Isolation, characterization, and plasmid pUPI126-mediated indole-3-acetic acid production in acinetobacter strains from rhizosphere of wheat

Plant Growth-Promoting Ability of Mycorrhizal Fusarium Strain KB-3 Enhanced by Its IAA Producing Endohyphal Bacterium, Klebsiella aerogenes

Fusarium oxysporum KB-3 had been reported as a mycorrhizal fungus of Bletilla striata, which can promote the seed germination and vegetative growth. Endohyphal bacteria were demonstrated in the hyphae of the KB-3 by 16S rDNA PCR amplification and SYTO-9 fluorescent nucleic acid staining. A strain Klebsiella aerogenes KE-1 was isolated and identified based on the multilocus sequence analysis. The endohyphal bacterium was successfully removed from the wild strain KB-3 (KB-3⁻), and GFP-labeled KE-1 was also transferred to the cured strain KB-3⁻ (KB-3⁺). The production of indole-3-acetic acid (IAA) in the culturing broths of strains of KE-1, KB-3, KB-3⁻, and KB-3⁺ was examined by HPLC. Their IAA productions were estimated using Salkowski colorimetric technique. The highest concentrations of IAA were 76.9 (at 48 h after inoculation), 31.4, 9.6, and 19.4 μg/ml (at 60 h after inoculation), respectively. Similarly, the three fungal cultural broths exhibited plant promoting abilities on the tomato root and stem growth. The results indicated that the ability of mycorrhizal Fusarium strain KB-3 to promote plant growth was enhanced because its endohyphal bacterium, Klebsiella aerogenes KE-1, produced a certain amount of IAA.

Bifunctional Double-Salt Ionic Liquids Containing both 4-Chloro-2-Methylphenoxyacetate and l-Tryptophanate Anions with Herbicidal and Antimicrobial Activity

The goal of this research was to obtain and characterize ionic liquids based on a bisammonium cation and both 4-chloro-2-methylphenoxyacetate (MCPA) and l-tryptophanate anions. The concept of including two structurally different anions was utilized to achieve improved biological activity, while crucial functional traits could be designed by modifying the cation. The synthesis process was efficient and resulted in high yields. Subsequent analyses (nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, and high-performance liquid chromatography (HPLC)) confirmed the chemical structure, purity, and molar ratio of ions in the obtained compounds. The described compounds are novel and have not been previously described in the literature. Evaluations of physicochemical properties indicated that the obtained double-salt ionic liquids (DSILs) exhibited high thermal stability, high solubility in water, and surface activity. A biological activity assessment using greenhouse tests revealed that the herbicidal efficiency of the studied DSILs was notably increased compared to the reference commercial herbicide (even by ∼50% in the case of oilseed rape), which could be attributed to their high wettability toward hydrophobic surfaces. The compounds also efficiently inhibited the growth of several microbial species, with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)/minimum fungicidal concentration (MFC) values at the level of several μg·mL-1. The length of the spacer and alkyl substituent in the cation notably influenced the physicochemical and biological properties of the DSILs, which allowed us to design the structures of the obtained compounds in accordance with needs. The presented results confirm the high application potential of the described DSILs and provide a new and promising path for obtaining new and efficient plant-protection agents.

Auxins of microbial origin and their use in agriculture

To maintain the world population demand, a sustainable agriculture is needed. Since current global vision is more friendly with the environment, eco-friendly alternatives are desirable. In this sense, plant growth-promoting rhizobacteria could be the choice for the management of soil-borne diseases of crop plants. These rhizobacteria secrete chemical compounds which act as phytohormones. Indole-3-acetic acid (IAA) is the most common plant hormone of the auxin class which regulates various processes of plant growth. IAA compound, in which structure can be found a carboxylic acid attached through a methylene group to the C-3 position of an indole ring, is produced both by plants and microorganisms. Plant growth-promoting rhizobacteria and fungi secrete IAA to promote the plant growth. In this review, IAA production and mechanisms of action by bacteria and fungi along with the metabolic pathways evolved in the IAA secretion and commercial prospects are revised.Key points• Many microorganisms produce auxins which help the plant growth promotion.• These auxins improve the plant growth by several mechanisms.• The auxins are produced through different mechanisms.

