International Journal of Antimicrobial Agents 41 (2013) 288–291 Contents lists available at SciVerse ScienceDirect International Journal of Antimicrobial Agents j our na l ho me p age: http://www.elsevier.com/locate/ijantimicag Short Communication In vitro susceptibility of European human Borrelia burgdorferi sensu stricto strains to antimicrobial agents Gorana Veinovi ´ c a,1 , Tjaˇ sa Cerar b , Franc Strle c , Stanka Lotriˇ c-Furlan c , Vera Maraspin c , Joˇ ze Cimperman c , Eva Ruˇ zi´ c-Sablji ´ c b,∗ a Department of Microbiology and Immunology, Faculty of Pharmacy, University of Belgrade, Vojvode Stepe 450, 11221 Belgrade, Serbia b Institute of Microbiology and Immunology, Faculty of Medicine, University of Ljubljana, Zaloˇ ska 4, 1000 Ljubljana, Slovenia c Department of Infectious Diseases, University Medical Centre Ljubljana, Japljeva 2, 1525 Ljubljana, Slovenia a r t i c l e i n f o Article history: Received 12 July 2012 Accepted 30 November 2012 Keywords: Borrelia burgdorferi sensu stricto Antimicrobial agents Susceptibility a b s t r a c t Broth microdilution and macrodilution assays were used to determine minimum inhibitory concentra- tions (MICs) and minimum bactericidal concentrations (MBCs) of six antimicrobial agents (ceftriaxone, cefuroxime sodium, azithromycin, amoxicillin, doxycycline and amikacin) for nine European human isolates of Borrelia burgdorferi sensu stricto (s.s.). Strains were obtained from patients diagnosed with Lyme borreliosis in Slovenia. Modified Kelly–Pettenkofer medium with a final inoculum of 10 5 Borrelia cells/mL and incubation periods of 72 h and of 3 weeks and 6 weeks were used in the determination of MICs and MBCs, respectively. Observed MICs indicated that all isolates were susceptible to all the tested antimicrobial agents with the exception of amikacin. Cefuroxime sodium (MIC 90 = 0.063 mg/L), azithromycin (MIC 90 = 0.22 mg/L) and ceftriaxone (MIC 90 = 0.25 mg/L) displayed the lowest MICs, followed by amoxicillin (MIC 90 = 1 mg/L) and doxycycline (MIC 90 = 2 mg/L); no strain was susceptible to amikacin (MIC 90 = 256 mg/L). MBCs after incubation for 3 weeks and 6 weeks were determined for amoxicillin (MBC 90 = 32 mg/L), doxycycline (MBC 90 = 32 mg/L) and amikacin (MBC 90 = 1024 mg/L) and were found to be high (but not defined) for azithromycin (MBC 90 > 0.88 mg/L), cefuroxime sodium (MBC 90 > 4 mg/L) and ceftriaxone (MBC 90 > 4 mg/L). In determination of borrelial susceptibility to antimicrobial agents, intrin- sic low susceptibility or methodological factors could result in low in vitro susceptibility of individual strains. This study is the first report on the antibiotic susceptibility of a series of European human isolates of B. burgdorferi s.s. © 2013 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved. 1. Introduction Lyme borreliosis, the most common tick-transmitted disease in the northern hemisphere, is caused by spirochaetes of the Borrelia burgdorferi sensu lato (s.l.) complex [1]. In Europe, the main aetio- logical agents are Borrelia afzelii, Borrelia garinii and B. burgdorferi sensu stricto (s.s.), but Borrelia spielmanii, Borrelia valaisiana, Borre- lia lusitaniae and Borrelia bissettii have also been reported as rare or potential causes of human disease. In North America, B. burgdorferi s.s. is the only human pathogen [1]. Treatment with antibiotics is necessary for all clinical manifestations of Lyme borreliosis [1,2]. As with other bacterial diseases, susceptibility of the causative agent ∗ Corresponding author. Tel.: +386 31 356 479; fax: +386 1 540 7434. E-mail address: eva.ruzic-sabljic@mf.uni-lj.si (E. Ruˇ zi´ c-Sablji ´ c). 1 Present address: Institute of Microbiology and Immunology, Faculty of Medicine, University of Ljubljana, Zaloˇ ska 4, 1000 Ljubljana, Slovenia. to the antibiotics used for therapy is a prerequisite for successful treatment. Although in vitro studies have indicated susceptibility of B. burgdorferi s.l. to several antimicrobial agents including amoxicillin, azithromycin, ceftriaxone, cefuroxime, doxycycline and penicillin among others [3–8], the susceptibility of individual Borrelia spp. to antibiotics has been defined only partially. In vitro susceptibil- ity testing has been limited because of the absence of standardised methodology as well as the small number of B. burgdorferi s.l. strains from patients with Lyme borreliosis that have been tested in indi- vidual studies. Moreover, such isolates were frequently not of low passage. Differences in the antibiotic susceptibilities of pathogenic Borrelia spp. have been reported, with some authors finding distinct effects of antimicrobial agents on strains within particular species, whereas others did not [4,8]. The aim of the present study was to determine the in vitro susceptibility of European B. burgdorferi s.s. strains isolated from patients in Slovenia to the antibiotics most frequently used for the treatment of Lyme borreliosis. 0924-8579/$ – see front matter © 2013 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved. http://dx.doi.org/10.1016/j.ijantimicag.2012.11.016