ISOLATION , CULTIVATION , AND IN VITRO SUSCEPTIBILITY TESTING OF BORRELIA BURGDORFERI SENSU LATO : A REVIEW

Lyme borreliosis is the most common vector-borne disease in the northern hemisphere. The agents of Lyme borreliosis are borrelia, bacteria of the family Spirochaetaceae, which are grouped in Borrelia burgdorferi sensu lato species complex. Borreliae are fastidious, slow-growing and biochemically inactive bacteria that need special attention and optimal conditions for cultivation. The isolation of Borrelia from clinical material and their cultivation is a time-consuming and demanding procedure. Cultivation lasts from 9 up to 12 weeks, which is much longer than is necessary to grow most other human bacterial pathogens. Although B. burgdorferi sensu lato is susceptible to a wide range of antimicrobial agents in vitro, up to now the susceptibility of individual Borrelia species to antibiotics is defined only partially.


INTRODUCTION
Lyme borreliosis is a multisystem disease caused by spirochete B. burgdorferi sensu lato that is transmitted to humans by ticks of the Ixodes complex and is manifested with diverse clinical signs and symptoms and with several variations in the course of the disease.A complete presentation of the disease has been divided into three clinical stages: early infection manifested as skin lesion erythema migrans at the site of the tick bite (stage 1), followed by involvement of the nervous system, joints and/or heart (stage 2) and late involvement of the nervous system, joints and skin that appear within months or years (Stanek and Strle, 2003;Steere, 2001).

Geographic distribution and incidence rate
Lyme borreliosis occurs in North America (from the Mexican border in the south to the southern Canadian provinces in the north), the whole Europe, parts of North Africa (Maghreb), and northern Asia (Russian Siberia and the Far East, Sakhalin, Japan, China and Korea).In North America, Lyme borreliosis has been recorded in almost all states.The existence of Lyme borreliosis in the southern hemisphere (South and Central America, Sub-Saharan Africa, southern Asia, Australia) has never been reliably confirmed (Hubálek, 2009).
The incidence of Lyme borreliosis is associated with the prevalence of the main vectors -ticks, which are classified as the Ixodes ricinus complex, also called Ixodes persulcatus complex (Hubálek, 2009;Steere et al., 2004).Species of borrelia (pathogenic, less pathogenic and non-pathogenic Borrelia species) and geographical distribution of ticks are presented in Table 1.About 85,000 and 19,000 (from 15,000 to 20,000) cases are reported annually in Europe and in the USA, respectively (estimated from available national data) (Lindgren and Jaenson, 2006;Steere, 2001).The highest incidence of Lyme borreliosis is in central Europe (e.g.Slovenia, 155/100,000) and the lowest in the UK (0.7/100,000) and Ireland (0.6/100,000) (Lindgren and Jaenson, 2006).
The registration of patients with Lyme borreliosis is necessary in only a few European countries, and it is assumed that the real incidence of this disease is most probably higher.
The outer cell membrane is very fluid and contains transmembrane and outer surface proteins (Osp).Outer surface proteins (Osp A, Osp B, Osp C, Osp D, Osp E in Osp F) are lipoproteins encoded by the linear and circular plasmids.They are important for maintening the structure of the membrane, while some of them have a role in enzymatic reactions or in transport through the membrane.Osp A, Osp B, and Osp C are the most important antigens of Borrelia.On their surface are many antigenic characteristics, which give to Borrelia the properties of heterogeneous bacteria (Dressler et al., 1993;Wilske et al., 1988).In addition, the outer membrane also contains lipopolysaccharides that are similar to the lipopolysaccharides of Gram-negative bacteria (Barbour and Hayes, 1986;Cox et al., 1996).Different outer surface proteins help Borrelia in adaptation and survival in different arthropod and mammalian environments (de Silva and Fikrig, 1997).Osp A is one of the major outer membrane lipoproteins of Borrelia and has been used for serological diagnosis as well as for vaccine development (Hilton et al., 1996;Kramer et al. 1996).(Rosa et al., 2005) Antibodies directed against Osp A appear in the later stage of Lyme borreliosis (Wilske et al., 1996).Osp B has adhesive properties like Osp A, and antibodies directed against this antigen appear in the late stage of disease (Dressler et al., 1993).Osp C is the predominant seroreactive antigen in the early stage of Lyme borreliosis, and important in the transmission of the spirochete from tick to mammal.Rapid synthesis of Osp C by Borrelia during tick feeding is an essential in the capacity of Borrelia to infect mammalian hosts, including humans, when transmitted by ticks, and after transmission from the tick may play a role in colonization of host tissues (Schwan et al., 1995;Schwan, 2003).The sequences of Osp C vary among strains and only a few of the groups of sequences are associated with dissemination, and Osp C alleles have been linked to infectivity as well as to invasiveness (Lagal et al., 2003;Seinost et al. 1999).