Bacterial communities associated with the surface of fresh sweet pepper (Capsicum annuum) and their potential as biocontrol

Fresh produce vegetables are colonized by different bacterial species, some of which are antagonistic to microbes that cause postharvest losses. However, no comprehensive assessment of the diversity and composition of bacteria inhabiting surfaces of fresh pepper plants grown under different conditions has been conducted. In this study, 16S RNA amplicon sequencing was used to reveal bacterial communities inhabiting the surfaces of red and green pepper (fungicides-treated and non-fungicides-treated) grown under hydroponic and open field conditions. Results revealed that pepper fruit surfaces were dominated by bacterial phylum Proteobacteria, Firmicutes, Actinobacteria, and, Bacteroidetes. The majority of the bacterial operation taxonomic units (97% similarity cut-off) were shared between the two habitats, two treatments, and the two pepper types. Phenotypic predictions (at phylum level) detected a high abundance of potentially pathogenic, biofilm-forming, and stress-tolerant bacteria on samples grown on open soils than those from hydroponic systems. Furthermore, bacterial species of genera mostly classified as fungal antagonists including; Acinetobacter, Agrobacterium, and Burkholderia were the most abundant on the surfaces. These results suggest that peppers accommodate substantially different bacterial communities with antagonistic activities on their surfaces, independent of employed agronomic strategies and that the beneficial bacterial strains maybe more important for peppers established on open fields, which seems to be more vulnerable to abiotic and biotic stresses.

Antagonistic endophytic bacteria associated with nodules of soybean (Glycine max L.) and plant growth-promoting properties

A total of 276 endophytic bacteria were isolated from the root nodules of soybean (Glycine max L.) grown in 14 sites in Henan Province, China. The inhibitory activity of these bacteria against pathogenic fungus Phytophthora sojae 01 was screened in vitro. Six strains with more than 63% inhibitory activities were further characterized through optical epifluorescence microscopic observation, sequencing, and phylogenetic analysis of 16S rRNA gene, potential plant growth-promoting properties analysis, and plant inoculation assay. On the basis of the phylogeny of 16S rRNA genes, the six endophytic antagonists were identified as belonging to five genera: Enterobacter, Acinetobacter, Pseudomonas, Ochrobactrum, and Bacillus. The strain Acinetobacter calcoaceticus DD161 had the strongest inhibitory activity (71.14%) against the P. sojae 01, which caused morphological abnormal changes of fungal mycelia; such changes include fracture, lysis, formation of a protoplast ball at the end of hyphae, and split ends. Except for Ochrobactrum haematophilum DD234, other antagonistic strains showed the capacity to produce siderophore, indole acetic acid, and nitrogen fixation activity. Regression analysis suggested a significant positive correlation between siderophore production and inhibition ratio against P. sojae 01. This study demonstrated that nodule endophytic bacteria are important resources for searching for inhibitors specific to the fungi and for promoting effects for soybean seedlings.

Lignin peroxidase mediated silver nanoparticle synthesis in Acinetobacter sp

Metals present in environment render the bacteria to attain certain resistance machinery to survive, one of which is transformation of metal ions to nano forms. Various enzymes and proteins have been suggested to play significant role in synthesis of silver nanoparticles (AgNPs) in bacteria. In present study, we have purified lignin peroxidase from secreted enzyme extract of Acinetobacter sp. employing diethyl aminoethyl cellulose ion exchange and Biogel P-150 gel filtration column chromatography. The purified fraction has a specific activity of 1.571 U/mg with substrate n-propanol and 6.5-fold purification. The tetrameric enzyme, with molecular weight of 99 kDa, consisted of dimers of two polypetides of 23.9 and 24.6 kDa as revealed by native and SDS-PAGE. On exposure to purified enzyme, spherical polydispersed AgNPs of ~ 50 nm were obtained as observed under transmission electron microscope. Optimum activity of the purified enzyme was obtained at pH 2 and 60 °C with n-propanol as substrate. This is the first report describing the reduction of extracellular silver ions by lignin peroxidase purified from Acinetobacter sp.

