Characterization of halophiles isolated from solar salterns in Baja California, Mexico

Solar salterns are extreme hypersaline environments that are five to ten times saltier than seawater (150–300 g L⁻¹ salt concentration) and typically contain high numbers of halophiles adapted to tolerate such extreme hypersalinity. Thirty-five halophile cultures of both Bacteria and Archaea were isolated from the Exportadora de Sal saltworks in Guerrero Negro, Baja California, Mexico. 16S rRNA sequence analysis showed that these cultured isolates included members belonging to the Halorubrum, Haloarcula, Halomonas, Halovibrio, Salicola, and Salinibacter genera and what may represent a new archaeal genus. For the first time, metabolic substrate usage of halophile isolates was evaluated using the non-colorimetric BIOLOG Phenotype MicroArray™ plates. Unique carbon substrate usage profiles were observed, even for closely related Halorubrum species, with bacterial isolates using more substrates than archaeal cultures. Characterization of these isolates also included morphology and pigmentation analyses, as well as salinity tolerance over a range of 50–300 g L⁻¹ salt concentration. Salinity optima varied between 50 and 250 g L⁻¹ and doubling times varied between 1 and 12 h. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Chitinolytic Activity of Cold Tolerant Antagonistic Species of <Emphasis Type="Italic">Streptomyces</Emphasis> Isolated from Glacial Sites of Indian Himalaya

Seventy-eight isolates of actinomycetes were isolated from the soil samples collected from alpine zones of Pindari glacier region in Indian Himalaya. Following a plate based rapid screening using two test fungi, five efficient isolates (nos. HA1, HA2, HA6, HA40, and HA142) were selected for further characterization with special reference to their antagonistic properties. Based on phenotypic and genotypic characters, the isolates were identified up to species level. All the isolates belonged to the genus Streptomyces. The isolate nos. HA1 and HA2 were S. sampsonii and HA6, HA40 and HA142 were S. griseobrunneus, S. aurantiacus, and S. griseoluteus, respectively. The isolates showed strong antifungal properties against phytopathogenic test fungi in plate assays. All the isolates hydrolyzed glycol–chitin as a substrate in denaturing conditions showing variable amount of different isoforms.     

Biosystematics of alkaliphilic streptomycetes isolated from seven locations across a beach and dune sand system

Alkaliphilic streptomycetes were isolated from composite sand samples collected from six out of seven locations across a beach and dune sand system using starch-casein-nitrate agar supplemented with cycloheximide and buffered to pH 10.5. The isolates had colonial and chemotaxonomic properties consistent with their classification in the genus Streptomyces. They were assigned to 49 multimembered and 114 single-membered colour-groups given their ability to produce pigments on oatmeal and peptone-yeast-extract-iron agars and to corresponding taxa based on whole-genome rep-PCR banding patterns. Twenty-four isolates representing the colour and rep-PCR groups grew well from pH 5 to 11, and optimally at pH 9, as did phylogenetically close members of the Streptomyces griseus 16S rRNA gene clade. One hundred and twelve representative alkaliphilic streptomycetes formed a heterogeneous but distinct clade in the Streptomyces 16S rRNA gene tree. A 3-dimensional representation of 16S rRNA sequence data showed that the alkaliphilic streptomycetes formed a distinct group in multidimensional taxospace. It is evident that alkaliphilic streptomycetes are common in the beach and dune sand system and that representatives of this community form new centers of taxonomic variation within the genus Streptomyces that can be equated with species. GenBank accession numbers for the 16S rRNA gene sequences for the strains of the alkaliphilic streptomycetes Bd 095, Bd 064, Bd 077, Bd 013, Bd 108, Bd 088, Bd 012, Bd 187, Bd 128, Bd 174, Bd 167, Lt 005, Lt 006, Fd 015, Bd 099, Bd 059, Bd 159, Ht 015, Md 005, Ht 020, Bd 205, Md 063, Fd 004, Md 039 and Bd 092 are EU477215, EU477216, EU477217, EU477218, EU477219, EU477220, EU477221, EU477222, EU477223, EU477224, EU477225, EU477226, EU477227, EU477228, EU477229, EU477230, EU477231, EU477232, EU477233, EU477234, EU477235, EU477236, EU477237, EU477238 and EU477257, respectively.     

