Prey range characterization, ribotyping, and diversity of soil and rhizosphere Bdellovibrio spp. isolated on phytopathogenic bacteria

Thirty new Bdellovibrio strains were isolated from an agricultural soil and from the rhizosphere of plants grown in that soil. Using a combined molecular and culture-based approach, we found that the soil bdellovibrios included subpopulations of organisms that differed from rhizosphere bdellovibrios. Thirteen soil and seven common bean rhizosphere Bdellovibrio strains were isolated when Pseudomonas corrugata was used as prey; seven and two soil strains were isolated when Erwinia carotovora subsp. carotovora and Agrobacterium tumefaciens, respectively, were used as prey; and one tomato rhizosphere strain was isolated when A. tumefaciens was used as prey. In soil and in the rhizosphere, depending on the prey cells used, the concentrations of bdellovibrios were between 3 x 10(2) to 6 x 10(3) and 2.8 x 10(2) to 2.3 x 10(4) PFU g(-1). A prey range analysis of five soil and rhizosphere Bdellovibrio isolates performed with 22 substrate species, most of which were plant-pathogenic and plant growth-enhancing bacteria, revealed unique utilization patterns and differences between closely related prey cells. An approximately 830-bp fragment of the 16S rRNA genes of all of the Bdellovibrio strains used was obtained by PCR amplification by using a Bdellovibrio-specific primer combination. Soil and common bean rhizosphere strains produced two and one restriction patterns for this PCR product, respectively. The 16S rRNA genes of three soil isolates and three root-associated isolates were sequenced. One soil isolate belonged to the Bdellovibrio stolpii-Bdellovibrio starrii clade, while all of the other isolates clustered with Bdellovibrio bacteriovorus and formed two distantly related, heterogeneous groups.     

The rhizosphere effect on bacteria antagonistic towards the pathogenic fungus Verticillium differs depending on plant species and site

Rhizobacteria with antagonistic activity towards plant pathogens play an essential role in root growth and plant health and are influenced by plant species in their abundance and composition. To determine the extent of the effect of the plant species and of the site on the abundance and composition of bacteria with antagonistic activity towards Verticillium dahliae, bacteria isolated from the rhizosphere of two Verticillium host plants, oilseed rape and strawberry, and from bulk soil were analysed at three different locations in Germany over two growing seasons. A total of 6732 bacterial isolates screened for in vitro antagonism towards Verticillium resulted in 560 active isolates, among which Pseudomonas (77%) and Serratia (6%) were the most dominant genera. The rhizosphere effect on the antagonistic bacterial community was shown by an enhanced proportion of antagonistic isolates, by enrichment of specific amplified ribosomal DNA restriction analysis types, species and genotypes, and by a reduced diversity in the rhizosphere in comparison to bulk soil. Such an effect was influenced by the plant species and by the site of its cultivation. Altogether, 16S rRNA gene sequencing of 66 isolates resulted in the identification of 22 different species. Antagonists of the genus Serratia were preferentially isolated from oilseed rape rhizosphere, with the exception of one site. For isolates of Pseudomonas and Serratia, plant-specific and site-specific genotypes were found.     

热带药用植物根际放线菌的分离、鉴定及生物活性分析

从海南热带植物园采集12种药用植物的根际土样,采用选择性分离方法,分离得到400株根际放线菌。使用5种活性筛选模型对分离菌株进行生物活性评价,154株放线菌在一个或多个活性筛选模型中显示为阳性,菌株初筛阳性率达38.5%;根据菌株形态特征并结合代谢产物的生物活性,从中挑选出28株菌进行16S rRNA基因序列分析,发现其分属于链霉菌属、诺卡氏菌属、小单孢菌属和野野村菌属。     

Phylogeny, ribosomal RNA gene typing and relative abundance of new Pseudomonas species (sensu stricto) isolated from two pinyon-juniper woodland soils of the arid southwest U.S.

