Genetic and Functional Diversity among Fluorescent Pseudomonads Isolated from the Rhizosphere of Banana

Fluorescent pseudomonads from banana rhizospheric soil were isolated and screened for the production of enzymes and hormones such as phosphatase, indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, protease, and antifungal metabolites. Of 95 isolates, 50 (52%) isolates solubilized tri-calcium phosphate (TCP), 63 (66%) isolates produced plant growth hormone IAA, 10 (11%) isolates exhibited ACC deaminase, and 23 (24%) isolates produced protease. Isolates were screened for antifungal activity toward phytopathogenic fungi. Gene-specific primers have identified the putative antibiotic producing isolates. These putative isolates were grown in the production media and production of antibiotics was confirmed by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). Genotypic analysis by BOX (bacterial repetitive BOX element)-polymerase chain reaction (PCR) resulted into three distinct genomic clusters at a 50% similarity level and 62 distinct BOX profiles. Based on the sequence similarity of 16S rRNA and construction of subsequent phylogenetic tree analysis, isolates were designated as Pseudomonas monteilii, P. plecoglossicida, P. fluorescens, P. fulva, P. mosselii, P. aeruginosa, P. alcaligenes, and P. pseudoalcaligenes. Present study revealed the genetic and functional diversity among isolates of fluorescent pseudomonads associated with rhizospheric soil of banana and also identified P. monteilii as dominant species. The knowledge on genetic and functional diversity of fluorescent pseudomonads associated with banana rhizosphere is useful to understand their ecological role and for their utilization in sustainable agriculture.     

Comparative Genetic Diversity of the narG, nosZ, and 16S rRNA Genes in Fluorescent Pseudomonads

The diversity of the membrane-bound nitrate reductase (narG) and nitrous oxide reductase (nosZ) genes in fluorescent pseudomonads isolated from soil and rhizosphere environments was characterized together with that of the 16S rRNA gene by a PCR-restriction fragment length polymorphism assay. Fragments of 1,008 bp and 1,433 bp were amplified via PCR with primers specific for the narG and nosZ genes, respectively. The presence of the narG and nosZ genes in the bacterial strains was confirmed by hybridization of the genomic DNA and the PCR products with the corresponding probes. The ability of the strains to either reduce nitrate or totally dissimilate nitrogen was assessed. Overall, there was a good correspondence between the reductase activities and the presence of the corresponding genes. Distribution in the different ribotypes of strains harboring both the narG and nosZ genes and of strains missing both genes suggests that these two groups of strains had different evolutionary histories. Both dissimilatory genes showed high polymorphism, with similarity indexes (Jaccard) of between 0.04 and 0.8, whereas those of the 16S rRNA gene only varied from 0.77 to 0.99. No correlation between the similarity indexes of 16S rRNA and dissimilatory genes was seen, suggesting that the evolution rates of ribosomal and functional genes differ. Pairwise comparison of similarity indexes of the narG and nosZ genes led to the delineation of two types of strains. Within the first type, the similarity indexes of both genes varied in the same range, suggesting that these two genes have followed a similar evolution. Within the second type of strain, the range of variations was higher for the nosZ than for the narG gene, suggesting that these genes have had a different evolutionary rate.     

Conjugative Transfer of Chromosomal Genes between Fluorescent Pseudomonads in the Rhizosphere of Wheat