Streptomyces phytohabitans sp. nov., a novel endophytic actinomycete isolated from medicinal plant Curcuma phaeocaulis

A novel actinomycete, designated strain KLBMP 4601^T, was isolated from the root of the medicinal plant Curcuma phaeocaulis collected from Sichuan Province, south-west China. The strain produced extensively branched substrate and aerial hyphae that carried straight to flexuous spore chains. Chemotaxonomic properties of this strain were consistent with those of members of the genus Streptomyces. The cell wall of strain KLBMP 4601^T contained ll-diaminopimelic acid as the characteristic diamino acid. The major menaquinone was MK-9(H₄), with minor amounts of MK-9(H₆), MK-9(H₈) and MK-10(H₂). The major fatty acids were C_16:0, iso-C_16:0, C_18:1ω9c and C_16:1, iso G. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain KLBMP 4601^T belongs to the genus Streptomyces and is most closely related to Streptomyces armeniacus JCM 3070^T (97.9 %), Streptomyces pharmamarensis PM267^T (97.6 %) and Streptomyces artemisiae YIM 63135^T (97.5 %). The 16S rRNA gene sequence similarity between strain KLBMP 4601^T and other members of this genus were lower than 97.5 %. DNA–DNA hybridization studies of strain KLBMP 4601^T with the three closest species showed relatedness values of 36.3 ± 4.2 %, 27.3 ± 0.6 %, and 30.9 ± 2.5 %, respectively. On the basis of chemotaxonomic, phenotypic and genotypic characteristics, it is evident that strain KLBMP 4601^T represents a novel species of the genus Streptomyces, for which the name Streptomyces phytohabitans sp. nov. is proposed. The type strain is KLBMP 4601^T (=KCTC 19892^T = NBRC 108772^T).     

Genomics of Sponge-Associated Streptomyces spp. Closely Related to Streptomyces albus J1074: Insights into Marine Adaptation and Secondary Metabolite Biosynthesis Potential

A total of 74 actinomycete isolates were cultivated from two marine sponges, Geodia barretti and Phakellia ventilabrum collected at the same spot at the bottom of the Trondheim fjord (Norway). Phylogenetic analyses of sponge-associated actinomycetes based on the 16S rRNA gene sequences demonstrated the presence of species belonging to the genera Streptomyces, Nocardiopsis, Rhodococcus, Pseudonocardia and Micromonospora. Most isolates required sea water for growth, suggesting them being adapted to the marine environment. Phylogenetic analysis of Streptomyces spp. revealed two isolates that originated from different sponges and had 99.7% identity in their 16S rRNA gene sequences, indicating that they represent very closely related strains. Sequencing, annotation, and analyses of the genomes of these Streptomyces isolates demonstrated that they are sister organisms closely related to terrestrial Streptomyces albus J1074. Unlike S. albus J1074, the two sponge streptomycetes grew and differentiated faster on the medium containing sea water. Comparative genomics revealed several genes presumably responsible for partial marine adaptation of these isolates. Genome mining targeted to secondary metabolite biosynthesis gene clusters identified several of those, which were not present in S. albus J1074, and likely to have been retained from a common ancestor, or acquired from other actinomycetes. Certain genes and gene clusters were shown to be differentially acquired or lost, supporting the hypothesis of divergent evolution of the two Streptomyces species in different sponge hosts.     

