Multilocus Sequence Typing Implicates Rodents as the Main Reservoir Host of Human-Pathogenic Borrelia garinii in Japan^,

Abstract

Multilocus sequence typing of Borrelia garinii isolates from humans and comparison with rodent and tick isolates were performed. Fifty-nine isolates were divided into two phylogenetic groups, and an association was detected between clinical and rodent isolates, suggesting that, in Japan, human-pathogenic B. garinii comes from rodents via ticks.

TEXT

Lyme disease is a multisystemic disorder caused by infection with the tick-borne spirochetes Borrelia burgdorferi sensu lato (s.l.). B. burgdorferi sensu stricto (s.s.), Borrelia garinii, and Borrelia afzelii, which are the known pathogenic borreliae of humans. B. burgdorferi s.s. is geographically distributed throughout North America and Europe, whereas B. garinii and B. afzelii are distributed throughout Europe and Asia. These Borrelia species are transmitted by Ixodes ricinus in Europe and Ixodes persulcatus in Asia and Russia. (2, 16, 24). In Europe, serological characterization has revealed that B. garinii is composed of several OspA serotypes (27). At present, it is understood that one B. garinii serotype (B. garinii OspA serotype 4) is maintained by rodents (7), although other serotypes of B. garinii are maintained by birds (4, 5, 26). Strains classified into B. garinii OspA serotype 4 were found to be distinguishable from other B. garinii strains by multilocus sequence typing (MLST), which was recently established for Lyme disease borreliae (14, 15). In Japan, B. garinii is known to be the main pathogenic borrelia, and it is transmitted by I. persulcatus (29). However, the natural reservoir host of human pathogenic B. garinii remains unclear since I. persulcatus infests both rodents and birds in Japan (28). To resolve this question, MLST analysis was performed on clinical, tick, and rodent isolates, and phylogenetic relationships among these strains were investigated.

Nineteen B. garinii strains were obtained for MLST analysis from Lyme disease patients with erythema migrans in Japan. Human isolates were cultured from erythema migrans lesions as previously described (22). As for tick and rodent isolates, 40 strains were examined. The sources of these strains are listed in Table 1. Eighteen strains were isolated from I. persulcatus ticks, which were collected from Japan (15 strains) and Russia (3 strains). Twenty-two strains were isolated from rodents. Of these, 10 were from Myodes rufocanus subsp. bedfordiae and 8 from Apodemus speciosus (both sets collected in Japan), and 4 were from A. uralensis (collected in China) (Table 1). The cultivation of borreliae was carried out at 34°C in modified Barbour-Stoenner-Kelly (BSK) medium (using minimal essential medium alpha [BioWest, Germany] as a substitute for CMRL-1066) (1). These strains were stored at −80°C until use. Cultivated bacterial cells (late-log phase) were used in DNA preparation. The genomic DNA of isolated strains was prepared by using a DNA extraction kit (DNeasy blood and tissue kit; Qiagen, Germany) according to the manufacturer's instructions. The PCR assay was performed according to Margos et al. (14, 15). After DNA amplification of eight loci (clpA, ATP-dependent Clp protease subunit A gene; clpX, ATP-dependent Clp protease subunit X gene; nifS, aminotransferase gene; pepX, dipeptidyl aminopeptidase gene; pyrG, CTP synthase; recG, DNA recombinase gene; rplB, 50S ribosomal protein L2 gene; and uvrA, excinuclease ABC subunit A gene), PCR products were purified by using ExoSAP-IT (GE Healthcare UK, Ltd., United Kingdom) and were directly sequenced (ABI Prism 3130xl Genetic Analyzer; Life Technologies Corporation). All sequences were deposited in GenBank (see the table in the supplemental material). In addition, reference sequences of each sequence type (ST) were downloaded from the MLST website (www.mlst.net). After concatenation of the sequences, Bayesian phylogenetic inference was performed (15). The phylogenetic tree was created according to Margos et al. (15), using TreeView software (ver. 1.6.6).

Table 1.

Borrelia garinii strains used in this study

B. garinii strain (no. of isolates)	Isolation source	Location	Reference
Tick isolates (18)
HP1, HP3, HT18, HT59, N346, HkIP1, HkIP2	Ixodes persulcatus	Hokkaido, Japan	3, 10, this study
NP4, NP8, NP76, NP81, NT24, NT25, NT31	I. persulcatus	Nagano, Japan	10, 17
FujiP2	I. persulcatus	Shizuoka, Japan	11
Ip90	I. persulcatus	Khabarovsk, Russia	23
Mp7, Np189	I. persulcatus	Moscow, Russia	18
Rodent isolates (22)
Ear isolates (17)
HkCR1, HkCR3, HkCR4, HkCR5, HkCR6, HkCR7, HkCR9, HkCR11, HkCR12,	Myodes rufocanus subsp. bedfordiae	Hokkaido, Japan	9
FsAE1, FsAE2	Apodemus speciosus	Fukushima, Japan	8
FiEE11	A. speciosus	Fukui, Japan	8
sai8E	A. speciosus	Aomori, Japan	This study
ChYAE2	A. uralensis	Yakeshi, China	13
CTA1b, CTA4a, CTA5b	A. uralensis	Urumqi, China	25
Spleen isolate (1)
ASF	A. speciosus	Hokkaido, Japan	19
Bladder isolates (4)
HokkaidoCRB35B	M. rufocanus subsp. bedfordiae	Hokkaido, Japan	This study
HokkaidoAS7B	A. speciosus	Hokkaido, Japan	This study
sai6B, sai7B	A. speciosus	Aomori, Japan	This study
Human skin isolates (19)
Hiratsuka	Erythema migrans	Niigata, Japan	This study
J-14, J-15, J-16, J-17, J-18, J-20T, J-21, J-32, J-33, J-34, J-35, J-37, J-38, J-39, J-40, J-41, J-42	Erythema migrans	Hokkaido, Japan	Miyamoto et al., unpublished data
HH1	Erythema migrans	Hokkaido, Japan	Sato et al., unpublished data

