgorferi varied among the three antibiotics with ceftiofur having the lowest MIC and metronidazole the highest MIC. The MIC values observed for ceftiofur in the study fall within the range of reported serum and tissue concentrations for the drug metabolite following ceftiofur sodium administration as crystalline-free acid. Minocycline and metronidazole treatments, as currently used in equine practice, could fall short of attaining MIC concentrations for B. burgdorferi.

Diet-Induced Obesity Does Not Alter Tigecycline Treatment Efficacy in Murine Lyme Disease

Obese individuals more frequently suffer from infections, as a result of increased susceptibility to a number of bacterial pathogens. Furthermore, obesity can alter antibiotic treatment efficacy due to changes in drug pharmacokinetics which can result in under-dosing. However, studies on the treatment of bacterial infections in the context of obesity are scarce. To address this research gap, we assessed efficacy of antibiotic treatment in diet-induced obese mice infected with the Lyme disease pathogen, Borrelia burgdorferi. Diet-induced obese C3H/HeN mice and normal-weight controls were infected with B. burgdorferi, and treated during the acute phase of infection with two doses of tigecycline, adjusted to the weights of diet-induced obese and normal-weight mice. Antibiotic treatment efficacy was assessed 1 month after the treatment by cultivating bacteria from tissues, measuring severity of Lyme carditis, and quantifying bacterial DNA clearance in ten tissues. In addition, B. burgdorferi-specific IgG production was monitored throughout the experiment. Tigecycline treatment was ineffective in reducing B. burgdorferi DNA copies in brain. However, diet-induced obesity did not affect antibiotic-dependent bacterial DNA clearance in any tissues, regardless of the tigecycline dose used for treatment. Production of B. burgdorferi-specific IgGs was delayed and attenuated in mock-treated diet-induced obese mice compared to mock-treated normal-weight animals, but did not differ among experimental groups following antibiotic treatment. No carditis or cultivatable B. burgdorferi were detected in any antibiotic-treated group. In conclusion, obesity was associated with attenuated and delayed humoral immune responses to B. burgdorferi, but did not affect efficacy of antibiotic treatment.

Persister Development by Borrelia burgdorferi Populations In Vitro

Doxycycline is an antibiotic commonly used to treat Lyme disease and other bacterial infections. The MIC and minimum bactericidal concentration (MBC) for Borrelia burgdorferi have been investigated by different groups but were experimentally established in this study as a function of input cell density. We demonstrated that B. burgdorferi treated in the stationary phase has a higher probability of regrowth following removal of antibiotic. In addition, we determined experimentally and mathematically that the spirochetes which persist posttreatment do not have a longer lag phase but exhibit a lower growth rate than untreated spirochetes. Finally, we found that treating the spirochetes by pulse-dosing did not eliminate growth or reduce the persister population in vitro. From these data, we propose that B. burgdorferi persister development is stochastic and driven by slowed growth.

Inter- and intra-specific pan-genomes of Borrelia burgdorferi sensu lato: genome stability and adaptive radiation

BACKGROUND

Lyme disease is caused by spirochete bacteria from the Borrelia burgdorferi sensu lato (B. burgdorferi s.l.) species complex. To reconstruct the evolution of B. burgdorferi s.l. and identify the genomic basis of its human virulence, we compared the genomes of 23 B. burgdorferi s.l. isolates from Europe and the United States, including B. burgdorferi sensu stricto (B. burgdorferi s.s., 14 isolates), B. afzelii (2), B. garinii (2), B. "bavariensis" (1), B. spielmanii (1), B. valaisiana (1), B. bissettii (1), and B. "finlandensis" (1).

RESULTS

Robust B. burgdorferi s.s. and B. burgdorferi s.l. phylogenies were obtained using genome-wide single-nucleotide polymorphisms, despite recombination. Phylogeny-based pan-genome analysis showed that the rate of gene acquisition was higher between species than within species, suggesting adaptive speciation. Strong positive natural selection drives the sequence evolution of lipoproteins, including chromosomally-encoded genes 0102 and 0404, cp26-encoded ospC and b08, and lp54-encoded dbpA, a07, a22, a33, a53, a65. Computer simulations predicted rapid adaptive radiation of genomic groups as population size increases.