Bacteria released in large numbers for biocontrol or bioremediation purposes might exchange genes with other microorganisms. Two model systems were designed to investigate the likelihood of such an exchange and some factors which govern the conjugative exchange of chromosomal genes between root-colonizing pseudomonads in the rhizosphere of wheat. The first model consisted of the biocontrol strain CHA0 of Pseudomonas fluorescens and transposon-facilitated recombination (Tfr). A conjugative IncP plasmid loaded with transposon Tn5, in a CHA0 derivative carrying a chromosomal Tn5 insertion, promoted chromosome transfer to auxotrophic CHA0 recipients in vitro. A chromosomal marker (pro) was transferred at a frequency of about 10(sup-6) per donor on wheat roots under gnotobiotic conditions, provided that the Tfr donor and recipient populations each contained 10(sup6) to 10(sup7) CFU per g of root. In contrast, no conjugative gene transfer was detected in soil, illustrating that the root surface stimulates conjugation. The second model system was based on the genetically well-characterized strain PAO of Pseudomonas aeruginosa and the chromosome mobilizing IncP plasmid R68.45. Although originally isolated from a human wound, strain PAO1 was found to be an excellent root colonizer, even under natural, nonsterile conditions. Matings between an auxotrophic R68.45 donor and auxotrophic recipients produced prototrophic chromosomal recombinants at 10(sup-4) to 10(sup-5) per donor on wheat roots in artificial soil under gnotobiotic conditions and at about 10(sup-6) per donor on wheat roots in natural, nonsterile soil microcosms after 2 weeks of incubation. The frequencies of chromosomal recombinants were as high as or higher than the frequencies of R68.45 transconjugants, reflecting mainly the selective growth advantage of the prototrophic recombinants over the auxotrophic parental strains in the rhizosphere. Although under field conditions the formation of chromosomal recombinants is expected to be reduced by several factors, we conclude that chromosomal genes, whether present naturally or introduced by genetic modification, may be transmissible between rhizosphere bacteria.     

Characterization of Phosphate-Solubilizing Fluorescent Pseudomonads from the Rhizosphere of Seabuckthorn Growing in the Cold Deserts of Himalayas

Isolation and characterization of fluorescent pseudomonads with high phosphate-solubilizing ability is reported from the alkaline and calcium-rich soils with low P availability in the cold desert region of Lahaul and Spiti in the trans-Himalayas of India. Of 216 phosphate-solubilizing isolates, 12 exhibiting high solubilization of tricalcium phosphate (TCP) in NBRIP liquid culture were identified as Pseudomonas trivialis, P. poae, P. fluorescens, and Pseudomonas spp. on the basis of phenotypic features, whole-cell fatty acids methyl ester (FAME) profiles, and 16S rDNA sequencing. These isolates also showed relatively high solubilization of North Carolina rock phosphate (NCRP) in comparison to the solubilization of Mussoorie rock phosphate (MRP) and Udaipur rock phosphate (URP). The solubilization of phosphate substrates by P. trivialis and P. poae is reported for the first time.     

Genetic Diversity and Biological Control Activity of Novel Species of Closely Related Pseudomonads Isolated from Wheat Field Soils in South Australia

Rhizobacteria closely related to two recently described species of pseudomonads, Pseudomonas brassicacearum and Pseudomonas thivervalensis, were isolated from two geographically distinct wheat field soils in South Australia. Isolation was undertaken by either selective plating or immunotrapping utilizing a polyclonal antibody raised against P. brassicacearum. A subset of 42 isolates were characterized by amplified 16S ribosomal DNA restriction analysis (ARDRA), BIOLOG analysis, and gas chromatography-fatty acid methyl ester (GC-FAME) analysis and separated into closely related phenetic groups. More than 75% of isolates tested by ARDRA were found to have >95% similarity to either Pseudomonas corrugata or P. brassicacearum-P. thivervalensis type strains, and all isolates had >90% similarity to either type strain. BIOLOG and GC-FAME clustering showed a >70% match to ARDRA profiles. Strains representing different ARDRA groups were tested in two soil types for biological control activity against the soilborne plant pathogen Gaeumannomyces graminis var. tritici, the causative agent of take-all of wheat and barley. Three isolates out of 11 significantly reduced take-all-induced root lesions on wheat plants grown in a red-brown earth soil. Only one strain, K208, was consistent in reducing disease symptoms in both the acidic red-brown earth and a calcareous sandy loam. Results from this study indicate that P. brassicacearum and P. thivervalensis are present in Australian soils and that a level of genetic diversity exists within these two novel species but that this diversity does not appear to be related to geographic distribution. The result of the glasshouse pot trial suggests that some isolates of these species may have potential as biological control agents for plant disease.     

