Evidence for genetic exchange and recombination of Rhizobium symbiotic plasmids in a soil population

A soil population of 16 Rhizobium leguminosarum bv. trifolii isolates was characterized by using three Sym (for symbiotic) plasmid-specific DNA hybridization probes: (i) an R. leguminosarum bv. trifolii-specific, repeated-sequence probe; (ii) a nifHDK gene probe, and (iii) a nod gene probe. A predominant Sym plasmid family was identified among the isolates. Three other unrelated Sym plasmid families were also identified. The isolates were also classified either by using a chromosomal DNA hybridization probe or by serological relatedness to 25 different R. leguminosarum bv. trifolii antisera. With either method, it was possible to group the 16 soil isolates into identical or related families. However, the correlation between the two techniques was not high. Irrespective of the means used to classify the bacterial host strain, it was possible to identify the same Sym plasmids in unrelated strains, as well as unrelated Sym plasmids in identical host strains. These data indicate that, within this soil population, there has been genetic exchange of Sym plasmids, and in one instance the hybridization pattern indicates that in vivo recombination of two different Sym plasmids may have occurred. Symbiotic effectiveness tests on red, strawberry, and subterranean clovers clearly differentiated the isolates. In general, the pattern of response was similar within groupings on the basis of Sym plasmid and chromosomal profiles but different between such groups.     

Diversity and genetic structure of a natural population of Rhizobium leguminosarum bv. trifolii isolated from Trifolium subterraneum L.

A collection of 121 isolates of Rhizobium leguminosarum biovar (bv.) trifolii was obtained from root nodules of Trifolium subterraneum L. (subclover) plants growing in an established pasture. The collection consisted of a single isolate from each of 18 plants sampled from seven microplots. The following year, a further 28 and 27 isolates were collected from the first and seventh sampling points, respectively. Analysis of restriction fragment length polymorphisms (RFLPs) of both chromosomal and Sym (symbiotic) plasmid DNA and multilocus enzyme electrophoresis (MLEE) were used to assess the diversity, genetic relationships and structure of this population. Symbiotic effectiveness tests were used to examine the symbiotic phenotype of each isolate collected in the first year. Analysis of RFLPs of the first year isolates revealed 13 chromosomal types and 25 Sym plasmid types. Similar Sym plasmid types were grouped into 14 families containing 1–6 members. No new chromosomal types and six new Sym plasmid types were detected in the second year. The symbiotic effectiveness of the first year isolates of the same Sym plasmid type was similar. Significant differences in symbiotic effectiveness were detected between different Sym plasmid types in the same plasmid family. Representative isolates of each chromosomal type Sym plasmid type identified in the first year were analysed using multilocus enzyme electrophoresis. Mean genetic diversity per locus was high (0.559). Enzyme electrophoresis revealed 17 electrophoretic types (ETs). Ouster analysis of the enzyme data revealed large genetic diversity amongst the ETs. Strong linkage disequilibrium was observed for the population as a whole, i.e. clonal population structure, but significantly less disequilibrium was observed among a cluster of ETs suggesting that recombination occurred between ETs within the cluster. Our results revealed that a population of naturally occurring isolates of Rhizobium leguminosarum bv. trifolii can be genetically diverse and support the possibility that recombination plays a role in generating new genotypes.     

Isolation and partial characterization of plasmids found in three Halobacterium volcanii isolates

Three new isolates of Halobacterium volcanii were screened for the presence of plasmids. Each of the different isolates was found to contain one plasmid. These plasmids do not show any homology to each other, nor to the previously isolated plasmid pHV2. Partial restriction maps of these plasmids were determined. One of the plasmids contains chromosomal repetitive sequences as judged by the existence of homologous sequences in the chromosomal DNA of the three isolates. Using the protoplast fusion technique, we showed that at least one of the newly isolated plasmids is compatible with pHV2.     