Biosynthesis of gold and selenium nanoparticles by purified protein from Acinetobacter sp. SW 30

Synthesis of nanoparticles is an enzymatic reduction process in microorganisms. In the present study, a protein, lignin peroxidase has been purified by DEAE-Cellulose anion exchange chromatography and Biogel P-150 gel filtration chromatography from the cell suspension of Acinetobacter sp. SW30 responsible for the synthesis of gold nanoparticles (AuNP) and selenium nanoparticles (SeNP). The purified fraction has a specific activity of 29.4U/mg/min with 959 fold purification. Native and SDS PAGE confirmed that purified lignin peroxidase is monomeric enzyme with 97.4KDa molecular weight. The enzyme synthesized spherical crystalline AuNP (10±2nm) and amorphous SeNP (100±10nm). It has maximum activity at pH 2 and temperature 40°C, with 1.0mMKm value, when n-propanol was used as a substrate. Activity was completely inhibited by sodium thiosulphate and zinc sulphate. This is the first report on association of lignin peroxidase in the synthesis of AuNP and SeNP from Acinetobacter sp. SW30.

Green synthesis of selenium nanoparticles using Acinetobacter sp. SW30: optimization, characterization and its anticancer activity in breast cancer cells

The aim of this study was to synthesize selenium nanoparticles (SeNPs) using cell suspension and total cell protein of Acinetobacter sp. SW30 and optimize its synthesis by studying the influence of physiological and physicochemical parameters. Also, we aimed to compare its anticancer activity with that of chemically synthesized SeNPs in breast cancer cells. Cell suspension of Acinetobacter sp. SW30 was exposed to various physiological and physicochemical conditions in the presence of sodium selenite to study their effects on the synthesis and morphology of SeNPs. Breast cancer cells (4T1, MCF-7) and noncancer cells (NIH/3T3, HEK293) were exposed to different concentrations of SeNPs. The 18 h grown culture with 2.7×10⁹ cfu/mL could synthesize amorphous nanospheres of size 78 nm at 1.5 mM and crystalline nanorods at above 2.0 mM Na₂SeO₃ concentration. Polygonal-shaped SeNPs of average size 79 nm were obtained in the supernatant of 4 mg/mL of total cell protein of Acinetobacter sp. SW30. Chemical SeNPs showed more anticancer activity than SeNPs synthesized by Acinetobacter sp. SW30 (BSeNPs), but they were found to be toxic to noncancer cells also. However, BSeNPs were selective against breast cancer cells than chemical ones. Results suggest that BSeNPs are a good choice of selection as anticancer agents.

Bioengineering of Dioscorea nipponica with rhizospheric Proteus spp. for enhanced tuber size and diosgenin content

Rhizobacterial production of plant hormones, ACC deaminase, fixation of nitrogen, solubilization of phosphate and antimicrobial metabolites play very important role in the health and growth of plants. Hence exploration of plant probiotic prospectives of promising rhizobacterial isolates from biodiversity rich areas can have enormous applications to engineer both the biomass and active ingredients of plants. In the present study, rhizospheric Proteus spp. R6 and R60 isolated from Pseudarthria viscida and Glycosmis arborea were analyzed for tuber and diosgenin enhancement effects in Dioscorea nipponica under field conditions for 1 year. Among the two Proteus spp. selected, both were positive for ACC deaminase, siderophore, nitrogen fixation, IAA and ammonia production. However, the isolate R6 was found to have additional phosphate solubilizing activity. Quantitative analysis of IAA by HPTLC showed its maximum production by Proteus sp. R60 (714.47 ± 8.7 µg/mL) followed by Proteus sp. R6 (588.06 ± 7.0 µg/mL). The tubers formed from the Proteus sp. R6 treated samples were identified to have significant enhancement in size, root number and diosgenin content when compared to control. Interestingly, HPLC analysis has confirmed twofold higher diosgenin content in Proteus sp. R6 treated samples than control during 1 year period of its field growth. The obtained results are of great importance as it involved the utilization of rhizospheric bacteria to improve tuber size which suggests its potential use in developing cost-effective, eco-friendly and multifunctional biofertilizer.