Antagonistic,Anti-oxidant,Anti-inflammatory and Anti-diabetic Probiotic Potential of Lactobacillus agilis Isolated From the Rhizosphere of the Medicinal Plants

Potential probiotic bacteria can be used as a biotherapeutic agent and a sustainable alternative to antibiotics, as an anti-oxidative, anti-inflammatory, and anti-diabetic agent without causing any serious side effects. Mostly human-friendly Lactic acid bacteria (LAB) have been isolated from the animal-human origin to be used as biotherapeutic agents or to produce useful metabolites (nutraceutical). However, less information is known about the role of medicinal plants associated LAB as biotherapeutic agents. The isolation of 115 human-friendly Lactobacillus strains was done from the rhizosphere of the medicinal plants Ocimum tenuiflorum, Azadirachta indica, Ficus carica. The obtained bacteria were then tested for their safe status before being using it for a beneficial purpose. Out of 115 strains, 29 (25%) were negative for blood hemolytic activities. Among these 29 isolates, three isolates did not show a breakdown of gelatin and were recognized as safe. Antibiotic resistance assay showed resistance of two of them against antibiotics discs of Streptomycin (10 µg), Ciprofloxacin (20 µg), Vancomycin (30 µg), Metronidazole (10 µg), Ampicillin (5 µg), Chloramphenicol (30 µg), Kanamycin (30 µg), Erythromycin (15 µg), Penicillin (10 µg) and Tetracycline (30 µg). The bacterial isolate (T-2) was found safe that was identified as Lactobacillus agilis by sequence analysis of 16 s rRNA gene and processed in vitro as an anti-bacterial, anti-oxidant, anti-diabetic, and anti-inflammatory agent. Free radical scavenging activities and inhibition of α-amylase activities for Lactobacillus agilis were found relative to standard drug values as 68% and 73% and 51.3% and 65.3%, respectively. The in-vitro anti-inflammatory assay showed 61.6% (Lactobacillus agilis) while showed 69% (aspirin) activity for denaturation albumin protein. The results suggested that Lactobacillus agilis can be used as a potential probiotic strain as well as can be used to produce nutraceuticals.     

Genotypic and Phenotypic Diversity of phlD-Containing Pseudomonas Strains Isolated from the Rhizosphere of Wheat

Production of 2,4-diacetylphloroglucinol (2,4-DAPG) in the rhizosphere by strains of fluorescent Pseudomonas spp. results in the suppression of root diseases caused by certain fungal plant pathogens. In this study, fluorescent Pseudomonas strains containing phlD, which is directly involved in the biosynthesis of 2,4-DAPG, were isolated from the rhizosphere of wheat grown in soils from wheat-growing regions of the United States and The Netherlands. To assess the genotypic and phenotypic diversity present in this collection, 138 isolates were compared to 4 previously described 2,4-DAPG producers. Thirteen distinct genotypes, one of which represented over 30% of the isolates, were differentiated by whole-cell BOX-PCR. Representatives of this group were isolated from eight different soils taken from four different geographic locations. ERIC-PCR gave similar results overall, differentiating 15 distinct genotypes among all of the isolates. In most cases, a single genotype predominated among isolates obtained from each soil. Thirty isolates, representing all of the distinct genotypes and geographic locations, were further characterized. Restriction analysis of amplified 16S rRNA gene sequences revealed only three distinct phylogenetic groups, one of which accounted for 87% of the isolates. Phenotypic analyses based on carbon source utilization profiles revealed that all of the strains utilized 49 substrates and were unable to grow on 12 others. Individually, strains could utilize about two-thirds of the 95 substrates present in Biolog SF-N plates. Multivariate analyses of utilization profiles revealed phenotypic groupings consistent with those defined by the genotypic analyses.     

Community Profiling of Culturable Fluorescent Pseudomonads in the Rhizosphere of Green Gram (Vigna radiata L.)