CONCLUSIONS

Intra- and inter-specific pan-genome sizes of B. burgdorferi s.l. expand linearly with phylogenetic diversity. Yet gene-acquisition rates in B. burgdorferi s.l. are among the lowest in bacterial pathogens, resulting in high genome stability and few lineage-specific genes. Genome adaptation of B. burgdorferi s.l. is driven predominantly by copy-number and sequence variations of lipoprotein genes. New genomic groups are likely to emerge if the current trend of B. burgdorferi s.l. population expansion continues.

Comparison of in vitro activities of tigecycline, doxycycline, and tetracycline against the spirochete Borrelia burgdorferi

Tigecycline is a new glycylcycline that has recently been revealed to be very effective in vitro against a variety of Gram-negative and Gram-positive bacteria including multi-drug resistant microorganisms. Using a standardized microdilution susceptibility testing method, we determined the minimal inhibitory concentrations (MICs) and the minimal bactericidal concentrations (MBCs) of tigecycline, in parallel with doxycycline, tetracycline, and other antibiotic agents relevant for Lyme borreliosis treatment such as ceftriaxone and cefotaxime. The activity of all agents against 16 different Borrelia isolates belonging to all borrelial genospecies known to be pathogenic for humans was investigated and analyzed under standardized conditions. The overall rank order of MIC(90)s was tigecycline (≤0.016 mg/L) > ceftriaxone (0.03 mg/L) > cefotaxime (≤0.125 mg/L) > doxycycline (0.25 mg/L) > tetracycline (0.25 mg/L). The rank order of MBC(90)s was tigecycline (0.5 mg/L) > ceftriaxone (2 mg/L) > tetracycline (16 mg/L) > doxycycline (16 mg/L) > cefotaxime (>16 mg/L). High in vitro activity of the new glycylcycline against Borrelia was further substantiated by time-kill experiments performed with B. afzelii isolate EB1. Parallel testing of tigecycline and ceftriaxone demonstrated a bacteriostatic effect for 0.016 mg/L of tigecycline and for 0.03 mg/L for ceftriaxone after 72 h of incubation. Moreover, tigecycline was bactericidal at a concentration of 0.25 mg/L showing a >3 log(10) unit reduction of the initial inoculum, whereas for ceftriaxone a concentration of 2 mg/L was needed.

Ineffectiveness of tigecycline against persistent Borrelia burgdorferi

The effectiveness of a new first-in-class antibiotic, tigecycline (glycylcycline), was evaluated during the early dissemination (1 week), early immune (3 weeks), or late persistent (4 months) phases of Borrelia burgdorferi infection in C3H mice. Mice were treated with high or low doses of tigecycline, saline (negative-effect controls), or a previously published regimen of ceftriaxone (positive-effect controls). Infection status was assessed at 3 months after treatment by culture, quantitative ospA real-time PCR, and subcutaneous transplantation of joint and heart tissue into SCID mice. Tissues from all saline-treated mice were culture and ospA PCR positive, tissues from all antibiotic-treated mice were culture negative, and some of the tissues from most of the mice treated with antibiotics were ospA PCR positive, although the DNA marker load was markedly decreased compared to that in saline-treated mice. Antibiotic treatment during the early stage of infection appeared to be more effective than treatment that began during later stages of infection. The viability of noncultivable spirochetes in antibiotic-treated mice (demonstrable by PCR) was confirmed by transplantation of tissue allografts from treated mice into SCID mice, with dissemination of spirochetal DNA to multiple recipient tissues, and by xenodiagnosis, including acquisition by ticks, transmission by ticks to SCID mice, and survival through molting into nymphs and then into adults. Furthermore, PCR-positive heart base tissue from antibiotic-treated mice revealed RNA transcription of several B. burgdorferi genes. These results extended previous studies with ceftriaxone, indicating that antibiotic treatment is unable to clear persisting spirochetes, which remain viable and infectious, but are nondividing or slowly dividing.