Molecular characterization of rhizospheric soil streptomycetes isolated from indigenous Turkish plants and their antimicrobial activity

As part of a research program whose aim is to determine the diversity of streptomycetes in order to discover new bioactive secondary metabolites, rhizosphere soils of three indigenous plants were analyzed. A total of 55 actinomycetes were isolated using three different medium from the samples. The rhizospheric soil of the plant Aethionema dumanii gave the highest number of actinomycetes, i.e., 42% versus 27% and 31% for the soils from Salvia aytachii and Achillea ketenoglui, respectively. The AIA is the most favorable medium for the isolation of the actinomycetes from different rhizospheric soils. 16S rDNA sequence analysis revealed that while some isolates belong to different cluster groups such as Streptomyces lydicus, S. rochei, S. microflavus, S. griseoflavus, S. albidoflavus and S. violaceusniger, the majority of the sequences did not considerable clustered with the member of different Streptomyces groups. The in vitro antimicrobial activities of the crude organic and aqueous extracts of isolates were screened using a disc diffusion assay against a panel of bacteria and C. albicans. A total of 22 isolates showed antimicrobial activity. The antibacterial action of the extracts is more pronounced on Gram-positive than on Gram-negative bacteria in most cases. About 18% of the actinomycetes showed also antifungal activity. Study of the influence of two different culture media on production of bioactive molecules showed that the higher antimicrobial activity was obtained in M2 when compared to TSB. The results from this study provide evidence that the streptomycetes in the rhizosphere soils could be promising sources for antimicrobial bioactive agents.     

Culturable Actinobacteria from the Marine Sponge Hymeniacidon perleve: Isolation and Phylogenetic Diversity by 16S rRNA gene-RFLP Analysis

A total of 106 actinobacteria associated with the marine sponge Hymeniacidon perleve collected from the Yellow Sea, China were isolated using eight different media. The number of species and genera of actinobacteria recovered from the different media varied significantly, underlining the importance of optimizing the isolation conditions. The phylogenetic diversity of the actinobacteria isolates was assessed using 16S rRNA gene amplification–restriction fragment length polymorphism (RFLP) analysis of the 106 strains with different morphologies. The RFLP fingerprinting of selected strains by HhaI-digestion of the 16S rRNA genes resulted in 11 different patterns. The HhaI-RFLP analysis gave good resolution for the identification of the actinobacteria isolates at the genus level. A phylogenetic analysis using 16S rRNA gene sequences revealed that the isolates belonged to seven genera of culturable actinobacteria including Actinoalloteichus, Micromonospora, Nocardia, Nocardiopsis, Pseudonocardia, Rhodococcus, and Streptomyces. The dominant genus was Streptomyces, which represented 74% of the isolates. Three of the strains identified are candidates for new species.     

Characterization and Identification of a Novel Marine Streptomyces sp. Produced Antibacterial Substance

Strain GB-2 is a marine microbe with broad-spectrum antimicrobial activity, isolated from soil taken from the coastal city Lianyungang in the JiangSu province of China. Analysis of its morphological, physiological, and biochemical characteristics as well as chemical components of the cell wall strongly suggested that the strain GB-2 belonged to the Streptomyces sp. Analysis of the nucleotide sequence of the 16S rRNA gene of Streptomyces sp. GB-2 strain showed a strong similarity (98%) with the 16 rRNA gene of Streptomyces fradiae. Application to antibacterial substance of strain Streptomyces sp. GB-2 by various separation steps led to isolation of one active molecule having a retention time of 9.495 min, P _9.495 min, which possessed antibacterial activity against Bacillus cereus and Escherichia coli. Through analysis by liquid chromatography–mass spectrometry and mass/mass spectrometry of the peak, the molecular weight of the antibacterial substance (P _9.495 min sample) was 447.5 Da and it was determined to be sisomicin according to the analysis of ion fragments. Nuclear magnetic resonance spectrum of the peak also demonstrated that the antibacterial substance was sisomicin. This study is the first to introduce the finding of sisomicin produced from marine Streptomyces sp. This work provides a preference for the production of sisomicin in pharmaceutical industries and a probability for studying the biodiversity of marine microbe.     

Diversity of the culturable lichen-derived actinobacteria and the taxonomy of Streptomyces parmotrematis sp. nov.