Study on microbial diversity in the unexplored rhizosphere is important to understand their community structure, biology and ecological interaction with the host plant. This research assessed the genetic and functional diversity of fluorescent pseudomonads [FP] in the green gram rhizophere. One hundred and twenty types of morphologically distinct fluorescent pseudomonads were isolated during vegetative as well as reproductive growth phase of green gram. Rep PCR, ARDRA and RISA revealed two distinct clusters in each case at 75, 61 and 70% similarity coefficient index respectively. 16S rRNA partial sequencing analysis of 85 distantly related fluorescent pseudomonads depicted Pseudomonas aeruginosa as the dominant group. Out of 120 isolates, 23 (19%) showed antagonistic activity towards phytopathogenic fungi. These bacterial isolates showed varied production of salicylic acid, HCN and chitinase, 2, 4-diacetylphloroglucinol (DAPG), phenazine-1-carboxylic acid (PCA) and pyoluteorin (PLT). Production efficiency of inherent level of plant growth promoting (PGP) traits among the 120 isolates demonstrated that 10 (8%) solubilised inorganic phosphates, 25 (20%) produced indoles and 5 (4%) retained ACC deaminase activity. Pseudomonas aeruginosa GGRJ21 showed the highest production of all antagonistic and plant growth promoting (PGP) traits. In a greenhouse experiment, GGRJ21 suppressed root rot disease of green gram by 28–93% (p = 0.05). Consistent up regulation of three important stress responsive genes, i.e., acdS, KatA and gbsA and elevated production efficiency of different PGP traits could promote GGRJ21 as a potent plant growth regulator.     

Genetic Diversity Among Cold-Tolerant Fluorescent Pseudomonas Isolates from Indian Himalayas and Their Characterization for Biocontrol and Plant Growth-Promoting Activities

In Uttarakhand, the Organic State of India, where soils in most farming situations are deficient in nutrients and loss of crops due to soil- and seed-borne pathogens is rampant, use of native plant growth-promoting rhizobacteria (PGPRs) possessing biocontrol (BC) activities holds promise. In view of this, 600 native cold-tolerant rhizospheric bacterial isolates were collected from Uttarakhand Himalayas, of which 336 were confirmed as fluorescent Pseudomonas spp. On the basis of specific biochemical tests, these were characterized into three major groups: P. fluorescens (308 isolates), P. aeruginosa (20 isolates), and P. putida (8 isolates). Most of the isolates could grow at 8°C after 12 h of incubation, confirming their cold tolerance. In vitro biocontrol assays revealed that of 336 isolates, 74 were antagonistic to Rhizoctonia solani and 91 to Fusarium solani, the two major pathogens associated with root-rot complex in vegetables widespread in the region. Simultaneously, good HCN producers (33 isolates), siderophore producers (80 isolates), and P solubilizers (49 isolates) were also identified, which could increase the biocontrol and plant growth-promoting efficacies of the putative PGPRs. Among the different species and biovars, P. fluorescens biovar-I had the maximum number of potential isolates with BC and plant growth-promoting (PGP) activities. In French bean, under polyhouse and field conditions, five isolates (Pf-173, Pf-193, Pf-547, Pf-551, and Pf-572) showed good BC and PGP activities as up to 93% reduction in root rot was achieved. A combination of all five isolates was found to be best with respect to BC and PGP activities. In a set of 59 fluorescent Pseudomonas isolates, RAPD-PCR analysis, using three random oligodecamer primers, revealed high diversity and formed ten distinct clusters, corresponding to the host of origin (annual or perennial) or habitat (farming situations) of the isolates. The amount of diversity revealed in the set of fluorescent Pseudomonas isolates could represent enormous diversity that exists in the wild that could be exploited for improved BC and PGP activities of the PGPRs. For the first time, this study led to a large-scale characterization and repositioning of fluorescent pseudomonads from the Indian Himalayas.     

