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Journal of Clinical Microbiology, April 2005, p. 1879-1884, Vol. 43, No. 4
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.4.1879-1884.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Jennifer M. Anderson,1
Jinho Park,2,
Paul G. Auwaerter,3
John N. Aucott,3
Douglas E. Norris,1 and
J. Stephen Dumler1,2*
W. Harry Feinstone Department of Molecular Microbiology and Immunology, The Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland 21205,1 Division of Medical Microbiology, Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205,2 Divisions of General Internal Medicine and Infectious Diseases, The Johns Hopkins University School of Medicine, Lutherville, Maryland 210933
Received 3 September 2004/ Returned for modification 1 November 2004/ Accepted 2 December 2004
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B. burgdorferi plasmid-encoded outer surface protein C genes (ospC) are implicated in transmission and dissemination of Borrelia between ticks and mammals (12, 18, 20, 24, 32). Diversity of ospC has lead to the classification of B. burgdorferi clones into human invasive and "noninvasive" groups (14, 16, 25, 28), but these studies have not evaluated ospC as an invasive factor in small mammals. Because of ospC heterogeneity, it has been used as marker for clonality in epidemiological studies that suggest that human and small mammal isolates from the same region are restricted to only a single or a few clones (14, 16). Neither geographic nor ospC heterogeneity of B. burgdorferi infecting humans and mice has been extensively studied in the mid-Atlantic States. The present study was designed to test two hypotheses: (i) that only limited ospC diversity exists among B. burgdorferi infecting humans and mice from the same localities in northern Maryland, and (ii) that ospC genotypes associated with invasiveness in humans extends to Maryland and to small mammals.
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Growth of human Borrelia isolates. Low-passage (<2 passages) bacteria were propagated from dimethyl sulfoxide-preserved frozen patient isolates. Approximately 0.5 ml of each frozen isolate was transferred to 5 ml of modified BSK-II medium (31) and incubated at 34°C in 5% CO2 until sufficient growth was confirmed by fluorescent microscopy using acridine-orange staining. Two control cultures were used: B. burgdorferi strain 297 (courtesy of Dennis Grab, the Johns Hopkins University School of Medicine) and a randomly selected human isolate previously prepared in the lab and used for internal control purposes (lab strain).
Mouse B. burgdorferi isolates. Mice were trapped using 60 Sherman live traps (H. B. Sherman, Tallahassee, FL) baited with bird seed and placed within 300 yards of each patient's home. After administration of a light anesthetic, a 2-mm-diameter ear tissue sample was taken using a biopsy tool (Accuderm Inc., Ft. Lauderdale, FL) and placed in BSK-II medium for transport. A blood sample was taken from each capture by retro-orbital sampling and placed directly on ice for transport. Captured rodents were released at the site of capture. Trapping and sample collection were performed according to Hofmeister et al. (9) as approved by the John Hopkins University Institutional Animal Care and Use Committee.
Mouse blood and skin cultures were incubated at 34°C in 5% CO2 and examined weekly for up to 8 weeks. Acridine orange-positive samples were subcultured in BSK-II medium supplemented with 50 µg/ml of ciprofloxacin and 20 µg/ml of rifampin to reduce contamination with other bacterial species. When appropriate, 0.45-µm-pore-diameter syringe-driven filter units were used (11) to remove contaminating bacteria. After 2 to 3 weeks of log phase growth, aliquots of each culture were cryopreserved at 80°C. Despite the use of antimicrobials and filtering, contamination persisted for many cultures, resulting in poor Borrelia recovery in 11 field samples later excluded from further study. Despite all attempts, many isolates grew poorly in the medium preparations used.
DNA extraction. During log phase growth, 1.5 ml of each Borrelia culture was centrifuged at 13,800 x g for 25 min. The bacterial pellet was resuspended in 200 µl of phosphate-buffered saline, and DNA was prepared using a QIAamp DNA Mini kit (QIAGEN, Inc.) and the manufacturer's protocols. DNA preparations were stored at 20°C until analyzed.
PCR methods and conditions.
The 3' and 5' ends of ospC were PCR amplified using conserved primers as described previously (25). PCR was performed using an eLONGase kit (Invitrogen Inc.) per manufacturer's instructions with the addition of 100 pmol of each primer and using 2 µl of B. burgdorferi isolate DNA that was not otherwise quantitated. The reaction was completed as follows: initial denaturation at 96°C for 2 min, followed by 40 cycles of denaturation at 95°C for 40 s, annealing at 54°C for 35 s, and extension at 72°C for 1 min. The 5' ospC amplification did not produce a reliable product from all samples; therefore, all subsequent analyses were based on amplification of a
314-bp fragment from the 3' end. A negative control (deionized water) was included in each PCR. PCR amplification of a B. burgdorferi-specific fragment from the flaB gene was used to confirm that small mammal isolates from which ospC was amplified were B. burgdorferi (13).