A total of 37 actinobacteria were isolated from eighteen lichen samples collected in Thailand. Based on the 16S rRNA gene sequences, they were identified into five genera including Actinoplanes (1 strain), Actinomadura (1 strain), Pseudosporangium (1 strain), Wangella (1 strain) and Streptomyces (33 strains). Among these isolates, strain Ptm05^T, Ptm01 and Ptm12 showed low 16S rRNA gene similarity and was selected for the further taxonomic study using the polyphasic approach. These strains showed the highest 16S rRNA gene sequence similarity with Streptomyces sparsogenes ATCC 25498^T (97.44–97.72%). Strain Ptm05^T was selected for the type strain. The chemical cell composition of the strain was similar to the members of Streptomyces genus. LL-diaminopimelic acids were detected in the peptidoglycan. Menaquinones were MK-9(H₈) and MK-9(H₆). Phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol, phosphatidylinositol mannoside, one unidentified phospholipid, one unidentified glycolipid and one unidentified lipid were detected as the polar lipids. The predominant cellular fatty acids are anteiso-C_15:0, iso-C_15:0, iso-C_16:0, iso-C_17:0 and C_16:0. The dDNA-DNA hybridization values among strain Ptm05^T and its closely related Streptomyces type strains were 17.2–18.0%. In addition, the ANIb and ANIm between strain Ptm05^T and related Streptomyces type strains were ranged from 75.69 to 76.13% and 85.21 to 85.35%, respectively. Based on phenotypic and genomic evidence, strain Ptm05^T (=?TBRC 14546^T?=?NBRC 115203^T) represents the novel species of the genus Streptomyces for which the name Streptomyces parmotrematis sp. nov. is proposed. This study showed that the lichens are the promising source of the novel actinobacterial taxa.

     

Biologically Active Endophytic Streptomycetes from <Emphasis Type="Italic">Nothofagus</Emphasis> spp. and Other Plants in Patagonia

Endophytic streptomycetes have been isolated and characterized from several species of Nothofagus and other plants growing in the southern reaches of Patagonia. No endophytic streptomycete was obtained from any plant species studied in Northern Patagonia. However, from Southern Patagonia, biologically active Streptomyces spp. from several plant species were isolated. Each isolate, as studied by scanning electron microscopy (SEM), has small hyphae, some produce typical barrel-shaped spores in culture and each has some unique hyphal surface structures. Interestingly, although none has any detectable antibacterial killing properties, each has demonstrable killing activity against one or more pathogenic fungi including representative plant pathogenic organisms such as Phytophthora erythroseptica, Pythium ultimum, Sclerotinia sclerotiorum, Mycosphaerella fijiensis, and Rhizoctonia solani. The 16S rDNA sequences of the isolates were distinct from all other genetic accessions of Streptomyces in GenBank. However, isolate C-2 from Chiliotrichum diffusum (Compositae) is identical, in all respects, to isolate C-4 obtained from Misodendrum punctulatum (Loranthaceae). These results confirm that endophytic streptomycetes represent a novel source of biologically active microorganisms.     

<Emphasis Type="Italic">Streptomyces fildesensis</Emphasis> sp. nov., a novel streptomycete isolated from Antarctic soil

A novel actinomycete strain, GW25-5^T, was isolated from a soil sample collected from the Fildes Peninsula, King George Island, West Antarctica. The strain was characterized by white to grey aerial mycelia, which were differentiated to straight to flexuous spore chains, with rod-shaped smooth spores. The cell wall of strain GW25-5^T contained LL-diaminopimelic acid (A₂pm) and traces of meso-A₂pm. Whole-cell sugars were galactose and minor amounts of mannose and glucose. The predominant menaquinones were MK-9(H₆) (49%), MK-9(H₈) (24%) and MK-9(H₄) (12%). The phospholipids contained DPG, PE, PI, PIM and PL(s). The major cellular fatty acids were iso-C_16:0 and anteiso-C_15:0. Genomic DNA G+C content of strain GW25-5^T was 70.0 mol%. BLAST result showed that strain GW25-5 has the 16S rRNA gene sequence highest similarity of 97.5% with members of genus Streptomyces and phylogenetic analysis indicated that this strain belongs to the genus Streptomyces. DNA–DNA relatedness values of strain GW25-5^T with the closest species of Streptomyces purpureus LMG 19368^T and Streptomyces beijiangensis YIM 6^T were significantly lower than 70% of the threshold value for the delineation of genomic species. A polyphasic taxonomic investigation based on a judicious combination of genotypic and phenotypic characteristics revealed that the organism represents a novel species of the genus Streptomyces. Thus, we propose strain GW25-5^T as the type strain of this novel species, Streptomyces fildesensis (=CGMCC 4.5735^T = YIM 93602^T = DSM 41987^T = NRRL B 24828^T).     