Plasmid RFLP profiling and DNA homology inThermus isolated from hot springs of different geographical areas

Thirty-four strains belonging to various species of the genus Thermus (T. aquaticus, "T. thermophilus," "T. brockianus," T. scotoductus, and genomic species 2) isolated from hot springs of different geographical areas were examined for plasmid content and restriction fragment length polymorphism (RFLP) of plasmid DNAs. The four strains of the numerical taxonomy cluster E of genomic species 2 did not harbor plasmid DNA. Overall examination of the HindIII-RFLP profiling of plasmid DNA showed considerable variability between and within genomic species, with the exception of presumed clonal isolates. In spite of this heterogeneity, HindIII plasmid digests within a numerical taxonomic cluster gave a subset of restriction fragments of similar or identical length. Strains belonging to genomic species 2 or unclassified isolates from S. Pedro do Sul that harbored plasmid DNA (7 of the 14 strains studied) exhibited strong DNA homology between plasmid regions. No homologous sequences to these plasmid regions were found in chromosomal DNA from strains isolated from S. Pedro do Sul in which no plasmids were detected. The strains belonging to T. scotoductus formed two plasmid DNA homology groups, as estimated by probing with a plasmid fragment that coincided with the two numerical taxonomy clusters proposed previously. Among the other species, homology of plasmid regions was also found between some strains. Strong homology was also found between plasmid regions from some strains of different taxonomic groups, isolated from the same and from different sources, suggesting that these sequences are highly conserved in plasmids present in Thermus. For plasmid-containing strains, results of plasmid RFLP profiling/DNA homology appear promising for the typing of Thermus at the level of biotypes or of individual strains, namely, for monitoring the diversity and frequency of isolates from a particular hot spring. Received: 24 October 1994 / Accepted: 6 March 1995     

Plasmid distribution in European <Emphasis Type="Italic">Diaporthe helianthi</Emphasis> isolates

Diaporthe helianthi is the causal agent of sunflower stem canker, a serious pathogen of sunflower in Europe, which has been sporadically recorded in Italy. A collection of 26 Diaporthe helianthi isolates deriving from different geographic origins was analysed in order to determine the presence of extra-chromosomal genetic determinants and their molecular diversity. Extra-chromosomal bands in total genomic DNAs were identified in every French and the Yugoslavian isolate and in only one Italian isolate, while no Romanian and Argentinean isolates resulted to host any plasmids. When tested for their chemicophysical nature, they were recognised as linear plasmids sized about 2.3 Kb. A more detailed analysis was performed on a plasmid purified from a French isolate (plasmid F). Its intracellular localisation resulted as mitochondrial. Plasmid F was also exploited as a probe in Southern hybridisation experiments, in which it recognised only plasmids present in the genomes of French and Yugoslavian isolates (countries were the disease has a heavy incidence) indicating a strong correlation to geographic origin. An RFLP hybridisation analysis performed on genomic DNAs revealed a homogeneous restriction pattern in all French and Yugoslavian isolates, suggesting molecular homology among plasmids present in those isolates.     

Diversity of carbofuran-degrading soil bacteria and detection of plasmid-encoded sequences homologous to the mcd gene

Abstract: Fifty-five bacterial isolates, from English and French soils with different histories of carbofuran field treatment, which hydrolysed the N -methylcarbamate insecticide carbofuran to carbofuran 7-phenol were characterised phenotypically and genetically. The isolates were compared by using 125 physiological tests and morphological features, plasmid profiles and restriction fragment length polymorphism (RFLP) patterns of total DNA using the rRNA operon of Escherichia coli as a DNA probe. Cluster analysis of both phenotypic characters and RFLP patterns showed a high degree of diversity amongst the bacteria. Ten distinct plasmid profiles with 2–4 plasmids ranging in size from 84 to about 438 kb were visualised in 50 isolates. The majority of isolates had one of two types of plasmid profiles. Plasmid profiles and Eco RI restricted total DNA patterns were hybridised with an internal fragment of the carbofuran hydrolase ( mcd ) gene and 22 diverse soil isolates exhibited sequence homology with this gene probe. Our results indicate that sequences homologous to the mcd gene are located on a conserved Eco RI fragment (12 or 14 kb) of a plasmid (100, 105, 115 or 124 kb) found in diverse soil isolates from geographically distant areas. Thirty-three isolates did not exhibit detectable homology to the mcd gene probe and the hydrolase enzymes and genes in these isolates need further investigation.     

Plasmid DNA from methanogenic bacteria

In total, 73 strains of methanogen isolates from our laboratory and 6 from culture collections were examined for the presence of plasmid DNA. Five strains were found to contain detectable plasmids. Multiple plasmids were found in two isolates, while three strains contained only one plasmid each. A physical map of the plasmid pT3 was constructed by use of six different restriction endonucleases. All sites were aligned with a single BgII site, and the position of the restriction sites was determined by double or sequential digestion of the plasmid DNA.     