Sequencing and phylogenetic analysis. B. burgdorferi ospC PCR products (25 µl) were electrophoresed in 2% agarose gels, stained with ethidium bromide, and excised for purification using a QIAquick gel extraction kit (QIAGEN Inc.). Sequencing was performed using the forward and reverse PCR primers (25) and a fluorescent automated method (ABI 3100 genetic analyzer). The resulting forward and reverse ospC sequences were initially assembled using the Contig Assembly Program in BioEdit (8) to generate consensus sequences. Discrepancies were resolved by inspection of the original chromatograms or by additional sequence analysis.
To validate the use of the 3' ospC sequences for phylogenetic comparisons, 21 B. burgdorferi sequences corresponding to the major ospC groups (25, 30) were retrieved from GenBank (Table 1) and aligned using ClustalX (10). Primer sequences flanking the region of the alignment corresponding to the 3' amplicon were removed, and the resulting sequences were gap stripped and realigned for phylogenetic analysis. Full-length and 3' ospC trees were generated using the neighbor-joining algorithm, and topologies of the trees were compared. The consensus sequence assemblies of the study isolates were aligned with the reference 3' ospC sequences. These combined sequences were then trimmed for optimal pairwise alignments and used to generate a neighbor-joining tree with bootstrapping 1,000 times. The tree was viewed with TreeView software (19).
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TABLE 1. Accession numbers of the 21 major ospC group reference strains (from GenBank), used for alignment purposes, and their geographical origin/source
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TABLE 2. Total number of animals and patient samples tested and number of acridine orange-positive and number of ospC PCR-positive samples
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650-bp sequence of ospC was initiated, as previously described (25), but amplification was inconsistent. Therefore, the 3' and 5' ends of ospC were separately amplified. Although the 5' ospC amplifications often failed, the 3' reactions were more often successful and were used as the basis for phylogenetic analysis. To validate this approach, full-length sequences of 21 previously published ospC reference sequences and sequences corresponding to the predicted 3' amplicons were separately aligned and resulted in similar tree topologies, for which each reference group identified a unique clade. B. burgdorferi ospC PCR and sequencing. All six B. burgdorferi DNA templates from human plasma cultures and one of the two human skin cultures produced an amplicon of approximately 314 bp with the 3' ospC primers. Only 18 of the 71 Borrelia cultures derived from small animals amplified with the ospC primers. From the 18 PCR positives, 2 were from blood and 16 were from ear biopsy cultures. Both controls (strain 297 and lab strain) amplified in all PCRs; negative controls (deionized water) did not produce detectable products. Unambiguous forward and reverse sequences were obtained from all except for three amplicons (one blood and two ear biopsies).
B. burgdorferi ospC heterogeneity among human and mouse isolates from the six properties. A total of 45 sequences, including 21 references from GenBank, 2 laboratory controls, and 22 obtained from human and small mammal cultures, were aligned to generate a phylogenetic tree. Sequences derived from human isolates were found in four distinct clusters, while sequences originating from animal reservoirs were found in seven clusters (Fig. 1). Table 3 summarizes the findings and shows the 7 human and 15 small mammal isolates and the sites from which they were collected. Only at site 2 were B. burg-dorferi isolates with the same ospC genotype observed in both human (isolate PL 2) and mouse (isolate EP2-12) isolates.
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FIG. 1. Phylogenetic tree derived from the nucleotide sequences of ospC genes of humans (prefix PL or SK) and mouse isolates and the reference strains (italics) of Borrelia burgdorferi, using Borrelia turicatae (GenBank AF129434) ospC-like gene as outgroup. Lab strain and strain 297 were used as controls for amplification. The bar represents the number of nucleotide substitutions per 1,000 bp. Bootstrap values based on 1,000 replicates are shown for each node.
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TABLE 3. Site of the patients' specimens and small animal samples that tested ospC PCR positive
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TABLE 4. Human and animal strains of Baltimore County, MD, and York County, PA, and the ospC groups with which each clustered
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PCR amplification of full-length ospC was unreliable for samples derived in this study; therefore, a modified method derived by comparing the sequences from
240 bp of the 3' end of ospC was used to yield a phylogenetic tree that allowed distinct identification of each reference group as demonstrated for the tree of the full-length gene (14). When applied to B. burgdorferi isolates from humans and mice collected within a limited geographic region in the Northern Maryland and Southern Pennsylvania, these data demonstrate that there is a high degree of ospC diversity. Matching human and animal isolates were only recovered from a single location/site, but this may be an artifact of small sample size. With the high degree of ospC polymorphism observed in this study, larger numbers of mammals may need to be captured to determine whether all or most mouse-infective clones are also infective for humans. Altogether, five of the six sites contained two or more B. burgdorferi ospC clones, including four at one site and three at another. Only a single clone was identified in site 5 (Pennsylvania). Unfortunately, ospC typing of animal isolates from that location was unsuccessful and would have likely revealed additional polymorphism.