Table 1.Species of borrelia and geographical distribution of ticks (Steere et al., 2004;Stanek and Strle 2003).The genome of Borrelia species consists of a linear chromosome of approximately one million base pairs (Mb) and various linear and circular plasmids (Saint Girons et al., 1992).B. burgdorferi sensu stricto strain B31 was the first spirochete whose complete genome was sequenced.The genome of this strain consists of a linear chromosome of 910, 725 bp and 17 linear and circular plasmids of 533,000 bp (Fraser et al., 1997).
The chromosome of Borrelia is important for the identification of species, and methods for identification are based on the analysis of plasmid or chromosomal DNA (e.g.restriction of entire DNA with different restriction enzymes, hybridization, DNA sequencing, etc.) (Belfaiza et al., 1993;Picken et al., 1996;Wang et al., 1999).B. burgdorferi sensu lato strains differ among themselves in relation to different plasmid profiles.They have an unusual plasmid content of linear and circular plasmids that may vary in number and size.This feature is very important in comparing and distinguishing strains within the same species (Xu and Johnson, 1995).
The spreading of Borrelia through the skin and other tissue is facilitated by the binding of human plasminogen, which can then be converted to active plasmin.Borrelia with bound plasmin is able to degrade fibronectin, penetrate the endothelium, and activate matrix metalloprotease-9 (MMP-9), and collagenase 1 (MMP-1) (Coleman et al., 1995;Coleman et al., 1999;Gebbia et al., 2001).

Isolation of B. burgdorferi sensu lato from clinical materials and growth conditions in vitro
The isolation of Borrelia from clinical materials is a golden standard for confirming borrelial infection, especially useful during the first several weeks of infection when serodiagnostic tests are insensitive (Ružić-Sabljić et al., 2006;Steere, 2001).
Borrelia can be isolated from different clinical materials such as skin, blood, cerebrospinal fluid (CSF), etc. during early as well as chronic stages of Lyme borreliosis (Ružić-Sabljić et al., 2002;Wilske and Preac-Mursic, 1993).The clinical material for isolation must be taken from patients under aseptic conditions, before antimicrobial therapy, in as large a quantity as possible (e.g. 2 mL of CSF, 10mL of blood) and inoculated into the medium as soon as possible (Wilske and Preac-Mursic, 1993;Wilske and Pfister, 1995).In addition to the above-mentioned, temperature is very important during the transportation of clinical material from patient to laboratory.Room temperature was described as suitable for the transport of samples infected with Borrelia during the period from one to 11 days, while refrigerator temperature (5ºC) was described as inadequate (Berger et al., 1992;Campbell et al., 1994).
Isolation, as well cultivation of borrelia, is a demanding, time-consuming and expensive procedure characterized by a modest level of sensitivity (Kollars et al., 1997;Maraspin et al., 2001;Nadelman et al., 1996;Strle et al., 1996a;Wormser et al., 2000a), and few laboratories are equipped to carry it out.
The generation time of Borrelia is long and ranges from 7 to 20 h; it is influenced by available nutri-ents, conditions of cultivation and the adaptation of Borrelia to the artificial medium (Preac-Mursic and Wilske, 1993).Cultivation lasts from 9 to 12 weeks, which is much longer than needed to grow most other human bacterial pathogens (Ružić-Sabjić et al., 2002;Wormser et al., 2000a).On the other hand, borrelia requires complex media for in vitro cultivation, due to their inability to synthesize any amino acids, nucleosides, nucleotides, fatty acids, or other cellular building blocks (Fraser et al., 1997).
Many factors can influence in vitro Borrelia growth, such as medium ingredients, pH of medium, temperature of incubation, contaminants, sample cell density, the capacity of particular borrelial species to grow, number of different Borrelia strains in the sample, previous antibiotic therapy, local anesthesia at the site of skin biopsy, size of skin biopsy specimens, and conditions during the transport of samples to the laboratory (Barbour 1984;Callister et al., 1990;Campbell et al., 1994;Hubálek et al., 1998;Jobe et al., 1993;Kollars et al., 1997;Pollack et al., 1993;Ružić-Sabljić et al., 2006;Wormser et al., 2000a;Yang et al., 2001).
