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Journal of Bacteriology, August 2004, p. 5138-5146, Vol. 186, No. 15
0021-9193/04/$08.00+0 DOI: 10.1128/JB.186.15.5138-5146.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Yanping Han,1,
Yajun Song,1 Zongzhong Tong,2 Jin Wang,1 Zhaobiao Guo,1 Decui Pei,1 Xin Pang,1 Junhui Zhai,1 Min Li,3 Baizhong Cui,3 Zhizhen Qi,3 Lixia Jin,3 Ruixia Dai,3 Zongmin Du,1 Jingyue Bao,2 Xiuqing Zhang,2 Jun Yu,2 Jian Wang,2 Peitang Huang,1 and Ruifu Yang1*
Laboratory of Analytical Microbiology, National Centre for Biomedical Analysis, Army Center for Microbial Detection and Research, Institute of Microbiology and Epidemiology, Academy of Military Medical Sciences, Beijing 100071,1 Beijing Genomics Institute, Chinese Academy of Sciences, Beijing 100101,2 Qinghai Institute for Endemic Diseases Prevention and Control, Xining 811602, China3
Received 8 February 2004/ Accepted 31 May 2004
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Y. pestis can be divided into three biovars, i.e., antiqua, mediaevalis, and orientalis, according to their ability to reduce nitrate and utilize glycerol (1). These three biovars are thought to be responsible for the three major plague pandemics: the Justinian plague, the Black Death, and the modern plague, respectively (1). The third plague pandemic was believed to have originated from Yunnan Province, China, in 1855. It then spread around the world with the aid of modern transportation (25). Human plague has been successfully controlled since the 1950s in China. However, 11 natural plague foci still remain in China, covering more than 277 counties in 19 provinces with an area of more than 1 million km2 (10, 13).
We assumed that the host niche, compound interactions between the environment, the reservoirs, the vectors, and the pathogen, would determine the traits (host range, virulence, biochemical features, genetics contents, etc.) of Y. pestis in a specific geographic region and that strains of Y. pestis from different origins should slightly differ in genome content. However, the challenge was how to track down the genetic differences, how to use these differences as markers for genomotyping, and how to illustrate genomotypic and phenotypic microevolution of Y. pestis. The recently decoded whole-genome sequences of Y. pestis CO92 (17), KIM (5), and 91001 (Y. Song and R. Yang, unpublished data) (accession numbers AE017042, AE017043, AE017044, AE017045, and AE017046) provide the unprecedented opportunity to overcome this challenge. Here, we report our results on the considerable genome dynamics in natural populations of Y. pestis due to gene acquisition and loss, as determined by using DNA microarray-based comparative genomic analysis in conjunction with PCR-based screening. Further, we outlined the parallel microevolution of the Y. pestis genome and propose its intimate link with niche adaptation of Y. pestis in natural foci.
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TABLE 1. Natural isolates of Y. pestis used in microarray analysis
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TABLE 2. Y. pseudotuberculosis strains used as controls in this study
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TABLE 3. Genes unique to Y. pestis strain 91001 represented on the microarray
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PCR analysis. All of the difference region (DFR) genes (see below) were used in PCR amplification to validate the deletion events, identified by the microarray methods, in the 36 Y. pestis isolates listed in Table 1. Then, one or more genes were chosen from each DFR to stand for the corresponding DFR, and PCR amplification of the selected genes was performed on the 260 isolates of Y. pestis to screen the distribution of DFRs in these strains. All of the DNAs to be tested were arrayed in 96-well PCR plates. Each primer pair was pretested, with the genomic DNA of strain 91001 or CO92 as a template, to ensure successful amplification. PCR products were analyzed by 1.2% agarose gel electrophoresis with ethidium bromide staining.
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TABLE 4. DFRs in the genome of Y. pestis
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TABLE 5. Genomovars of Yersinia pestis based on DFR profiling
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FIG. 1. Deduced pattern of microevolution of Y. pestis based on DFR profiling. A phylogenetic tree of the 14 genomovars of Y. pestis (Table 5) was constructed by using the PHYLIP Mix algorithm with the genomovar of the Ype ancestor (Table 5) as an outgroup (data not shown). The tree was then used as a backbone to develop the evolutionary relationships among the genomovars under the assumption that it is most likely that as few DFR alleles as possible are changed at a time. The figure shows the loss or acquisition of DFRs and islands and the conversion of genomovars and biovars. In addition, strains from Microtus foci, belonging to biovar mediaevalis according to the traditional biovar assignment, were proposed as a new biovar, microtus, because of their unique pathogenic, biochemical, and molecular features. The figure also shows some point mutations leading to the inactivation of certain genes, which most likely accounts for the metabolic variations between the four Y. pestis biovars; these results will be interpreted in detail in another study (Zhou and Yang, unpublished data).