Rhizosphere-inhabiting Pseudomonas species interact with plant roots and may be important for plant performance under stressful environmental conditions. A comparison was conducted of culturable Pseudomonas isolates associated with pinyon rhizosphere and between-tree interspace areas in a hot, dry, volcanic cinder field and an adjacent sandy loam soil, in order to identify Pseudomonas species which may be involved in pinyon pine survival under stressful conditions. From a collection of 800 isolates, eleven isolates exhibiting different colony morphology were selected for 16S ribosomal RNA gene sequencing. Phylogenetic analysis of rDNA sequences from the eleven field isolates, forty-six described Pseudomonas species, and thirty-four previously characterized environmental isolates indicated that the isolates from the cinders and sandy loam soil clustered into three groups. The field isolates were distinct from any of the named species or other environmental isolates. Oligonucleotide primer pairs that differentiated three field isolate groups were designed from the 16S rDNA sequences, and eight hundred Pseudomonas field isolates cultured from pinyon rhizospheres and interspaces in the cinders and sandy loam soils were typed into the three groups using PCR assays. The composition of Pseudomonas populations in four environments was significantly different. The relative abundance of the three rDNA-based groups appeared to be affected by both the soil type and the pinyon rhizosphere.     

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.     

Diversity of pseudomonads isolated from three different plant rhizospheres

AIMS: To study the diversity of the Pseudomonas populations isolated from three different plant rhizospheres, namely pearl millet, cotton and paddy, grown in saline soils along the coastline of Southern India. METHODS AND RESULTS: The Pseudomonas populations were analysed for their biochemical characters and genetic diversity using molecular tools including RAPD and PCR-RFLP. The biochemical characterization, antibiotic resistance assay and RAPD profiles revealed a largely homogeneous population. Even in PCR-RFLP restriction studies, only two groups of isolates were seen. One group was predominant in all three rhizospheres, while the other minor group consisted of salt-sensitive isolates restricted to the paddy rhizosphere alone. CONCLUSIONS: It was observed that increasing salinity caused a predominant selection of salt-tolerant species, in particular Ps. pseudoalcaligenes and Ps. alcaligenes, irrespective of the host rhizosphere. SIGNIFICANCE AND IMPACT OF THE STUDY: This study has reinstated the importance of the soil over the host plant with regard to rhizosphere populations. It has also resulted in the isolation of several salt-tolerant Pseudomonas strains, which are being screened for their biological control activity against common plant pathogens of the coastal agri-ecosystem.     

Relationship of Pseudomonas syringae pv. tabaci races to the rhizosphere of Wisconsin-grown tobacco

Isolates of Pseudomonas syringae pv. tabaci, including 21 strains of the wildfire pathogen and 2 strains of the angular leafspot pathogen, were isolated from 143 rhizosphere and soil samples collected from 11 tobacco fields in Wisconsin. These pathogens were isolated by inoculating rhizosphere and soil washings into tobacco leaves and isolating the bacteria from wildfire or angular leafspot lesions that developed on the leaves. The wildfire isolates were from the rhizospheres of tobacco and Panicum capillare and from soil. While the majority of these were from wildfire-diseased fields, one isolate was from a field without disease symptoms; both angular leafspot isolates were from fields without angular leafspot symptoms. The majority of wildfire isolates were race 1, but three were race 0, and one was a new race. In three fields multiple races of wildfire were found. Both angular leafspot isolates were race 1. Two wildfire and one angular leafspot isolates were from fields where the cultivars were resistant to the races isolated.     

Influence of Two Plant Species (Flax and Tomato) on the Distribution of Nitrogen Dissimilative Abilities within Fluorescent Pseudomonas spp

The distribution of nitrogen-dissimilative abilities among 317 isolates of fluorescent pseudomonads was studied. These strains were isolated from an uncultivated soil and from the rhizosphere, rhizoplane, and root tissue of two plant species (flax and tomato) cultivated on this same soil. The isolates were distributed into two species, Pseudomonas fluorescens (45.1%) and Pseudomonas putida (40.4%), plus an intermediate type (14.5%). P. fluorescens was the species with the greatest proportion of isolates in the root compartments and the greatest proportion of dissimilatory and denitrifying strains. According to their ability to dissimilate nitrogen, the isolates have been distributed into nondissimilatory and dissimilatory strains, nitrate reducers and true denitrifiers with or without N(inf2)O reductase. The proportion of dissimilatory isolates was significantly enhanced in the compartments affected by flax and tomato roots (55% in uncultivated soil and 90 and 82% in the root tissue of flax and tomato, respectively). Among these strains, the proportion of denitrifiers gradually and significantly increased in the root vicinity of tomato (44, 68, 75, and 94% in uncultivated soil, rhizosphere, rhizoplane, and root tissue, respectively) and was higher in the flax rhizoplane (66%) than in the uncultivated soil. A higher proportion of N(inf2)O reducers was also found in the root compartments. This result was particularly clear for tomato. It is hypothesized that denitrification could be a selective advantage for the denitrifiers in the root environment and that this process could contribute to modify the specific composition of the bacterial communities in the rhizosphere.     