Rapid discrimination of potato scab-causing <Emphasis Type="Italic">Streptomyces</Emphasis> species based on the RNase P RNA gene sequences

Scab disease significantly damages potatoes and other root crops. Some Streptomyces species have been reported as potato-scab pathogens. Identification of the phytopathogenic Streptomyces is mainly done on the basis of the 16S rRNA gene, but use of this gene has some limitations for discriminating these strains because they share high similarities of 16S rRNA gene sequences. We tested the RNase P RNA (rnpB) gene as a taxonomic marker to clarify the relationship among closely related scab-causing Streptomyces strains. The rnpB genes were analyzed for 41 strains including 9 isolates from Jeju, Korea. There were 4 highly variable regions including nucleotide gaps in the rnpB genes. Interspecies similarity of the rnpB gene in tested Streptomyces strains was lower than about 97%, while the intraspecies similarity was higher than about 98%. Phylogenetic analysis demonstrated that the rnpB tree has similar topology to the 16S rRNA gene tree, but produces a more divergent phyletic lineage. These results revealed that the rnpB gene could be used as a powerful taxonomic tool for rapid differentiation of closely related Streptomyces species. In addition, it was also suggested that the variable regions marked as α, β, γ, and δ in the rnpB gene could be useful markers for the detection of specific Streptomyces species.     

Influence of Long Term Irrigation with Pulp and Paper Mill Effluent on the Bacterial Community Structure and Catabolic Function in Soil

Microbial communities play a vital role in maintaining soil health. A multiphasic approach to assess the effect of pulp and paper mill effluent on both the structure and function of microbial soil communities is taken. Bacterial communities from agricultural soils irrigated with pulp and paper mill effluent were compared to communities form soils irrigated with well water. Samples were taken from fields in the state of Uttarakhand, India, where pulp and paper mill effluent has been used for irrigation for over 25 years. Comparisons of bacterial community structure were conducted using sequencing of the 16S rRNA gene from both isolates and clone libraries attained from the soil. Community-level physiological profiling was used to characterize the functional diversity and catabolic profile of the bacterial communities. The multiphasic approach using both physiological and molecular techniques proved to be a powerful tool in evaluating the soil bacterial community population and population differences therein. A significant and consistent difference in the population structure and function was found for the bacterial communities from soil irrigated with effluent in comparison to fields irrigated with well water. The diversity index parameters indicated that the microbial community in pulp and paper mill effluent irrigated fields were more diverse in both structure and function. This suggests that the pulp and paper mill effluent is not having a negative effect on the soil microbial community, but in fact may have a positive influence. In terms of soil health, this finding supports the continued use of pulp and paper mill effluent for irrigation. This is however only one aspect of soil health which was evaluated. Further studies on soil resistance and robustness could be undertaken to holistically evaluate soil health in this situation.     

Actinobacterial diversity from marine sediments collected in Mexico

Seventeen different media known to support the growth and isolation of members of the class Actinobacteria were evaluated as selective isolation media for the recovery of this microbial group from marine sediments samples collected in the Gulf of California and the Gulf of Mexico. A general selective isolation procedure was employed for six sediments and nearly 300 actinomycetes were recovered from the selective isolation plates. Full 16S rRNA gene sequencing revealed that the isolates belonged to several actinobacterial taxa, notably to the genera Actinomadura, Dietzia, Gordonia, Micromonospora, Nonomuraea, Rhodococcus, Saccharomonospora, Saccharopolyspora, Salinispora, Streptomyces, “Solwaraspora” and Verrucosispora. Previous works on marine sediments have been restricted to the isolation of members of the genera Micromonospora, Rhodococcus and Streptomyces. This study provides further evidence that Actinobacteria present in marine habitats are not restricted to the Micromonospora-Rhodococcus-Streptomyces grouping. Indeed, this first systematic study shows the extent of actinobacterial diversity that can be found in marine sediments collected in Mexico and probably, worldwide. The 16S rRNA gene sequences of marine isolates A1, AA2, AA6, AB1, AB2, AG1, AI2, AK1, AL2, AO1, AO3, AR1, AW1, B1, BB1, BC1, C5, R1, R2, R3, AV1, AE1, AI1, AN1 and AP1 determined in this study have been deposited under GenBank accession numbers EU714241–EU714258 and FJ462359–FJ462365, respectively.     