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There are 21 genomic islands (including the virulence-related pgm locus), probably acquired from other organisms through horizontal gene transfer, on the chromosome of strain CO92 (17). Our experimental data indicate that 18 of them are harbored in both Y. pseudotuberculosis and Y. pestis, while the other 3 (island 09, island 14, and island 15) are present only in Y. pestis (Table 6). Apparently, these three islands were acquired by Y. pestis during the course of speciation (Fig. 1). All three of these islands encode prophages. Some bacteriophages encode bacterial proteins that enable the bacteria to invade host tissues, avoid the host immune defense, and damage host cells (3). With the integration of the bacteriophage genome into the bacterial chromosome, the virulence factors encoded by the prophage can convert their bacterial host from a nonpathogenic strain to a virulent one or to a strain with increased virulence (3).
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TABLE 6. Distribution of genomic islands of Y. pestis in Y. pseudotuberculosis
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There was an accumulation of pseudogenes in Y. pestis during the course of speciation (Fig. 1), which is the outcome of the switch of Y. pestis from an enteric lifestyle to a mammalian blood-borne lifestyle (17). For example, yadA and inv are both inactive in Y. pestis but encode functional adhesin and invasion in Y. pseudotuberculosis, enabling the enteropathogen to specifically adhere to surfaces of the host intestines and invade the lining epithelial cells (21-24). The lipopolysaccharide (LPS) of Y. pseudotuberculosis has been shown to possess an O antigen, which is an essential virulence determinant (19), while Y. pestis expresses rough LPS lacking the O antigen, due to the inactivation of several genes in the O-antigen gene cluster (11). Smooth LPS production may be unnecessary for Y. pestis virulence, and the metabolic burden has been alleviated by the inactivation of the O-antigen biosynthetic operon (16).
Parallel microevolution of the Y. pestis genome. The third human plague pandemic, beginning in the mid-19th century in China's Yunnan Province and then spreading globally, eventually affected more than 60 countries and regions in Asia, Europe, the Americas, and Africa (1). It is believed that the third pandemic was caused by the emergence of the orientalis strains (25). Indeed, all of the strains isolated from plague focus F in Yunnan Province and its neighboring regions are orientalis strains. All of the orientalis strains tested in this study fell into genomovar 09, which has evolved from genomovar 07 (biovar antiqua) by acquiring DFR 13, which is specific for the orientalis strains (Fig. 1). This strongly supports the notion that biovar orientalis directly evolved from antiqua rather than arising from mediaevalis (11). This fact also suggests that horizontal gene acquisition may play a key role in dramatic evolutionary segregation within species.
For reasons that are probably due to human population density and animal husbandry practices, Y. pestis strains causing the modern plague have been considered to have emerged from China. The data presented here confirm that the modern plague originated from Yunnan Province in China, due to the arising of strains of biovar orientalis from biovar antiqua rather than mediaevalis. Genomovar 09 of Y. pestis seems to be the oldest ancestor of biovar orientalis strains.
DFR 13 (island 15) encodes a filamentous prophage that is integrated into the chromosomal dif locus (7). This conserved prophage was also found in the high-virulence clone Escherichia coli O18:K1:H7, with high homology at nucleotide level (7). E. coli O18:K1:H7, the most virulent among all K1 strains in animal models of sepsis and meningitis, is responsible for almost all cases of neonatal meningitis in the United States and for the preponderance of uncomplicated cystitis cases in North American women. The acquisition of this prophage seems to be linked to the differential disease potential of E. coli O18:K1:H7 (7). Likewise, Y. pestis biovar orientalis, believed to be newly emerged and to be responsible for the third human plague pandemic, possesses major genomic differences compared with its antiqua progenitor.
In contrast to selective genome expansion by horizontal gene acquisition, genome reduction occurs through the loss of DFRs (Fig. 1). The lost genes are always nonessential to bacterial survival (unable to provide a selective benefit of bacterial growth efficiency or fitness in the host) in a specific host niche; that is, if genes are rendered useless due to redundancy within the host niche, then loss bias occurs (14). The parallel loss of DFRs in Y. pestis genomes leads to the discrete segregation between the progenitor and offspring strains, and this genome reduction gradually causes the offspring strains to inhabit a more specific host niche, not overlapping with its progenitor.
Transmission and expansion of Y. pestis in natural foci. In our study, each genomovar is confined in a specific geographic region, commonly a plague focus or a part of a focus with a unique set of natural environment, reservoirs, and vectors (Table 7). Most of the geographic regions with different primary reservoirs have unique genomovars. Sometimes there is more than one genomovar in a single focus within a single primary reservoir, but each of the genomovars corresponds to a unique set of natural environment and primary vector(s). The microevolution of the genomovars is consistent with the expansion of plague foci. Hence, we present a paradigm of the transmission, colonization, and expansion of Y. pestis in China (Fig. 2). Y. pestis strains from Central Asia and the Far East of Russia may have migrated into the Xinjiang-Tibet region and northeast China, respectively, and then separated into three branches with the expansion of plague foci.