Analysis of rhizobacterial communities in perennial Graminaceae from polluted water meadow soil, and screening of metal-resistant, potentially plant growth-promoting bacteria

This study investigates the impact of long-term heavy metal contamination on the culturable, heterotrophic, functional and genetic diversity of rhizobacterial communities of perennial grasses in water meadow soil. The culturable heterotrophic diversity was investigated by colony appearance on solid LB medium. Genetic diversity was measured as bands in denaturing gradient gel electrophoresis (DGGE) obtained directly from rhizosphere soil and rhizoplane DNA extracts, and from the corresponding culturable communities. In the two rhizospheric fractions the DGGE profiles of the direct DNA extracts were similar and stable among replicates, whereas in the enriched cultures the profiles of the fractions differed, but among the replicates they were similar. One hundred isolates were collected into 33 different operational taxonomic units by use of amplified internal transcribed spacers and into 19 heavy metal-resistant phenotypes. The phylogenetic position of strains belonging to 18 operational taxonomic units, representing more than 80% of the isolates, was determined by 16S rRNA gene sequencing. Several heavy metal-resistant strains were isolated from rhizoplane. Finally, metal-resistant rhizobacteria were tested for plant growth-promoting characteristics; some were found to contain 1-aminocyclopropane-1-carboxylic acid deaminase and/or to produce indole acetic acid and siderophores. Two strains resistant to cadmium and zinc, Pseudomonas tolaasii RP23 and Pseudomonas fluorescens RS9, had all three plant growth-promoting characteristics. Our findings suggest that bacteria can respond to soil metal contamination, and the described methodological approach appears promising for targeting potential plant growth-promoting rhizobacteria.     

Diversity of streptomycetes among specific Greek terrestrial ecosystems

The diversity of streptomycetes isolated from different Greek terrestrial ecosystems using phenotypic identification, and the relationship between the number of species and the number of isolates as a diversity index, was studied. A total of 344 Streptomyces strains have been isolated and identified from diverse sites in the Greek territory, such as heavily disturbed agricultural areas and preserved forest areas, and from specific rhizosphere ecosystems. According to phenotypic identification, these strains belonged to 19 different cluster groups with a Willcox probability > 0·8. Streptomyces cyaneus , Strep. albidoflavus , Strep. diastaticus and Strep. exfoliatus were the most common cluster groups isolated from at least six different habitats. On the other hand, there were cluster groups that appeared in only one or two habitats, such as Strep. griseoflavus , Strep. rimosus , Streptoverticillium blastmyceticum , Nocardia mediterranea and Strep. fulvissimus . The diversity indices among the different cluster groups of each sampling area indicated that the different habitats can be sub-divided into two main groups: rhizosphere habitats and non-rhizosphere habitats, showing that the rhizosphere is one of the most important factors which determines the population structure of a specific soil area.     

Prey Range Characterization, Ribotyping, and Diversity of Soil and Rhizosphere Bdellovibrio spp. Isolated on Phytopathogenic Bacteria

Thirty new Bdellovibrio strains were isolated from an agricultural soil and from the rhizosphere of plants grown in that soil. Using a combined molecular and culture-based approach, we found that the soil bdellovibrios included subpopulations of organisms that differed from rhizosphere bdellovibrios. Thirteen soil and seven common bean rhizosphere Bdellovibrio strains were isolated when Pseudomonas corrugata was used as prey; seven and two soil strains were isolated when Erwinia carotovora subsp. carotovora and Agrobacterium tumefaciens, respectively, were used as prey; and one tomato rhizosphere strain was isolated when A. tumefaciens was used as prey. In soil and in the rhizosphere, depending on the prey cells used, the concentrations of bdellovibrios were between 3 × 10² to 6 × 10³ and 2.8 × 10² to 2.3 × 10⁴ PFU g⁻¹. A prey range analysis of five soil and rhizosphere Bdellovibrio isolates performed with 22 substrate species, most of which were plant-pathogenic and plant growth-enhancing bacteria, revealed unique utilization patterns and differences between closely related prey cells. An approximately 830-bp fragment of the 16S rRNA genes of all of the Bdellovibrio strains used was obtained by PCR amplification by using a Bdellovibrio-specific primer combination. Soil and common bean rhizosphere strains produced two and one restriction patterns for this PCR product, respectively. The 16S rRNA genes of three soil isolates and three root-associated isolates were sequenced. One soil isolate belonged to the Bdellovibrio stolpii-Bdellovibrio starrii clade, while all of the other isolates clustered with Bdellovibrio bacteriovorus and formed two distantly related, heterogeneous groups.     