Plant-Dependent Genotypic and Phenotypic Diversity of Antagonistic Rhizobacteria Isolated from Different Verticillium Host Plants

To study the effect of plant species on the abundance and diversity of bacterial antagonists, the abundance, the phenotypic diversity, and the genotypic diversity of rhizobacteria isolated from potato, oilseed rape, and strawberry and from bulk soil which showed antagonistic activity towards the soilborne pathogen Verticillium dahliae Kleb. were analyzed. Rhizosphere and soil samples were taken five times over two growing seasons in 1998 and 1999 from a randomized field trial. Bacterial isolates were obtained after plating on R2A (Difco, Detroit, Mich.) or enrichment in microtiter plates containing high-molecular-weight substrates followed by plating on R2A. A total of 5,854 bacteria isolated from the rhizosphere of strawberry, potato, or oilseed rape or bulk soil from fallow were screened by dual testing for in vitro antagonism towards Verticillium. The proportion of isolates with antagonistic activity was highest for strawberry rhizosphere (9.5%), followed by oilseed rape (6.3%), potato (3.7%), and soil (3.3%). The 331 Verticillium antagonists were identified by their fatty acid methyl ester profiles. They were characterized by testing their in vitro antagonism against other pathogenic fungi; their glucanolytic, chitinolytic, and proteolytic activities; and their BOX-PCR fingerprints. The abundance and composition of Verticillium antagonists was plant species dependent. A rather high proportion of antagonists from the strawberry rhizosphere was identified as Pseudomonas putida B (69%), while antagonists belonging to the Enterobacteriaceae (Serratia spp., Pantoea agglomerans) were mainly isolated from the rhizosphere of oilseed rape. For P. putida A and B plant-specific genotypes were observed, suggesting that these bacteria were specifically enriched in each rhizosphere.     

Effects of Fungal Root Pathogens on the Population Dynamics of Biocontrol Strains of Fluorescent Pseudomonads in the Wheat Rhizosphere

The influences of Gaeumannomyces graminis var. tritici (which causes take-all of wheat), Rhizoctonia solani AG-8 (which causes rhizoctonia root rot of wheat), Pythium irregulare, P. aristosporum, and P. ultimum var. sporangiiferum (which cause pythium root rot of wheat) on the population dynamics of Pseudomonas fluorescens 2-79 and Q72a-80 (bicontrol strains active against take-all and pythium root rot of wheat, respectively) in the wheat rhizosphere were examined. Root infection by either G. graminis var. tritici or R. solani resulted in populations of both bacterial strains that were equal to or significantly larger than their respective populations maintained on roots in the absence of these pathogens. In contrast, the population of strain 2-79 was significantly smaller on roots in the presence of any of the three Pythium species than on noninfected roots and was often below the limits of detection (50 CFU/cm of root) on Pythium-infected roots after 40 days of plant growth. In the presence of either P. aristosporum or P. ultimum var. sporangiiferum, the decline in the population of Q72a-80 was similar to that observed on noninfected roots; however, the population of this strain declined more rapidly on roots infected by P. irregulare than on noninfected roots. Application of metalaxyl (which is selectively inhibitory to Pythium spp.) to soil naturally infestated with Pythium spp. resulted in significantly larger rhizosphere populations of the introduced bacteria over time than on plants grown in the same soil without metalaxyl. It is apparent that root infections by fungal pathogens may either enhance or depress the population of fluorescent pseudomonads introduced for their control, with different strains of pseudomonads reacting differentially to different genera and species of the root pathogens.     

向日葵菌核病根际拮抗菌的筛选与鉴定及其拮抗机理的初步研究

利用梯度稀释和对峙试验的方法,从向日葵根际土壤筛选到18株对向日葵菌核病病原菌—核盘菌(Sclerotiniasclerotiorum)有拮抗效果的细菌,其中1株具有较强的拮抗效果,命名为XRK4。经过形态观察及16S rDNA序列分析,将XRK4鉴定为地衣芽孢杆菌(Bacillus licheniformis)。研究表明XRK4具有β-1,3-1,4-葡聚糖酶活性。设计引物以XRK4的基因组DNA为模板扩增得到长度为732 bp的β-1,3-1,4-葡聚糖酶的完整基因,其开放阅读框架编码243个氨基酸。XRK4可能因产生葡聚糖酶,降解病原菌细胞壁而具有拮抗活性。     