Verminephrobacter aporrectodeae sp. nov. subsp. tuberculatae and subsp. caliginosae, the specific nephridial symbionts of the earthworms Aporrectodea tuberculata and A. caliginosa

Clone library-based studies have shown that almost all lumbricid earthworm species harbour host-specific symbiotic bacteria belonging to the novel genus Verminephrobacter in their nephridia (excretory organs). To date the only described representative from this genus is Verminephrobacter eiseniae, the specific symbiont of the earthworm Eisenia fetida. In this study two novel rod-shaped, non-endosporeforming, betaproteobacterial symbionts were isolated from the nephridia of two closely related earthworm species. Both isolates were affiliated with the genus Verminephrobacter by 16S rRNA gene sequence analysis. Similarly to V. eiseniae, the two isolates grew aerobically with a preference for low oxygen concentrations on a range of sugars, fatty acids and amino acids and fermentatively on glucose and pyruvate. These phenotypes match well with the conditions reported or inferred for the nephridial environment. Based on 16S rRNA gene similarity, DNA–DNA hybridization value and phenotypic characteristics the two isolates are clearly distinct from V. eiseniae. Phenotypic characteristics could not clearly differentiate the two strains as separate species but a low DNA–DNA hybridization value of 57.3%, their earthworm host specificity, differing temperature ranges and pH optima suggest that they represent two subspecies of a novel species of Verminephrobacter. For this species, the name V. aporrectodeae sp. nov. is proposed, with the two subspecies V. aporrectodeae subsp. tuberculatae (type strain, At4^T = DSM 21361^T = LMG 25313^T) and V. aporrectodeae subsp. caliginosae (type strain, Ac9^T = DSM 21895^T = LMG 25312^T) isolated from the nephridia of the earthworms Aporrectodea tuberculata and A. caliginosa, respectively.     

Use of quantitative real-time PCR to monitor population dynamics of ammonia-oxidizing bacteria in batch process

A quantitative real-time PCR (QPCR) assay with the TaqMan system was used to quantify 16S rRNA genes of β-proteobacterial ammonia-oxidizing bacteria (AOB) in a batch nitrification bioreactor. Five different sets of primers, together with a TaqMan probe, were used to quantify the 16S rRNA genes of β-proteobacterial AOB belonging to the Nitrosomonas europaea, Nitrosococcus mobilis, Nitrosomonas nitrosa, and Nitrosomonas cryotolerans clusters, and the genus Nitrosospira. We also used PCR followed by denaturing gradient gel electrophoresis (DGGE), cloning, and sequencing of their 16S rRNA genes to identify the AOB species. Seed sludge from an industrial wastewater treatment process controlling high-strength nitrogen wastewater (500 mg/L NH₄ ⁺–N) was used as the inoculum for subsequent batch experiment. The Nitrosomonas nitrosa cluster was the predominant AOB (2.3 × 10⁵ copies/mL) in the start-up period of the batch experiment. However, from the exponential growth period, the Nitrosomonas europaea cluster was the most abundant AOB, and its 16S rRNA gene copy number increased to 8.9 × 10⁶ copies/mL. The competitive dominance between the two AOB clusters is consistent with observed differences in ammonia tolerance and substrate affinity. Analysis of the DGGE results indicated the presence of Nitrosomonas europaea ATCC19718 and Nitrosomonas nitrosa Nm90, consistent with the QPCR results.     