Borrelia has an ability for in vitro transformation of normal, mobile spirochetes to cystic forms under unfavorable conditions in their environment (Brorson and Brorson, 1997).The authors evaluated the behavior of Borrelia under controlled conditions and Borrelia was cultivated in commercial BSK-H medium, which contained 6% rabbit serum, and in BSK-H medium without rabbit serum.In the medium without rabbit serum, borreliae were transformed into cystic forms, but after the cystic forms were transferred to the same culture medium with rabbit serum, they were transformed into regular, mobile spirochetes after 6 weeks, and their regeneration time was normal.This means that similar phenomenon may occur in vitro under other conditions unfavorable for Borrelia (e.g. the presence of antibiotics).Similarly, when normal, mobile spirochetes were inoculated into cerebrospinal fluid, the spirochetes were converted to cysts (spheroplast Lforms), but when these cystic forms were transferred to BSK-H medium, the cysts were converted back to normal, mobile spirochetes after incubation.When neuroborreliosis is suspected, it is necessary to realize that Borrelia species can be present in cystic form, and these cysts have to be recognized by microscopy (Brorson and Brorson, 1998).

Description of culture media
The preparation of culture media is demanding and expensive.Different culture media have been introduced and evaluated for borrelial cultivation, but for routine work most frequently three liquid media were reported: modified Kelly-Pettenkofer (MKP), Barbour-Stoenner-Kelly II (BSK-II) medium and commercially available BSK-H (Sigma, USA) medium (Barbour 1984;Pollac et al., 1993;Preac-Mursic et al., 1986;).The majority of the ingredients in these media are equivalent (e.g.CMRL as a source of amino acids, vitamins, and other factors, N-acetyl-D-glucosamine-a precursor for bacterial cell wall biosynthesis, HEPES, neopeptone, pyruvic acid, citric acid, bovine serum albumin, rabbit serum, etc.).
On the other hand, media differ with regard to concentration, origin (diverse commercial sources), and the preparation of certain ingredients (Barbour, 1984;Pollack et al., 1993;Preac-Mursic et al., 1986;Ružić-Sabljić et al., 2006).For example, in contrast to BSK-H, BSK-II and MKP contain gelatin, but MKP lacks yeast extract and contains a higher concentration of rabbit serum that differs in its preparation (7.2% heat-inactivated rabbit serum in MKP versus 6% non-inactivated in BSK-II and BSK-H).There is also a difference in glucose concentration between the three media: 3, 5 and 6% for MKP, BSK-II, and BSK-H medium, respectively.The main differences between MKP, BSK-II and BSK-H are presented in Table 2.
The source and quality of albumin and specific preparation of rabbit serum can influence Borrelia growth in vitro and some preparations of rabbit serum contain antispirochetal immunoglobulin G that reduces or inhibits the growth of Borrelia (Pollack et al., 1993).Similarly, if identical BSK media contain different lots of bovine serum albumin from differ-ent manufacturers, they differ in their ability to support the growth of a small number of Borrelia strains (Callister et al., 1990).
Borrelia strains can also grow on solid media with agarose to solidify the liquid media under microaerophilic or anaerobic conditions (De Martino et al., 2006;Kurtti et al., 1987;Preac-Mursic et al., 1991).By using a solid medium, distinct morphological variations in colonies of B. burgdorferi sensu stricto strains can be observed.B. garinii in both media, but the results were found to be statistically significant only for the MKP medium, while B. garinii overgrew B. afzelii but significant differences were established only for the BSK-II medium.Strle et al. (1999a) compared patients in Europe and USA with culture-confirmed erythema migrans, and showed B. burgdorferi sensu stricto (iso-lated from USA patients) to be more virulent than B. afzelii (isolated from Slovenian patients).Erythema migrans spreads more slowly, the duration is longer, and the possibility of dissemination is less common in European patients, unlike erythema migrans in USA patients, which spreads more rapidly, the duration is shorter, and it is associated with more intensive inflammation and signs that often indicate the dissemination of Borrelia.