Diversity of bacteria and archaea in the rhizosphere of bioenergy crop Jatropha curcas

Plant-microbial interaction in rhizosphere plays vital role in shaping plant’s growth and ecosystem function. Most of the rhizospheric microbial diversity studies are restricted to bacteria. In natural ecosystem, archaea also constitutes a major component of the microbial population. However, their diversity is less known compared to bacteria. Experiments were carried out to examine diversity of bacteria and archaea in the rhizosphere of bioenergy crop Jatropha curcas (J. curcas). Samples were collected from three locations varying widely in the soil physico-chemical properties. Diversity was estimated by terminal restriction fragment length polymorphism (TRFLP) targeting 16S rRNA gene of bacteria and archaea. Fifteen bacterial and 17 archaeal terminal restriction fragments (TRFs) were retrieved from J. curcas rhizosphere. Bacterial indicative TRFs were Actinobacteria, Firmicutes, Acidobacteria, Verrumicrobiaceae, and Chlroflexi. Major archaeal TRFs were crenarchaeota, and euryarchaeota. In case of bacteria, relative fluorescence was low for TRF160 and high for TRF51, TRF 420. Similarly, for archaea relative fluorescence of TRF 218, and TRF 282 was low and high for TRF 278, TRF468 and TRF93. Principal component analysis (PCA) of bacterial TRFs designated PC 1 with 46.83% of variation and PC2 with 31.07% variation. Archaeal TRFs designated 90.94% of variation by PC1 and 9.05% by PC2. Simpson index varied from 0.530 to 0.880 and Shannon index from 1.462 to 3.139 for bacteria. For archaea, Simpson index varied from 0.855 to 0.897 and Shannon index varied from 3.027 to 3.155. Study concluded that rhizosphere of J. curcas constituted of diverse set of both bacteria and archaea, which might have promising plant growth promoting activities.

Plant growth promoting potential and phylogenetic characteristics of a lichenized nitrogen fixing bacterium, Enterobacter cloacae

Lichens are complex symbiotic association of mycobionts, photobionts, and bacteriobionts, including chemolithotropic bacteria. In the present study, 46 lichenized bacteria were isolated by conventional and enrichment culture methods on nitrogen-free bromothymol blue (NFb) medium. Only 11 of the 46 isolates fixed nitrogen on NFb and had reduced acetylene. All these 11 isolates had also produced siderophore and 10 of them the IAA. Further, ammonia production was recorded from nine of these nitrogen fixers (NF). On molecular characterization, 16 S rRNA sequencing recorded that, nine NF belonged to Proteobacteria, within Gammaproteobacteria, and were closely related to Enterobacter sp. with a maximum similarity to Enterobacter cloacae. Each one of our NF isolates was aligned closely to Enterobacter pulveris strain E443, Cronobacter sakazakii strain PNP8 and Providencia rettgeri strain ALK058. Notably, a few strains we examined found to possess plant growth promoting properties. This is the first report of Enterobacter sp. from lichens which may be inhabit lichen thalli extrinsically or intrinsically.

Kinetics of Synthesis of Gold Nanoparticles by Acinetobacter sp. SW30 Isolated from Environment

Cell biomass and metal salt concentration have great influence on morphology of biosynthesized nanoparticle. The aim of present study was to evaluate the effect of varying cell density and gold salt concentrations on synthesis of nanoparticles and its morphology, which has not been studied in bacteria till now. When cells of Acinetobacter sp. SW30 were incubated with different cell density and gold chloride concentrations, tremendous variation in color of colloidal solution containing gold nanoparticles (AuNP) was observed indicating variation in their size and shapes. Surprisingly, monodispersed spherical AuNP of size ~19 nm were observed at lowest cell density and HAuCl₄ salt concentration while increase in cell number resulted in formation of polyhedral AuNP (~39 nm). Significance of this study lays in the fact that the shape and dispersity of AuNP can be customized depending up on the requirement. FTIR spectrum revealed shift from 3221 to 3196 cm^-1 indicating the presence and role of amino acids in Au³⁺ reduction while possible involvement of amide I and II groups in stabilization of AuNP. The rate constant was calculated for cell suspension of 2.1 × 10⁹ cfu/ml challenged with 1.0 mM HAuCl₄, incubated at 30 °C and pH 7 using the slopes of initial part of the plot log (A_α - A_t) versus time as 1.99 × 10^-8 M. Also, this is the first study to report the kinetics of gold nanoparticle synthesis by Acinetobacter sp. SW30.