The overall analysis is hampered by several potential biases. Although 60% of the 118 animal samples were culture positive, 18 were ospC PCR positive and only 15 were successfully sequenced despite repeated attempts. The inability to amplify ospC from field (mammal) isolates may be due to additional sequence variability at the priming sites resulting in failed PCR. Alternatively, the inability to amplify ospC and fla from small mammal blood cultures in which Borrelia were visualized could be the result of significant blood contamination and inhibition of PCR in spite of DNA purification methods. Another potential pitfall relates to the selection of B. burgdorferi clones in culture, possibly leading to a selection bias (17). For illustration, among the two human skin B. burgdorferi isolates examined, one failed to amplify with ospC primers, yet amplified with primers that target ospA, suggesting that even greater ospC heterogeneity exists among B. burgdorferi pathogenic for humans than is presently evaluated. Likewise, the inability to obtain unambiguous direct sequences from 17% of amplicons suggested that some cultures may be mixed. This proportion is similar to that described by Seinost et al. (26) and further illustrates the degree of potential heterogeneity in these regions.
Seinost et al. suggested that ospC typing could differentiate non-pathogenic from B. burgdorferi that are pathogenic for humans and may further differentiate strains that cause localized skin infection from those that cause systemic/invasive disease (25). However, four of the six human plasma isolates in the present study did not cluster with the major ospC groups previously implicated in invasion (25). In contrast, 10 of the 15 animal isolates clustered with the major ospC groups designated as invasive, potentially reflecting the innate ability of these groups to disseminate, even in the animal host. Unlike Seinost's observations, the major ospC groups A and K did not predominate in humans from this region (25, 29) but were found in 6 of 15 small mammal isolates (Table 4). Qiu et al. found group K to be one of the most common clones on the U.S. East Coast, including 14 of 22 B. burgdorferi identified in I. scapularis from Maryland (23), whereas it only accounted for 18% of our ospC-positive samples.
B. burgdorferi ospC groups G, H, J, N, and T are usually associated only with erythema migrans and not invasion in humans (25). However, in this study, their invasiveness in both animals and humans was demonstrated by identification in disseminated sites. Although not well investigated, animal invasiveness in groups G and T suggests the potential for invasion in humans.
The ospC groups D, E, F, and L, previously found only in ticks (25) were not identified in this study, confirming how infrequently they are observed in humans or reservoirs. Of more interest is the absence of these ospC groups in mice as well, despite their presence in ticks collected from this region (23). As B. burgdorferi exists in an obligate relationship that requires transmission between the tick and mammalian host (5), it is expected that these isolates should be identified in small animals. Alternatively, one may hypothesize that other mammalian hosts beside P. leucopus may harbor the ospC groups that have been characterized as predominantly tick associated. Another alternative could be that such ospC groups depend on transovarial transmission, a mechanism that is usually considered highly inefficient for B. burgdorferi (21). An expanded investigation may shed light on the extent of the host distribution of major ospC groups.
These Maryland and southern Pennsylvania studies confirm a high degree of B. burgdorferi genetic diversity and a lack of concordance between strains identified in animals and humans from the same locations. However, since 10 of the 15 animal isolates clustered with ospC types previously characterized as "invasive," it is reasonable to conclude that mouse invasiveness may be equivalent to human infectivity. Moreover, the identification of invasion in both mice and humans by noninvasive ospC types suggest that the grouping proposal of Seinost et al. (25) needs a more rigorous evaluation. Inclusion of additional isolates representing both a variety of geographic regions and infected human as well as reservoir hosts may help to determine whether all or some subpopulations of B. burgdorferi strains can cause disseminated human infection.
In accordance with other investigators using animal models (29), it is possible that ospC diversity is less important as a predictor of mammalian and human invasion than proposed previously (3, 25). Thus, given the extensive diversity of B. burgdorferi ospC types that may infect humans, alternative genetic markers should be investigated that may better correlate with and explain B. burgdorferi disseminated disease.
This work was submitted in partial fulfillment of the Masters degree of M.Y.A. The isolation of B. burgdorferi from human patients was conducted with approval of The Johns Hopkins University School of Medicine (JHM IRB-2 RPN 01-01-12-05).
We thank Clara Lema, Amy Linhardt, and Peggy Coulter for their assistance with medium preparation and isolation and growth of B. burgdorferi from human patients.
Present address: Box 73265, Abu Dhabi, United Arab Emirates. ![]()
Present address: Department of Veterinary Internal Medicine, College of Veterinary Medicine, Chonbuk National University, Jeonju, Jeonbuk 561-756, Korea. ![]()
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