Similarly, in a comparison of the European and USA patients with culture-confirmed erythema migrans, Strle et al. (2011) showed B. burgdorferi sensu stricto isolated from USA patients to be more virulent than B. garinii isolated from Slovenian patients.Slovenian patients with erythema migrans caused by B. garinii developed larger lesions than USA patients, but systemic symptoms and abnormal physical findings, such as fever or regional lymphadenopathy, appear more frequently in USA patients with erythema migrans caused by B. burgdorferi sensu stricto than in European patients.Furthermore, B. burgdorferi sensu stricto induces normal macrophages to secrete higher levels of chemokines and cytokines than B. afzelii or B. garinii, indicating that B. burgdorferi sensu stricto induces a greater inflammatory response in macrophages than the other two Borrelia species (Strle et al., 2009).
Published in vitro susceptibility results on the minimal inhibitory concentracion (MIC) and minimal bactericidal concentracion (MBC) of antimicrobial agents for borreliae are difficult to compare because of methodological differences.Up to now, broth microdilution and macrodilution methods in different media (MKP, Barbour-Stoener-Kelly, BSK-II or standardized BSK-H) with various inoculum concentrations (10 4 -10 7 / mL), incubation periods (3-7 days for MIC, 1-6 weeks for MBC) were applied.Samples were mainly checked for the presence of Borrelia by the enumeration of cells and observation of borrelia motility using dark-field microscopy or by visual conformation of medium color changes in microtiter plates.
On the other hand, enumeration and observation of Borrelia is time-consuming and demanding, especially if more samples need to be checked simultaneously and subjectively.Boerner et al. (1995) reported that the MIC values for Borrelia are significantly influenced by the density of the inoculum and the mode of MIC determination (microscopical and macroscopical MIC reading).The authors showed that differences between MIC values obtained by testing 10 6 versus 10 7 Borrelia cells/mL were on average equivalent to 1.2 dilution steps for the macroscopical but only 0.2 dilution steps for the microscopical method, and that the high agreement of microscopical MICs has been due to the standardized enumeration method.Using an inoculum of 10 6 cells/mL, MICs determined macroscopically (which is less time-consuming than the microscopical method) were significantly lower than MICs determined microscopically because the inoculum of 10 6 cells/mL was too small to induce a distinct color change and sediment formation, whereas borrelial growth was often detectable microscopically.On the other hand, when using 10 7 cells/mL as the final inoculum, 75.9% of the MICs revealed identical values for both the macroscopical and the microscopical reading modes.This discrepancy in the methodology led to a wide range of published MIC and MBC results.For example, for amoxicillin, ceftriaxone, doxycycline, and azithromycin the MIC varies from ≤0.03 to 4 mg/L, from ≤0.01 to 4 mg/L, from 0.06 to 4 mg/L, and from 0.003 to 0.06 mg/L, respectively.Similarly, for amoxicillin, ceftriaxone, doxycycline, and azithromycin, the MBC varies from ≤0.03 to >16 mg/L, from 0.006 to 4 mg/L, from 0.4 to 32 mg/L, and from 0.003 to 4 mg/L, respectively.(Baradaran-Dilmaghani and Stanek, 1996;Dever et al., 1992;Hunfeld et al., 2000a;Hunfeld et al., 2000b;Levin et al., 1993;Morgenstern et al., 2009;Ružić-Sabljić et al., 2005;Sicklinger et al., 2003).Despite the problems in the methodology, in vitro susceptibility testing has also been limited because of the small number of tested B. burgdorferi sensu lato strains.Ružić-Sabljić et al. (2005) tested the susceptibility of B. afzelii strains to antimicrobial agents, and found that several isolates survived high concentrations of doxycycline, cefuroxime axetyl and amoxicillin and grew after 6 weeks but not after 3 weeks incubation.Their results showed that borrelia could survive exposure to antibiotics in vitro, but mechanisms of resistence are not obvious.One possible explanation for the survival of Borrelia in the presence of antibiotics could be the ability of Borrelia to persist in a latent phase.The ability could enable spirochetes to survive the presence of antimicrobial agents in their environment and begin to re-grow under suitable conditions, e.g. after antibiotic elimination in the body or after reduction or loss of activity of antibiotics during a prolonged incubation period in vitro.On the other hand, Preac-Mursic et al. (1996b) showed that Borrelia has the ability to transform into a cystic, spherical form (spheroplasts, L-form), which may offer protection from antimicrobial agents, allowing them to survive unfavorable conditions such as the presence of antibiotics.The spheroplast-L-form without cell walls can be a possible reason why Borrelia can survive in an organism for a long time and the cell-wall-dependent antibody titers disappear and emerge after reversion.Georgilis et al. (1992) reported that human skin fibroblasts could protect Borrelia from ceftriaxone in vitro.While this antibiotic lost efficiency when Borrelia was cultured in the presence of cells, ceftriaxone was lethal for spirochetes in the absence of human skin fibroblasts.Similarly, the study of Brouqui et al. (1996) confirmed the ability of Borrelia to survive in human cells in vitro in the presence of antimicrobial agents in their environment.When Borrelia was cultivated in an axenic medium in the presence of penicillin G or ceftriaxone, the number of bacteria decreased rapidly, whereas then they were co-cultivated with eukaryotic cells in the presence of penicillin G or ceftriaxone, no change in the viable borrelial count was observed.Doxycycline and erythromycin were found to act efficiently against Borrelia, especially when they were grown in the presence of eukaryotic cells.No viable Borrelia was found after incubation with these antibiotics, indicating no protective effect of eukaryotic cells on doxycycline and erythromycin actions.