Effect of Two Plant Species, Flax (Linum usitatissinum L.) and Tomato (Lycopersicon esculentum Mill.), on the Diversity of Soilborne Populations of Fluorescent Pseudomonads

Suppression of soilborne disease by fluorescent pseudomonads may be inconsistent. Inefficient root colonization by the introduced bacteria is often responsible for this inconsistency. To better understand the bacterial traits involved in root colonization, the effect of two plant species, flax (Linum usitatissinum L.) and tomato (Lycopersicon esculentum Mill.), on the diversity of soilborne populations was assessed. Fluorescent pseudomonads were isolated from an uncultivated soil and from rhizosphere, rhizoplane, and root tissue of flax and tomato cultivated in the same soil. Species and biovars were identified by classical biochemical and physiological tests. The ability of bacterial isolates to assimilate 147 different organic compounds and to show three different enzyme activities was assessed to determine their intraspecific phenotypic diversity. Numerical analysis of these characteristics allowed the clustering of isolates showing a high level (87.8%) of similarity. On the whole, the populations isolated from soil were different from those isolated from plants with respect to their phenotypic characteristics. The difference in bacteria isolated from uncultivated soil and from root tissue of flax was particularly marked. The intensity of plant selection was more strongly expressed with flax than with tomato plants. The selection was, at least partly, plant specific. The use of 10 different substrates allowed us to discriminate between flax and tomato isolates. Pseudomonas fluorescens biovars II, III, and V and Pseudomonas putida biovar A and intermediate type were well distributed among the isolates from soil, rhizosphere, and rhizoplane. Most isolates from root tissue of flax and tomato belonged to P. putida bv. A and to P. fluorescens bv. II, respectively. Phenotypic characterization of bacterial isolates was well correlated with genotypic characterization based on repetitive extragenic palindromic PCR fingerprinting.     

Biosynthesis of phytohormones from novel rhizobacterial isolates and their in vitro plant growth-promoting efficacy

The health of the plant and soil fertility is dependent on the plant–microbes interaction in the rhizosphere. Microbial life tends to endure various rhizosphere plant–microbe interactions. Phytohormones such as auxins, cytokinins, gibberellic acid, ethylene and abscisic acid are termed as the classical group of hormones. Out of the 70 rhizobacterial strains isolated from the Coleus rhizosphere, three different rhizobacterial strains Pseudomonas stutzeri MTP40, Stenotrophomonas maltophilia MTP42 and Pseudomonas putida MTP50 having plant growth-promoting attributes were isolated and characterized for its phytohormone-producing ability. The phytohormones such as indole 3-acetic acid (IAA), gibberellic acid and cytokinin (kinetin and 6-benzyladenosine) were affirmed in culture supernatant of the above isolates. IAA was detected in all the three isolates, where in highest production was found in S. maltophilia MTP42 (240?µg/mL) followed by P. stutzeri MTP40 (250?µg/mL) and P. putida MTP50 (233?µg/mL). Gibberellic acid production was found maximum in MTP40 (34?µg/mL), followed by MTP42 (31?µg/mL) and MTP50 (27?µg/mL). The cytokinin production from the isolates, namely, MTP40, MTP42 and MTP50 were 13, 11 and 7.5?µg/mL, respectively. The isolates showing the production of plant growth enhancing phytohormones can be commercialized as potent bioformulations.     

Comparative analysis of bacterial diversity in the rhizosphere of tomato by culture-dependent and -independent approaches

The microbiome in the rhizosphere–the region surrounding plant roots–plays a key role in plant growth and health, enhancing nutrient availability and protecting plants from biotic and abiotic stresses. To assess bacterial diversity in the tomato rhizosphere, we performed two contrasting approaches: culture-dependent and -independent. In the culture-dependent approach, two culture media (Reasoner’s 2A agar and soil extract agar) were supplemented with 12 antibiotics for isolating diverse bacteria from the tomato rhizosphere by inhibiting predominant bacteria. A total of 689 bacterial isolates were clustered into 164 operational taxonomic units (OTUs) at 97% sequence similarity, and these were found to belong to five bacterial phyla (Proteobacteria, Actinobacteria, Bacteroidetes, Acidobacteria, and Firmicutes). Of these, 122 OTUs were retrieved from the antibiotic-containing media, and 80 OTUs were recovered by one specific antibiotic-containing medium. In the culture-independent approach, we conducted Illumina MiSeq amplicon sequencing of the 16S rRNA gene and obtained 19,215 high-quality sequences, which clustered into 478 OTUs belonging to 16 phyla. Among the total OTUs from the MiSeq dataset, 22% were recovered in the culture collection, whereas 41% of OTUs in the culture collection were not captured by MiSeq sequencing. These results showed that antibiotics were effective in isolating various taxa that were not readily isolated on antibiotic-free media, and that both contrasting approaches provided complementary information to characterize bacterial diversity in the tomato rhizosphere.     