Amplification of the respiratory NADH dehydrogenase of Escherichia coli by gene cloning.

A relatively simple method has been used to clone the gene coding for the respiratory NADH dehydrogenase (NADH-ubiquinone oxidoreductase) of Escherichia coli from unfractionated chromosomal DNA. The restriction endonucleases EcoRI, BamI and HindIII were used to construct three hybrid plasmid pools from total E. coli DNA and the amplifiable plasmids pSF2124 and pGM706. Three different restriction endonucleases were used to increase the chances of cloning the ndh gene intact. Mobilization by the plasmid F was used to transfer the hybrid plasmids into ndh mutants and selection was made for Apr and complementation of ndh. DNA fragments complementing ndh were isolated from both the EcoRI and HindIII hybrid plasmid pools. The strain carrying the hybrid plasmid constructed with EcoRI produced about 8--10 times the normal level of the respiratory NADH dehydrogenase in the cytoplasmic membrane. Treating the cells with chloramphenicol to increase the plasmid copy number allowed the level of NADH dehydrogenase in the membrane to be increased to 50--60 times the level in the wild type. The results indicate the potential of gene cloning for the specific amplification of particular proteins prior to their purification.     

Restriction Fragment Length Polymorphism Analyses of the Plasmid DNAs in Strains of Xanthomonas campestris pv. vesicatoria from Different Geographic Areas

Restriction fragment length polymorphisms (RFLPs) of plasmid DNAs in Xanthomonas campestris pv. vesicatoria were analysed using 77 strains from the United states, Argentina, Australia, Taiwan, and Korea. One or more plasmids were detected in all tested strains, irrespective of geographic origin, host plant from which isolated, or chemical resistance. All Korean strains contained a few plasmids of similar high molecular weight, whereas some small plasmids occured only in strains from the United States, Argentina, and Taiwan. After digesting total plasmid DNAs with each of four restriction endonucleases, 18 fragments with sizes from about 1 to 23 kb were visualized. Seventy-seven strains of diverse geographic origins, with different levels of resistance to streptomycin and copper, were classified into the 14 RFLP groups based on the restriction endonuclease digestion patterns of their plasmid DNAs. Strains belonging to each group shared DNA fragments of identical size, suggesting the possible presence of similar plasmids in these strains. A 5.8-kb EcoRI plasmid DNA probe prepared from the United States strain 81-23 hybridized to EcoRI plasmid digests from all tested strains. Other plasmid DNA fragments of the strain81-2,3 used as probes had no homology to plasmid DNA fragments from several strains around the world. The variation in hybridization profiles of plasmid DNA was very similar to the results obtained by RFLP analysis of plasmid DNA digested by four restriction enzymes. Most of the Korean strains tested were highly sensitive to streptomycin and copper, whereas most strains from other geographic areas showed a high level of resistance to one or two of the chemicals. Cluster analysis of genetic distance between the strains based on the data obtained generated the dendrograms that separated all Korean strains from the other strains, suggesting that plasmid DNA of the Korean strains may be genetically very different from those of the others.     

Comparative genome hybridization reveals substantial variation among clinical isolates of Borrelia burgdorferi sensu stricto with different pathogenic properties

Clinical and murine studies suggest that there is a differential pathogenicity of different genotypes of Borrelia burgdorferi, the spirochetal agent of Lyme disease. Comparative genome hybridization was used to explore the relationship between different genotypes. The chromosomes of all studied isolates were highly conserved (>93%) with respect to both sequence and gene order. Plasmid sequences were substantially more diverse. Plasmids lp54, cp26, and cp32 were present in all tested isolates, and their sequences and gene order were conserved. The majority of linear plasmids showed variation both in terms of presence among different isolates and in terms of sequence and gene order. The data strongly imply that all B. burgdorferi clinical isolates contain linear plasmids related to each other, but the structure of these replicons may vary substantially from isolate to isolate. These alterations include deletions and presumed rearrangements that are likely to result in unique plasmid elements in many isolates. There is a strong correlation between complete genome hybridization profiles and other typing methods, which, in turn, also correlate to differences in pathogenicity. Because there is substantially less variation in the chromosomal and circular plasmid portions of the genome, the major differences in open reading frame content and genomic diversity among isolates are linear plasmid driven.     