In some studies, antimicrobial agents (e.g.β-lactams) showed moderate or weak in vitro activity against Borrelia, but are mainly effective in vivo.Nevertheless, the in vitro activity of many antimicrobial agents against Borrelia has not always correlated with clinical experience (Hassler et al., 1990;Luft et al., 1989).
A possible interaction between the ingredients of media, such as bovine albumin and antimicrobial agents, as well as the poor chemical stability of some antibiotics during the prolonged incubation period necessary for Borrelia susceptibility testing, may lead to the reduction or loss of their activity (Boerner et al., 1995;Dever et al., 1992;Reisinger et al., 1995).On the other hand, inadequately prepared and stored antibiotic solutions may also be one of reasons for the weak in vitro activity of some antibiotics against Borrelia strains.
Before performing an antibiogram (in vitro susceptibility testing of antimicrobial agents against bacteria), it is necessary to prepare and store the antibiotic solution appropriately.Jorgensen and Turnidge (2003) indicated that tubes with antimicrobial agents should be tightly capped and stored at 4 to 8°C until needed in order to minimize their evaporation and deterioration.The dilution of most antimicrobial agents should be used within 5 days of preparation and certain β-lactam antibiotics are too labile for prolonged storage in final concentration.Dever et al. (1992) reported that penicillin concentration reduced during incubation period at 34°C in BSK-II medium.The levels of penicillin G were reduced to 17% and less than 2% of the initial concentration after 72 h and 7 days, respectively.Thus, concentration of penicillin was undetectable after 7 days of incubation, while the concentration of ceftriaxone was also diminished, but was still detectable (47% of the initial concentration remained) after the same period.Similary, Kersten et al. (1995) reported decreased penicillin G concentrations of about 20% during 24 h of incubation; at 48 h more than 60% of the initial concentration was detectible at all the tested concentrations, while the remaining concentrations after 72 h were 50% and 20% of the initial concentration.After 48 h of incubation, doxycycline concentrations were more than 80% of initial concentrations.At 72 h, drug concentrations of between 64 and 76% of the initial values could be detected, indicating that doxycycline was quite stable under culture conditions.
The interaction between BSK-II medium and penicillin G led to a decrease in efficacy of 85.8% of this antibiotic after 72 h of incubation at 34°C, while other penicillins (mezlocillin and piperacillin) also showed a marked decrease with time of incubation.On the other hand, the activity of doxycycline and erythromycin increased against Borrelia strains when tested in BSK-medium (Boerner et al., 1995).
It is important to note that the effectiveness of some antibiotics such as penicillin and ceftriaxone in vitro and in vivo is temperature-dependent (Reisinger et al., 1996).The in vitro susceptibility of Borrelia strains to penicillin and ceftriaxone increased up to 16-fold after raising the temperature from 36°C to 38°C.This means that an increase in body tem-perature may be beneficial during the antimicrobial treatment of Lyme borreliosis.