Diazotrophic diversity in the rhizosphere of two exotic weed plants, Prosopis juliflora and Parthenium hysterophorus

This study is aimed at assessing culturable diazotrophic bacterial diversity in the rhizosphere of Prosopis juliflora and Parthenium hysterophorus, which grow profusely in nutritionally-poor soils and environmentally-stress conditions so as to identify some novel strains for bioinoculant technology. Diazotrophic isolates from Prosopis and Parthenium rhizosphere were characterized for nitrogenase activity by Acetylene Reduction Assay (ARA) and 16S rRNA gene sequencing. Further, the culture-independent quantitative PCR (qPCR) was performed to compare the abundance of diazotrophs in rhizosphere with bulk soils. The proportion of diazotrophs in total heterotrophs was higher in rhizosphere than bulk soils and 32 putative diazotrophs from rhizosphere of two plants were identified by nifH gene amplification. The ARA activity of the isolates ranged from 40 to 95 nmol ethylene h⁻¹ mg protein⁻¹. The 16S rRNA gene analysis identified the isolates to be members of alpha, beta and gamma Proteobacteria and firmicutes. The qPCR assay also confirmed that abundance of nif gene in rhizosphere of these two plants was 10-fold higher than bulk soil.     

Selection and Characterization of Microorganisms Utilizing Thaxtomin A, a Phytotoxin Produced by Streptomyces scabies

Thaxtomin A is the main phytotoxin produced by Streptomyces scabies, a causal agent of potato scab. Thaxtomin A is a yellow compound composed of 4-nitroindol-3-yl-containing 2,5-dioxopiperazine. A collection of nonpathogenic streptomycetes isolated from potato tubers and microorganisms recovered from a thaxtomin A solution were examined for the ability to grow in the presence of thaxtomin A as a sole carbon or nitrogen source. Three bacterial isolates and two fungal isolates grew in thaxtomin A-containing media. Growth of these organisms resulted in decreases in the optical densities at 400 nm of culture supernatants and in 10% reductions in the thaxtomin A concentration. The fungal isolates were identified as a Penicillium sp. isolate and a Trichoderma sp. isolate. One bacterial isolate was associated with the species Ralstonia pickettii, and the two other bacterial isolates were identified as Streptomyces sp. strains. The sequences of the 16S rRNA genes were determined in order to compare thaxtomin A-utilizing actinomycetes to the pathogenic organism S. scabies and other Streptomyces species. The nucleotide sequences of the γ variable regions of the 16S ribosomal DNA of both thaxtomin A-utilizing actinomycetes were identical to the sequence of Streptomyces mirabilis ATCC 27447. When inoculated onto potato tubers, the three thaxtomin A-utilizing bacteria protected growing plants against common scab, but the fungal isolates did not have any protective effect.     

Isolation of Endophytic Actinomycetes from Different Cultivars of Tomato and their Activities Against Ralstonia solanacearum in Vitro

The populations of endophytic actinomycetes from healthy and wilting tomato plants (tomato cultivars resistant and susceptible to Ralstonia solanacearum) grown in three different sites from Guangzhou, Guangdong Province, South China were investigated by cultivation methods. Most of the isolates belonged to streptomycetes. The Aureus group of Streptomyces was the most frequently isolated group. The population composition of Streptomyces varied according to tomato cultivars, physiological status and soil types. The proportions of antagonistic Streptomyces strains from healthy plants were higher than that from wilting plants (P < 0.05), although the difference among the proportions of antagonistic Streptomyces strains from different cultivars of healthy tomato was not significant, the similar result was found from wilting plants. No significant difference was found in the proportions of siderophere-producing Streptomyces strains from the same site (P > 0.05), but the difference was found from the different sampling sites (P < 0.05). The percentage of bacterial cell wall-degrading streptomycetes from wilting tomato was higher than that from healthy plants (P < 0.05). These results indicated that the cultivar of the host plant, physiological status and sampling sites would influence the proportion of endophytic streptomycetes with different physiological traits. Diversity of endophytic Streptomyces and their physiological diversity should be involved in developing potential biocontrol agents.