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TABLE 7. Distribution of genomovars in natural plague foci
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FIG. 2. Deduced transmission and expansion of Y. pestis in China. Biovar antiqua strains of Y. pestis from Central Asia and the Far East of Russia might have migrated into the Xinjiang-Tibet region and northeast China, respectively, and then separated into three branches with the expansion of plague foci. Branch 1 contains genomovars 01 to 10, including the expansion of foci A to G and focus K. Biovar antiqua strains evolved into orientalis strains in focus F. Branch 2 contains genomovars 10 to 13, including the expansion of foci H to K. Biovar antiqua strains evolved into mediaevalis strains in focus I. Branch 3, the unique one, contains only genomovar 14 (biovar microtus), including two distantly separated plague foci (foci L and M). Yunnan Province is the birthplace of biovar orientalis strains causing the modern plague, and Hong Kong is where the modern plague spread globally via marine shipping during the 1890s.
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It makes sense to say that the origin of ancestral Y. pestis is associated with only one kind of rodent, but Y. pestis has the potential to be transmitted to other species of animals. Once the bacteria are exposed to new animals in new geographic regions by animal contact or vector-borne routes, new host niches come gradually into being. The genetic variations, including gene acquisition, gene loss, point mutation, and genome rearrangement, occur randomly in the genome of Y. pestis. When Y. pestis temporarily colonizes a new host niche, the specific host niche acts as a constant and directional natural selection, leading to the stabilization and vertical inheritance of the beneficial genetic variations in the genome of the Y. pestis colonized in it, which we may define as directional microevolution of the genome. A specific host niche determines not only the long-term existence of Y. pestis but also the genomovar of Y. pestis itself; that is, the expansion of plague foci is a course of the stepwise adaptation of Y. pestis to the new host niches.
The host niches in different natural plague foci, each as a unique natural selection, direct the parallel adaptation of Y. pestis to the corresponding hosts and environment, which is a course of pathogen generalization from a newly emerged single-host species to a multihost pathogen. We can also say that the parallel adaptation to various niches drives Y. pestis strains to diversify into different biovars or genomovars. Certain genomovars are limited to certain geographic regions (or host niches) through a course of within-species segregation, or so-called pathogen specialization, with the advantage of avoiding niche overlapping.
Concluding remarks. Our study revealed that the genome of Y. pestis, a newly emerged pathogen, is at an intermediate stage of genetic flux, with evidence of selective genome expansion by horizontal acquisition of plasmids or chromosomal islands and genome reduction by loss of DNA regions. The revealed genome dynamics in natural populations of Y. pestis offer an unprecedented opportunity to establish the link between bacterial genome microevolution and niche adaptation under a Darwinian framework. The transmission, colonization, and expansion of Y. pestis in natural foci constitute a parallel, directional, and gradual process of adaptation to the complex of interactions between the environment, the hosts, and the pathogen, which is based on natural selection against the accumulation of small changes within genome.
While this study was in preparation, Hinchliffe et al. described genomic comparisons of Y. pestis and Y. pseudotuberculosis strains performed by using a CO92 gene-specific microarray (8). They identified dozens of DNA loci that were absent or divergent in more than one of the tested strains of Y. pestis and Y. pseudotuberculosis. Quite a number of these results are shared by our study; e.g., DFR 06, DFR 07, DFR 09, DFR 10, DFR 13, DFR 16, and DFR 17 were also found here to be absent from the relevant Y. pestis strains tested by Hinchliffe et al. One major difference of our study is that we selected isolates representing the natural populations of Y. pestis in confined areas that cover a number of typical plague foci. The extensive distribution of typical natural foci, the long-term collection of Y. pestis isolates, and the existing work on the ecology and epidemiology of plague in China enable us to theorize on the bacterial genome microevolution and its links to niche adaptation, taking advantage of the descriptive interpretation of the genomic differences in strains of Y. pestis and Y. pseudotuberculosis from diverse origins. Another difference is that the whole-genome sequencing data for Y. pestis 91001 allowed us to construct a mixed DNA microarray containing a number of genes that are absent in strain CO92, enabling us to capture many more details in the process of genome microevolution.
Financial support for this work came from the National High Technology Research and Development Program of China (Program 863, no. 2001-AA223061) and the National Natural Science Foundation of China (no. 30371284).
D.Z. and Y.H. contributed equally to this work. ![]()
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