In order to provide constant and appropriate concentrations of antimicrobial agents for a prolonged incubation period, Stiernstedt et al. (1999) developed the dialysis culture method for the determination of MICs and MBCs of benzylpenicillin for Borrelia.In this method, borrelial suspensions were enclosed in sealed dialysis membrane bags and put into tubes with BSK medium with the appropriate two-fold serial dilution of the antibiotic, and control tubes with only BSK medium.The dialysis membrane bags were transferred every day for 6 days to new tubes with BSK medium and freshly added antibiotic, and the MIC was determined on day 7.However, a shortcoming of this method is that it is difficult to standardize.Some studies reported differences in the antibiotic susceptibilities of pathogenic Borrelia species; MIC and MBC values may vary from one Borrelia species to another (Hunfeld et al., 2000a;Hunfeld et al., 2000b;Morgenstern et al., 2009;Preac-Mursic et al., 1996a;Sicklinger et al., 2003).In previous studies, B. afzelii and B. burgdorferi sensu stricto species were less susceptible to some antimicrobial agents, whereas B. garinii species was more susceptible than other Borrelia species to many antibiotics.
In the study of Hunfeld et al. (2000a), the MICs of penicillin for B. afzelii isolates were ten times higher than those for B. burgdorferi sensu stricto, B. valaisiana, and B. bissettii isolates, and 100 times higher than for other Borrelia isolates.Similary, Hunfeld et al. (2000b) found that the MICs of amoxicillin were lower for B. garinii isolates than for B. afzelii, B. burgdorferi sensu stricto, and B. valaisiana, while the MICs of cefixime for B. garinii isolates proved to be lower than those for B. afzelii isolates.Sicklinger et al. (2003) also reported differences in antibiotic susceptibility between Borrelia species.B. burgdorferi sensu stricto showed higher susceptibility to amoxicillin than B. afzelii and B. garinii isolates; B. afzelii was more susceptible to ceftriaxone than the other two Borrelia species, while B. garinii proved to be the most susceptible to azithromycin.Morgenstern et al. (2009)  garinii strains had different reactions to the antibiotics; B. garinii strains appeared to be more susceptible to antibiotics.Furthermore, the authors found that differences in antibiotic susceptibility also exist within a single species.
In contrast to the above-mentioned studies that showed differences in antibiotic susceptibility among Borrelia species, the study by Baradaran-Dilmaghani and Stanek (1996) did not.Different antimicrobial agents (azithromycin, amoxicillin, ceftriaxone, cefotaxime, doxycycline, penicillin G sodium, roxithromycin, and trimethoprim-sulfamethoxazole) were tested against thirty B. afzelii, B. garinii and B. burgdorferi sensu stricto strains from various sources (blood, cerebrospinal fluid-CSF, heart, skin and tick).MIC and MBC were determined after 72 h and 96 h of incubation, respectively, and did not show significant differences between the tested strains.Interestingly, a difference was found only for doxycycline.The MICs for B. afzelii isolates were lower than for B. garinii isolates, unlike the study of Sicklinger et al. (2003) where only doxycycline did not show any differences in its effect on B. afzelii, B. garinii, and B. burgdorferi sensu stricto.
Some studies have reported that in relapsed patients with early Lyme borreliosis, Borrelia isolates cultured after the conclusion of roxithromycin (Hansen et al., 1992), ceftriaxone (Pfister et al., 1991) and azithromycin (Hunfeld at al., 2005) treatment remained susceptible to these agents in vitro.On the other hand, Terekhova et al. (2002) reported susceptibility testing of laboratory strains and clinical isolates of Borrelia and demonstrated the existence of resistance to erythromycin in them.The results of this study indicated an important heterogeneity in the susceptibility of B. burgdorferi strains to erythromycin and suggested that erythromycin resistance could develop in Borrelia strains isolated from Lyme borreliosis patients that have been pre-exposed to the antibiotic, based on the existence of resistant subpopulations in vitro.