不同桑树基因型的遗传多样性研究

调查了印度 25 个不同农业气候条件下的桑树(Morus spp.)基因型的 14 个形态测量性状, 以研究它们的遗传多样性分布特点。研究发现在 14 个性状中存在广泛的遗传变异。25 个不同基因型的桑树被分为 10 个簇。实验结果表明 7 种基因型桑树 Baragura-2, Gorabandha-2, Kalimpong, Herbertpur, Kollegal, Resham majri-7 和 UP-14 的遗传资源是重要的。对不同基因型桑树性状的关联分析显示, 对桑树叶片长度、新鲜叶的总量、叶片面积和单个叶的重量这些性状的直接筛选将有助于提高桑树叶片的产量。     

Potential of Siderophore Production by Bacteria Isolated from Heavy Metal: Polluted and Rhizosphere Soils

Recently, heavy metals have been shown to have a stimulating effect on siderophore biosynthesis in various bacteria. In addition, several studies have found that siderophore production is greater in bacteria isolated from soil near plant roots. The aim of this study was to compare the production of siderophores by bacterial strains isolated from heavy metal-contaminated and uncontaminated soils. Chrome azurol sulphonate was used to detect siderophore secretion by several bacterial strains isolated from heavy metal-contaminated and rhizosphere-uncontaminated soils with both a qualitative disc diffusion method and a quantitative ultraviolet spectrophotometric method. Siderophore production by rhizosphere bacteria was significantly greater than by bacteria isolated from contaminated soil. The Pearson’s correlation test indicated a positive correlation between the amount of siderophore produced by bacteria isolated from the rhizosphere using the quantitative and qualitative detection methods and the amount of heavy metal in the soil. However, a significant negative correlation was observed between the amount of siderophore produced by bacteria isolated from heavy metal-contaminated soil and the amount of heavy metal (r value of ?0.775, P < 0.001).     

Functional Potential of Soil Microbial Communities in the Maize Rhizosphere

Microbial communities in the rhizosphere make significant contributions to crop health and nutrient cycling. However, their ability to perform important biogeochemical processes remains uncharacterized. Here, we identified important functional genes that characterize the rhizosphere microbial community to understand metabolic capabilities in the maize rhizosphere using the GeoChip-based functional gene array method. Significant differences in functional gene structure were apparent between rhizosphere and bulk soil microbial communities. Approximately half of the detected gene families were significantly (p<0.05) increased in the rhizosphere. Based on the detected gyrB genes, Gammaproteobacteria, Betaproteobacteria, Firmicutes, Bacteroidetes and Cyanobacteria were most enriched in the rhizosphere compared to those in the bulk soil. The rhizosphere niche also supported greater functional diversity in catabolic pathways. The maize rhizosphere had significantly enriched genes involved in carbon fixation and degradation (especially for hemicelluloses, aromatics and lignin), nitrogen fixation, ammonification, denitrification, polyphosphate biosynthesis and degradation, sulfur reduction and oxidation. This research demonstrates that the maize rhizosphere is a hotspot of genes, mostly originating from dominant soil microbial groups such as Proteobacteria, providing functional capacity for the transformation of labile and recalcitrant organic C, N, P and S compounds.     

Multilocus Sequence Analysis of Nectar Pseudomonads Reveals High Genetic Diversity and Contrasting Recombination Patterns

The genetic and evolutionary relationships among floral nectar-dwelling Pseudomonas ‘sensu stricto’ isolates associated to South African and Mediterranean plants were investigated by multilocus sequence analysis (MLSA) of four core housekeeping genes (rrs, gyrB, rpoB and rpoD). A total of 35 different sequence types were found for the 38 nectar bacterial isolates characterised. Phylogenetic analyses resulted in the identification of three main clades [nectar groups (NGs) 1, 2 and 3] of nectar pseudomonads, which were closely related to five intrageneric groups: Pseudomonas oryzihabitans (NG 1); P. fluorescens, P. lutea and P. syringae (NG 2); and P. rhizosphaerae (NG 3). Linkage disequilibrium analysis pointed to a mostly clonal population structure, even when the analysis was restricted to isolates from the same floristic region or belonging to the same NG. Nevertheless, signatures of recombination were observed for NG 3, which exclusively included isolates retrieved from the floral nectar of insect-pollinated Mediterranean plants. In contrast, the other two NGs comprised both South African and Mediterranean isolates. Analyses relating diversification to floristic region and pollinator type revealed that there has been more unique evolution of the nectar pseudomonads within the Mediterranean region than would be expected by chance. This is the first work analysing the sequence of multiple loci to reveal geno- and ecotypes of nectar bacteria.     