Isolated B. garinii strains were classified into two phylogenetic groups (preliminarily designated B. garinii ST group A and ST group B) by analysis of the concatenated DNA sequences of 8 loci (Fig. 1 and Table 2). The results indicated that B. garinii ST group B contained most of the Japanese clinical isolates (16/19 [84.2%]), all of the rodent isolates from Japan (18/18 [100%]) and China (4/4 [100%]), and 5 isolates from I. persulcatus collected in Japan (4/15 [26.7%]) and Russia (1/3 [33.3%]). Chi-square analysis indicated a confidence level of over 99% that B. garinii ST group B is predominant among Japanese clinical isolates and rodent isolates.

Fig. 1.

Bayesian phylogenetic inference of concatenated housekeeping gene sequences of B. garinii. The phylogenetic tree was constructed based on Bayesian phlylogenetic inference. The posterior probability values of the clades are provided. Bars labeled 0.05 depict 5% divergence. B. garinii human isolates are indicated in red, rodent isolates in green, and isolates from I. persulcatus in blue. The relapsing fever Borrelia spp. (B. duttonii Ly [NC_011229], B. hermsii DAH [NC_010673], B. recurrentis A1 [NC_011244], and B. turicatae 91E135 [NC_008710]) were used as outgroups (data not indicated). The accession numbers of alleles from Borrelia bissettii DN127, Borrelia andersonii 21123, Borrelia sinica CMN3, and Borrelia spielmanii PsigII 27/6 are listed in the table in the supplemental material.

Table 2.

ST groups of Borrelia garinii isolates from Ixodes persulcatus ticks, rodents, and humans

Isolation source	Country	B. garinii ST
		Group A	Group B
I. persulcatus	Japan	HkIP2, HP1, HP3, HT18, HT59, NP4, NP8, NP76, NP81, NT25, NT31	HkIP1, N346, NT24, FujiP2
	Russia	Ip90, Np189	Mp7
Rodents	Japan	None	HkCR1, HkCR3, HkCR4, HkCR5, HkCR6, HkCR7, HkCR9, HkCR11, HkCR12, HokkaidoCRB35B, HokkaidoAS7B, ASF, FsAE1, FsAE2, FiEE11, sai6B, sai7B, sai8E
	China	None	ChYAE2, CTA1b, CTA4a, CTA5b
Humans	Japan	J-21, J-37, J-38	Hiratsuka, J-14, J-15, J-16, J-17, J-18, J-20T, J-32, J-33, J-34, J-35, J-39, J-40, J-41, J-42, HH1

In this study, all B. garinii isolates from rodents were included in ST group B. In Europe, a recent report designated STs 84 and 85 as “Candidatus Borrelia bavariensis,” and the reservoir host was thought to be rodents (14). Since B. garinii STs 84 and 85 clustered with B. garinii ST group B, we hypothesized that rodents are the main reservoir host of this phylogenetic group. In this study, the STs of 9 clinical isolates (2 of ST128, 4 of ST131, and strains J-14, J-18, and J-41) were found among rodent isolates. Thus, it can be inferred that human-pathogenic B. garinii is maintained by rodents in Japan. B. garinii ST group B was also found among rodent isolates from China. In addition, STs 128 and 131 of B. garinii, which were originally recorded in the MLST database as Chinese isolates, are pathogenic to humans in Japan. These suggest that B. garinii ST group B may also represent a health threat of Lyme disease in China. In contrast, B. garinii ST group A was not isolated from rodents in this study, yet was found to include almost all of the B. garinii isolates in Europe. Given that several reports claim B. garinii is detectable from birds in Asia (10, 20, 21), we suspect that B. garinii ST group A is maintained by birds, as are most of the B. garinii isolates in Europe.

I. persulcatus and I. ricinus ticks are known vectors of pathogenic B. garinii in Asia and Russia and Europe, respectively. In this study, B. garinii ST group A, which is most often isolated in Europe, was infrequent in Japan (Fig. 1). The reason remains unclear, but the inhabitant species of ticks may be associated with this geographical difference. Furthermore, Korenberg et al. recently found that I. pavlovskyi and I. persulcatus ticks differ in their abilities to transmit borrelia (12). This finding may support the notion that the resident tick species contributes to the determination of the endemic species B. garinii.

In this study, it was observed that the B. garinii strains which infect humans in Japan, are often found in rodents, but not nearly as often in ticks. In Europe, it was reported that B. garinii STs 84 and 85 are pathogenic to humans, although they were infrequent among tick isolations (6). These STs are not found in Japan, but they are clustered with most of the Japanese clinical isolates. Therefore, we hypothesized from our data that B. garinii ST group B isolates may be more pathogenic to humans than isolates of B. garinii ST group A.

In conclusion, B. garinii could be divided into two phylogenetic groups by MLST analysis, and one group (B. garinii ST group B) was predominant among clinical and rodent isolates in Japan. These results suggest that rodents are the reservoir host for most human-pathogenic B. garinii isolates in Japan. We also revealed that Japanese clinical isolates may be distinct from most European isolates. This may be due to the different vectors of B. garinii in Asia and Russia versus Europe. Our findings may contribute to the elucidation of B. garinii-caused Lyme disease epidemiology.