In order to overcome methodological problems, Hunfeld et al. (2000a) introduced a colorimetric microdilution method for in vitro susceptibility testing of B. burgdorferi sensu lato against antimicrobial agents.For this susceptibility testing, the final inoculum concentration was 10 6 cells/mL.This is the standardized test based upon color changes (occurring in the presence of phenol red) and result from the accumulation of nonvolatile acid produced by actively metabolizing spirochetes after 72 h of incubation.Growth of Borrelia was detected by software-assisted kinetic measurement of the decrease of absorbance.In this method, the growth of samples and controls was determined for each well, based on the decrease in absorbance (A 562/630 ) after 72 h (E t72 ) in comparison to the initial absorbance values (E t0 ).In mathematical terms, if the absorbance values at 72 h decreased 5% or more compared with the initial absorbance values, the well was considered positive for borrelial growth (E t72 < E 0 minus 5%).The MIC was determined as the lowest concentration of antimicrobial agent with which no such significant color shift (decline of curve) could be detected.Colorimetric in vitro susceptibility testing has also been used in some other studies (Hunfeld et al., 2000b;Hunfeld et al. 2005;Kraiczy et al., 2001;Morgenstern et al., 2009).This MIC method is reliable, reproducible and convenient and can handle large numbers of isolates and antibiotics (Hunfeld et al., 2000a), but the problem related to its wider use is its unavailability to other laboratories.Despite the fact that there are differences in the experimental conditions and test methods applied during in vitro susceptibility testing of antimicrobial agents against Borrelia, this bacterium highly susceptible to antibiotic treatment and the majority of patients with early Lyme borreliosis profit from the recommended antibiotic treatment (Girschick et al., 2009;Strle, 1999b;Wormser et al., 2000b).Treatment of early Lyme borreliosis, such as skin lesion erythema migrans, with β-lactam agents (such as amoxicillin and cefuroxime) and tetracycline agents was described as very successful in >90% of the cases (Smith et al., 2002;Thanassi and Schoen, 2000).About 5 to 10% of patients with erythema migrans fail to respond to antibiotic therapy.This is more common with macrolide agents than with β-lactam and tetracycline agents (Hansen et al., 1992;Hunfeld et al., 2002;Luft et al., 1996;Smith et al., 2002;Wormser et al., 2000b).Some patients develop chronic persistent disease, and borreliae can be isolated in spite of previous treatment with antibiotics (Maraspin et al., 1995).Clinical and experimental data showed that after so-called adequate treatment in patients with Lyme borreliosis, Borrelia could persist in tissue, and failures of treatment have been reported for almost every suitable antimicrobial agent (Hassler et al., 1990;Preac-Mursic et al., 1989;Preac-Mursic et al., 1996a;Straubinger et al., 1997;Strle et al., 1993Strle et al., 1996b;Wormser et al., 2003).In some cases, the failure of treatment could be due to irreversible tissue damage during active borrelial infection or inflammation in association with the infection, the induction of autoimmune mechanisms, and possible misdiagnosis (Strle, 1999b).In any case, treatment with antibiotics is reasonable at all stages of Lyme borreliosis infection and for all clinical manifestations; recommendations for the treatment have been reported previously (Girschick et al., 2009;Steere, 2001;Strle, 1999b).

Table 2 .
The main differences between MKP, BSK-II and BSK-H media.

Gelatin Yeast extract Glucose (%) Rabbit serum BSK-II present
Three pathogenic Borrelia species (B.afzelii, B. garinii, and B. burgdorferi sensu stricto) grow well in both MKP and BSK-II media as was described previously (Ružić-Sabljić and Strle, 2004).On the other hand, Ružić-Sabljić et al. (2006) indicated a similar suitability of MKP and BSK-II media for routine laboratory work.The authors evaluated the isolation rate of B. afzelii, B. garinii, and B. burgdorferi sensu stricto from MKP and BSK-II media and showed comparable Borrelia isolation rates in both media.Sabljić and Strle (2004) compared the growth of B. afzelii, B. garinii, and B. burgdorferi sensu stricto in MKP and BSK-II media, and established that in a mixture of two species, B. burgdorferi sensu stricto behaves as the most aggressive species, followed by B. garinii and at lastly B. afzelii.B. burgdorferi sensu stricto overgrew B. afzelii and (Ružić-Sabljić and Strle 2004;Strle et al., 1999a, Strle et al., 2011) aggressive Borrelia species than B. afzelii i B. garinii(Ružić-Sabljić and Strle 2004;Strle et al., 1999a, Strle et al., 2011).Ružić- reported that macrolides (erythromycin and clarithromycin) and doxycycline showed the highest in vitro activity against B. spielmanii in contrast to B. afzelii, B. garinii and B. burgdorferi sensu stricto species, while B. spielmanii was less susceptible to amoxicillin than the other genospecies.Preac-Mursic et al. (1996a) investigated the killing effect in MKP medium and human serum during a 72 h exposure to amoxicillin, doxycycline, cefotaxime, ceftriaxone, azithromycin and penicillin G used in the treatment of Lyme borreliosis.B. afzelii and B.