Genetic Diversity of Escherichia coli Isolated from Urban Rivers and Beach Water

Repetitive element anchored PCR was used to evaluate the genetic profiles of Escherichia coli isolated from surface water contaminated with urban stormwater, sanitary sewage, and gull feces to determine if strains found in environmental samples reflect the strain composition of E. coli obtained from host sources. Overall, there was less diversity in isolates collected from river and beach sites than with isolates obtained from human and nonhuman sources. Unique strain types comprised 28.8, 29.2, and 15.0% of the isolate data sets recovered from stormwater, river water, and beach water, respectively. In contrast, 50.4% of gull isolates and 41.2% of sewage isolates were unique strain types. River water, which is expected to contain E. coli strains from many diffuse sources of nonpoint source pollution, contained strains most closely associated with other river water isolates that were collected at different sites or on different days. However, river sites impacted by sewage discharge had approximately 20% more strains similar to sewage isolates than did sites impacted by stormwater alone. Beach sites with known gull fecal contamination contained E. coli most similar to other beach isolates rather than gull isolates collected at these same sites, indicating underrepresentation of possible gull strains. These results suggest large numbers of strains are needed to represent contributing host sources within a geographical location. Additionally, environmental survival may influence the composition of strains that can be recovered from contaminated waters. Understanding the ecology of indicator bacteria is important when interpreting fecal pollution assessments and developing source detection methodology.     

Diversity of Pseudomonas spp. Isolated from Rice Rhizosphere Populations Grown along a Salinity Gradient

Along the coastline of Tamil Nadu, five sites were chosen to assess the diversity of Pseudomonas populations isolated from rice (Oryza sativa) cultivated along a salinity gradient. One of these sites was under organic farming while the other four were under inorganic farming. A total of 256 Pseudomonas strains isolated from these five sites were analyzed using both phenotypic (substrate utilization patterns and antibiotic resistance assay) and genotypic (PCR-RFLP of 16S rDNA) characteristics. The results derived from this study indicate that soil salinity affects rhizosphere Pseudomonas populations. It was observed that increasing salinity led to decreasing diversity. Fluorescent pseudomonads were the dominant species found in the non-saline site, while in the saline sites they were replaced by salt-tolerant species, in particular Pseudomonas alcaligenes and P. pseudoalcaligenes. An interesting observation was the increase in diversity found in the saline site under organic farming. Organic farming was found to be capable of mitigating the harmful effects of saline stress to a large extent, and restoring the Pseudomonas diversity, thereby making it comparable with the diversity encountered in the non-saline site.     

斜茎黄芪根瘤菌谷氨酰胺合成酶基因多样性研究

研究了22株代表性斜茎黄芪根瘤菌的谷氨酰胺合成酶基因多样性。首先对供试菌株进行了谷氨酰胺合成酶glnA和glnII基因扩增,结果显示从来自Mesorhizobium和Rhizobium属的多数代表菌株都可以扩增到约1kp、大小一致的glnA基因产物,而从Agrobacterium sp.的4株代表菌株未能得到glnA PCR扩增产物。基因glnII的扩增结果显示几乎从所有测试菌株都能够得到基因产物,来自Mesorhizobium septentrionale、M.temperatum和Mesorhizobium spp.的代表菌株都得到了单一的、大小约400bp～500bp的glnII PCR扩增产物,而从Agrobacterium sp.的4株代表菌株扩增得到的glnII PCR扩增产物明显不同于其它斜茎黄芪根瘤菌代表菌,它们都有一条约1 kb的特征PCR扩增产物条带,SDW052和R084还出现了另外2～3个扩增产物条带。此外,基因glnA的RFLP分析结果与我们先前的16S rRNA基因分析结果具有很好的一致性,这些结果都进一步证实了这些根瘤菌的染色体基因多样性。