Parallel variation in isoenzyme and nitrogen fixation markers in a Rhizobium population

Twenty isolates of Rhizobium leguminosarum bv. viceae were isolated at random from one field and examined for symbiotic plasmid fragment length polymorphisms and for isoenzyme patterns. The latter are most probably chromosome markers. With one exception both methods separated the isolates into the same 13 different groups. The largest group was represented 7 times according to isoenzymes and 8 times according to RFLP. This fixed non-random association of plasmid and chromosomal genotypes is consistent with a clonal population structure; it indicates limited exchange of plasmids under natural conditions. Seventeen isolates of 11 groups were highly effective and 2 isolates in one group almost ineffective.     

Megaplasmids in Thermus oshimai isolates from two widely separated geographical areas: restriction fragment profiling and DNA homology

Megaplasmid DNA was detected in ten isolates belonging to the recently described thermophilic eubacterial species Thermus oshimai and isolated from hot springs in Portugal (eight isolates) and Iceland (two isolates). The estimated size of the large plasmids purified from T. oshimai SPS-18 from S. Pedro do Sul, Portugal, and from isolate JK-91 from Hveragerdhi-Hengill, Iceland, was 214 and 275 kb, respectively. No sequence homologous to isolate SPS-18 megaplasmid is present in chromosomal DNA as indicated by Southern hybridization analysis. Overall examination of the HindIII fragment profiles of megaplasmid DNAs purified from isolates from the same geographical area gave similar but not always identical restriction profiles on agarose gels. Restriction fragment length polymorphism (RFLP) was higher for megaplasmids present in isolates purified from the Portuguese and Icelandic isolates than for megaplasmids from the same hot spring. Megaplasmid RFLP correlated with previous results obtained on the polymorphism of macrorestriction patterns of whole genomic DNA and with the RFLP of co-resident small plasmid DNA that was found in one half of the isolates examined. The 16-kb HindIII–HindIII fragment from isolate SPS-18 megaplasmid showed DNA–DNA homology with restriction fragments of similar size generated by the large plasmids present in all the other isolates, even in those from hot springs of widely separated geographical areas. This suggests a high degree of sequence conservation in T. oshimai megaplasmids. Received: 12 May 1997 / Accepted: 17 July 1997     

Plasmids Responsible for Horizontal Transfer of Naphthalene Catabolism Genes between Bacteria at a Coal Tar-Contaminated Site Are Homologous to pDTG1 from Pseudomonas putida NCIB 9816-4

The presence of a highly conserved nahAc allele among phylogenetically diverse bacteria carrying naphthalene-catabolic plasmids provided evidence for in situ horizontal gene transfer at a coal tar-contaminated site (J. B. Herrick, K. G. Stuart-Keil, W. C. Ghiorse, and E. L. Madsen, Appl. Environ. Microbiol. 63:2330–2337, 1997). The objective of the present study was to identify and characterize the different-sized naphthalene-catabolic plasmids in order to determine the probable mechanism of horizontal transfer of the nahAc gene in situ. Filter matings between naphthalene-degrading bacterial isolates and their cured progeny revealed that the naphthalene-catabolic plasmids were self-transmissible. Limited interstrain transfer was also found. Analysis of the restriction fragment length polymorphism (RFLP) patterns indicated that catabolic plasmids from 12 site-derived isolates were closely related to each other and to the naphthalene-catabolic plasmid (pDTG1) of Pseudomonas putida NCIB 9816-4, which was isolated decades ago in Bangor, Wales. The similarity among all site-derived naphthalene-catabolic plasmids and pDTG1 was confirmed by using the entire pDTG1 plasmid as a probe in Southern hybridizations. Two distinct but similar naphthalene-catabolic plasmids were retrieved directly from the microbial community indigenous to the contaminated site in a filter mating by using a cured, rifampin-resistant site-derived isolate as the recipient. RFLP patterns and Southern hybridization showed that both of these newly retrieved plasmids, like the isolate-derived plasmids, were closely related to pDTG1. These data indicate that a pDTG1-like plasmid is the mobile genetic element responsible for transferring naphthalene-catabolic genes among bacteria in situ. The pervasiveness and persistence of this naphthalene-catabolic plasmid suggest that it may have played a role in the adaptation of this microbial community to the coal tar contamination at our study site.     