南京小龙山钾矿区植物根际可培养细菌的遗传多样性分析

摘要:【目的】矿区优势植物可培养细菌生物多样性研究将有助于了解植物根际细菌与矿物,植物根系相互作用及对矿物风化和土壤形成的重要影响。【方法】采用纯培养法分离南京小龙山废钾矿区野生植物野塘蒿,千金子和栽培植物甘薯根内与根周围土壤的可培养细菌, 通过16S rDNA限制性酶切多态性分析(amplified rDNA restriction analysis, ARDRA)和16S rDNA序列分析研究了可培养细菌的多样性。【结果】分离纯化到60株具不同菌落形态的可培养细菌, 在60%相似水平上可分为18个OTU. 19株代表菌株分别属于3个门, 10个科, 11个属。多数菌株属于变形菌门(α-proteobacteria, 4株, 21.1%; β-proteobacteria, 2株, 10.5%; γ-proteobacteria, 6株, 31.6%)。假单胞菌属(Pseudomonas), 泛菌属(Pantoea)和根瘤菌属(Rhizobium)为优势种群。【结论】小龙山废矿区优势植物根围具有丰富的微生物种群多样性。     

Impact of plant species and site on rhizosphere-associated fungi antagonistic to Verticillium dahliae kleb

Fungi with antagonistic activity toward plant pathogens play an essential role in plant growth and health. To analyze the effects of the plant species and the site on the abundance and composition of fungi with antagonistic activity toward Verticillium dahliae, fungi were isolated from oilseed rape and strawberry rhizosphere and bulk soil from three different locations in Germany over two growing seasons. A total of 4,320 microfungi screened for in vitro antagonism toward Verticillium resulted in 911 active isolates. This high proportion of fungi antagonistic toward the pathogen V. dahliae was found for bulk and rhizosphere soil at all sites. A plant- and site-dependent specificity of the composition of antagonistic morphotypes and their genotypic diversity was found. The strawberry rhizosphere was characterized by preferential occurrence of Penicillium and Paecilomyces isolates and low numbers of morphotypes (n = 31) and species (n = 13), while Monographella isolates were most frequently obtained from the rhizosphere of oilseed rape, for which higher numbers of morphotypes (n = 41) and species (n = 17) were found. Trichoderma strains displayed high diversity in all soils, but a high degree of plant specificity was shown by BOX-PCR fingerprints. The diversity of rhizosphere-associated antagonists was lower than that of antagonists in bulk soil, suggesting that some fungi were specifically enriched in each rhizosphere. A broad spectrum of new Verticillium antagonists was identified, and the implications of the data for biocontrol applications are discussed.     

Effect of soil type and plant species on the fluorescent pseudomonads nitrate dissimilating community

The distribution of nitrogen dissimilative abilities among 618 isolates of fluorescent pseudomonads was studied. These strains were isolated from two uncultivated soils (C and D; collected at Chateaurenard and Dijon, France, respectively) and from rhizosphere, rhizoplane and root tissue of two plant species (flax and tomato) cultivated on these two soils. According to their ability to dissimilate nitrogen, the isolates have been distributed into three metabolic types: non-dissimilators, NO₂ ^- accumulators and denitrifiers. While the three metabolic types were recovered in all the compartments of soil D experiments, only two (non-dissimilators and denitrifiers) were recovered in all the compartments of soil C experiments. Even under the contrasting conditions of the two soil types, both plants were able to select the nitrate dissimilating community among the total community of fluorescent Pseudomonas, but the mode of this selection seems to be dependent on both plant and soil type. The soil type appears to be unable to significantly modulate the strong selective effect of tomato. Indeed, similar dissimilator to non-dissimilator ratios were found in the root tissue of this plant species cultivated in both soils. In contrast, the different dissimilator to non-dissimilator ratios observed in flax roots between soils C and D suggest that the selective effect of flax was modulated by the soil type. Taxonomic identifications showed that the 618 isolates were distributed among three species (P. chlororaphis, P. fluorescens, P. putida) plus an intermediate type between P. fluorescens and P. putida. However, no clear relationship between the distribution of the metabolic types (functional diversity) and the distribution of bacterial species has been found. This revised version was published online in June 2006 with corrections to the Cover Date.     