Characterization of Cultivable Bacteria from Brazilian Sponges

Among 1,236 colony-forming units (CFU) associated with 11 species of marine sponges collected from a Brazilian coast, a total of 100 morphologically different bacterial strains were analyzed. The phylogenetic diversity of the bacterial isolates was assessed by 16S rRNA gene amplification—restriction fragment length polymorphism (RFLP) analysis, using AluI restriction endonuclease. The RFLP fingerprinting resulted in 21 different patterns with good resolution for the identification of the bacterial isolates at the genus level. The genus Bacillus was the most commonly encountered genus, followed by Kocuria. Regarding the relationship between the morphotypes and species of marine sponges, Mycale microsigmatosa presented major diversity, followed by Dragmacidon reticulatum and Polymastia janeirensis. An antibiotic susceptibility profile of the 100 sponge-associated bacterial strains was determined by the disk diffusion method, and we observed a variable resistance profile, with 15 % of the bacteria being multiresistant. In addition, 71 of 100 strains were able to produce biofilm. These 71 strains were divided into 20 strong biofilm producers, 10 moderate biofilm producers, and 41 weak biofilm producers. The plasmid profile of the 100 bacterial strains was analyzed and 38 (38 %) of these samples possessed one or more plasmids. Studies like this are important to increase the information on these associated bacteria found off the coastline of Brazil, a place which has rich biodiversity that is still unknown.     

Plasmids in Corynebacterium glutamicum and their molecular classification by comparative genomics

Endogenous plasmids and selectable resistance markers are a fundamental prerequisite for the development of efficient recombinant DNA techniques in industrial microorganisms. In this article, we therefore summarize the current knowledge about endogenous plasmids in amino acid-producing Corynebacterium glutamicum isolates. Screening studies identified a total of 24 different plasmids ranging in size from 2.4 to 95 kb. Although most of the C. glutamicum plasmids were cryptic, four plasmids carried resistance determinants against the antibiotics chloramphenicol, tetracycline, streptomycin-spectinomycin, and sulfonamides. Considerable information is now available on the molecular genetic organization of 12 completely sequenced plasmid genomes from C. glutamicum. The deduced mechanism of plasmid DNA replication and the degree of amino acid sequence similarity among replication initiator proteins was the basis for performing a classification of the plasmids into four distinct C. glutamicum plasmid families.     

Genetic Diversity among Rhizobium leguminosarum bv. Trifolii Strains Revealed by Allozyme and Restriction Fragment Length Polymorphism Analyses

Allozyme electrophoresis and restriction fragment length polymorphism (RFLP) analyses were used to examine the genetic diversity of a collection of 18 Rhizobium leguminosarum bv. trifolii, 1 R. leguminosarum bv. viciae, and 2 R. meliloti strains. Allozyme analysis at 28 loci revealed 16 electrophoretic types. The mean genetic distance between electrophoretic types of R. leguminosarum and R. meliloti was 0.83. Within R. leguminosarum, the single strain of bv. viciae differed at an average of 0.65 from strains of bv. trifolii, while electrophoretic types of bv. trifolii differed at a range of 0.23 to 0.62. Analysis of RFLPs around two chromosomal DNA probes also delineated 16 unique RFLP patterns and yielded genetic diversity similar to that revealed by the allozyme data. Analysis of RFLPs around three Sym (symbiotic) plasmid-derived probes demonstrated that the Sym plasmids reflect genetic divergence similar to that of their bacterial hosts. The large genetic distances between many strains precluded reliable estimates of their genetic relationships.     

Construction of a Fibrobacter succinogenes Genomic Map and Demonstration of Diversity at the Genomic Level

The genomic cleavage map of the type strain Fibrobacter succinogenes S85 was constructed. The restriction enzymes AscI, AvrII, FseI, NotI, and SfiI generated DNA fragments of suitable size distribution that could be resolved by pulsed-field gel electrophoresis (PFGE). An average genome size of 3.6 Mb was obtained by summing the total fragment sizes. The linkages between the 15 AscI fragments of the genome were determined by combining two approaches: isolation of linking clones and cross-hybridization of restriction fragments. The genome of F. succinogenes was found to be represented by the single circular DNA molecule. Southern hybridization with specific probes allowed the eight genetic markers to be located on the restriction map. The genome of this bacterium contains at least three rRNA operons. PFGE of the other three strains of F. succinogenes gave estimated genome sizes close to that of the type strain. However, RFLP patterns of these strains generated by AscI digestion are completely different. Pairwise comparison of the genomic fragment distribution between the type strain and the three isolates showed a similarity level in the region of 14.3% to 31.3%. No fragment common to all of these F. succinogenes strains could be detected by PFGE. A marked degree of genomic heterogeneity among members of this species makes genomic RFLP a highly discriminatory and useful molecular typing tool for population studies. Received: 23 October 1996 / Accepted: 31 December 1996     