茅尾海红树根际土壤可培养细菌多样性及抑菌活性研究

为挖掘茅尾海红树植物根际土壤可培养细菌资源,研究其抑菌活性,该文使用培养基对茅尾海5种红树植物(红海榄、黄槿、无瓣海桑、桐花树、阔苞菊)的7份根际土壤进行富集培养,选用6种不同分离培养基对富集样品进行可培养细菌的分离纯化,基于菌株16S rRNA基因序列信息进行物种多样性分析,并采用纸片法筛选具有抑制表皮葡萄球菌、耐甲氧西林金黄色葡萄球菌、铜绿假单胞菌活性的菌株。结果表明:(1)从7份红树植物根际土壤样品中分离到120种可培养细菌,隶属于35科47属,其中链霉菌属(Streptomyces sp.)占菌种总数的 14.2%,同时发现5种潜在新菌株。(2)通过抑菌活性初筛,发现9种细菌的发酵粗提物对至少一种致病菌具有抑菌活性。综上表明,茅尾海红树植物根际土壤可培养细菌多样性丰富,并且部分菌株具有抑制人类致病菌活性。该研究结果为新型抗生素的开发与利用提供了菌种资源。     

Comparative identification by fatty acid analysis of soil,rhizosphere, and geocarposphere bacteria of peanut (Arachis hypogaea L.)

Bacterial isolates were collected from the geocarposphere, rhizosphere, and root-free soil of field grown peanut (Arachis hypogaea L.) at three sample dates, and the isolates were identified by analysis of fatty acid methyl-esters to determine if qualitative differences exist among the bacterial microflora of these zones. Five bacterial genera were associated with isolates from soil, while pod and root isolates constituted 16 and 13 genera, respectively, indicating that bacterial diversity was higher in the rhizosphere and geocarposphere than in soil. The dominant (most frequently identified) genus across all three samples dates was Flavobacterium, for pods, Pseudomonas for roots, and Bacillus, for root-free soil. Sixteen bacterial taxa were only isolated from the geocarposphere, 7 only from the rhizosphere, and 5 only from soil. These results show that specific bacterial taxa are preferentially adapted to colonization of the geocarposphere and suggest that the soil, rhizosphere, and geocarposphere constitute three distinct ecological niches. Bacteria which colonize the geocarposphere should be examined as potential biological control agents for pod-invading fungi such as the toxigenic strains of Aspergillus flavus and A. parasiticus.     

土霉素暴露对小麦根际抗生素抗性细菌及土壤酶活性的影响

通过培养的方法研究了土霉素暴露和小麦根际抗性细菌的数量、种类、分布特征及土壤酶活性之间的剂量效应关系。结果表明,土霉素暴露下小麦根际单一抗生素抗性细菌数量和抗土霉素—链霉素双重抗性细菌数都明显增加,且与暴露剂量呈正效应关系;同时,土壤磷酸酶、脱氢酶活性下降,但与土霉素的剂量效应关系不明显。从土霉素暴露的土壤中分离到50株抗性细菌,经形态观察、RFLP分组和16S rDNA序列测定与分析,将它们聚集在Actinobacteria、Bacilli、Alphaproteobacteria、Gammaproteobacteria 和Sphingobacteria类群。其中放线菌最多（15株）,占抗性菌总数的30 %;其次是Bacillus属细菌（9株）和Pseudomonas属细菌（8株）,分别占18 %和16 %。同时,具有抗性的人类机会致病菌Pseudomonas、Sphingomonas和Stenotrophomonas属细菌在土霉素暴露的样品中均被分离到,分别占抗性菌株总数的16 %、8 %和4 %。值得注意的是,随着土霉素暴露剂量的增加,小麦根际优势促生菌Bacillus属细菌的抗性检出率逐步降低;但具有抗生素抗性的人类机会致病菌Pseudomonas、Sphingomonas和Stenotrophomonas属细菌的检出率却明显增加,提示可能会进一步增大其机会致病性。