Increased Bean (Phaseolus vulgaris L.) Nodulation Competitiveness of Genetically Modified Rhizobium Strains

Rhizobium leguminosarum bv. phaseoli strain collections harbor heterogeneous groups of bacteria in which two main types of strains may be distinguished, differing both in the symbiotic plasmid and in the chromosome. We have analyzed under laboratory conditions the competitive abilities of the different types of Rhizobium strains capable of nodulating Phaseolus vulgaris L. bean. R. leguminosarum bv. phaseoli type I strains (characterized by nif gene reiterations and a narrow host range) are more competitive than type II strains (that have a broad host range), and both types are more competitive than the promiscuous rhizobia isolated from other tropical legumes able to nodulate beans. Type I strains become even more competitive by the transfer of a non-Sym, 225-kilobase plasmid from type II strain CFN299. This plasmid has been previously shown to enhance the nodulation and nitrogen fixation capabilities of Agrobacterium tumefaciens transconjugants carrying the Sym plasmid of strain CFN299. Other type I R. leguminosarum bv. phaseoli transconjugants carrying two symbiotic plasmids (type I and type II) have been constructed. These strains have a diminished competitive ability. The increase of competitiveness obtained in some transconjugants seems to be a transient property.     

Molecular analysis of methicillin-resistant Staphylococcus aureus reveals an absence of plasmid DNA in multidrug-resistant isolates

The number, diversity and restriction enzyme fragmentation patterns of plasmids harboured by 44 multidrug-resistant hospital-acquired methicillin-resistant Staphylococcus aureus (MR-HA-MRSA) isolates, two multidrug-resistant community-acquired MRSA (MR-CA-MRSA), 50 hospital-acquired MRSA (HA-MRSA) isolates (from the University Hospital Birmingham, NHS Trust, UK) and 34 community-acquired MRSA (CA-MRSA) isolates (from general practitioners in Birmingham, UK) were compared. In addition, pulsed-field gel electrophoresis (PFGE) type following SmaI chromosomal digest and SCCmec element type assignment were ascertained for each isolate. All MR-HA-MRSA and MR-CA-MRSA isolates possessed the type II SCCmec, harboured no plasmid DNA and belonged to one of five PFGE types. Forty-three out of 50 HA-MRSA isolates and all 34 CA-MRSA isolates possessed the type IV SCCmec and all but 10 of the type IV HA-MRSA isolates and nine CA-MRSA isolates carried one or two plasmids. The 19 non-multidrug-resistant isolates (NMR) that did not harbour plasmids were only resistant to methicillin whereas all the NMR isolates harbouring at least one plasmid were resistant to at least one additional antibiotic. We conclude that although plasmid carriage plays an important role in antibiotic resistance, especially in NMR-HA-MRSA and CA-MRSA, the multidrug resistance phenotype from HA-MRSA is not associated with increased plasmid carriage and indeed is characterised by an absence of plasmid DNA.     

用AFLP技术和16S rDNA PCR-RFLP分析毛苜蓿根瘤菌的遗传多样性

采用选择主增片断长度多态性（简称ＡＦＬＰ）ＤＮＡ指纹技术对采自我国云南省与西藏交界的高山地区的野生型豆科植物毛苜蓿根际土样分离的２９１株毛苜蓿（Ｍｅｄｉｃａｇｏ ｅｄｇｅｗｏｒｔｈｉｉ）根瘤菌进行遗传多样性的研究。从ＡＦＬＰ图谱中,揭示出毛苜蓿根瘤菌有较显著的遗传多样性,从２９１株中选择出９０年代表株用计算机进行树状图的分析。结果表明,所分析的菌株在７９％的相似性水平上聚类成３个群。对这９０年代表