Nanoparticles for Control of Biofilms of Acinetobacter Species

Biofilms are the cause of 80% of microbial infections. Acinetobacter species have emerged as multi- and pan-drug-resistant bacteria and pose a great threat to human health. These act as nosocomial pathogens and form excellent biofilms, both on biotic and abiotic surfaces, leading to severe infections and diseases. Various methods have been developed for treatment and control of Acinetobacter biofilm including photodynamic therapy, radioimmunotherapy, prophylactic vaccines and antimicrobial peptides. Nanotechnology, in the present scenario, offers a promising alternative. Nanomaterials possess unique properties, and multiple bactericidal mechanisms render them more effective than conventional drugs. This review intends to provide an overview of Acinetobacter biofilm and the significant role of various nanoparticles as anti-biofouling agents, surface-coating materials and drug-delivery vehicles for biofilm control and treatment of Acinetobacter infections.

Molecular characterization of outer membrane vesicles released from Acinetobacter radioresistens and their potential roles in pathogenesis

Acinetobacter radioresistens is an important member of genus Acinetobacter from a clinical point of view. In the present study, we report that a clinical isolate of A. radioresistens releases outer membrane vesicles (OMVs) under in vitro growth conditions. OMVs were released in distinctive size ranges with diameters from 10 to 150 nm as measured by the dynamic light scattering (DLS) technique. Additionally, proteins associated with or present into OMVs were identified using LC-ESI-MS/MS. A total of 71 proteins derived from cytosolic, cell membrane, periplasmic space, outer membrane (OM), extracellular and undetermined locations were found in OMVs. The initial characterization of the OMV proteome revealed a correlation of some proteins to biofilm, quorum sensing, oxidative stress tolerance, and cytotoxicity functions. Thus, the OMVs of A. radioresistens are suggested to play a role in biofilm augmentation and virulence possibly by inducing apoptosis.

Transcriptional regulation of the iac locus from Acinetobacter baumannii by the phytohormone indole-3-acetic acid

The iac locus is involved in indole-3-acetic acid (IAA) catabolism in Acinetobacter baumannii. Nine structural genes of iac are transcribed in the same direction, whereas iacR, which encodes a MarR-type transcriptional regulator, is transcribed in the opposite direction. The IacA protein, which is encoded by the second structural gene of the iac locus, is expressed in an IAA-dependent manner. Here, we characterized gene expression from this locus in wild type A. baumannii and in an iacR mutant; this revealed that the iacH promoter is negatively regulated by IacR. The transcriptional site of iacH was determined by using 5' rapid amplification of cDNA ends; one IacR-binding site was identified between positions -35 and +28 of the iacH promoter. Sequence analysis and an electrophoretic mobility shift assay indicated that recombinant IacR binds specifically to a sequence with dyad symmetry in the iacR-iacH overlapping promoters in the absence of IAA. In addition, a two-plasmid expression system in Escherichia coli showed that IAA probably serves as a ligand that binds to IacR and releases it from the iacH promoter, thereby allowing RNA polymerase to transcribe iac. Thus, iac is expressed in order to promote IAA degradation, whereas free IacR is required for iac repression. We conclude that IacR serves as a key regulator of IAA degradation in A. baumannii in the rhizosphere. These results provide new insights into the possible role of A. baumannii in the environment.

Characterization and Fungal Inhibition Activity of Siderophore from Wheat Rhizosphere Associated Acinetobacter calcoaceticus Strain HIRFA32

Acinetobacter calcoaceticus HIRFA32 from wheat rhizosphere produced catecholate type of siderophore with optimum siderophore (ca. 92 % siderophore units) in succinic acid medium without FeSO4 at 28 °C and 24 h of incubation. HPLC purified siderophore appeared as pale yellow crystals with molecular weight [M(+1)] m/z 347.18 estimated by LCMS. The structure elucidated by (1)H NMR, (13)C NMR, HMQC, HMBC, NOESY and decoupling studies, revealed that siderophore composed of 2,3-dihydroxybenzoic acid with hydroxyhistamine and threonine as amino acid subunits. In vitro study demonstrated siderophore mediated mycelium growth inhibition (ca. 46.87 ± 0.5 %) of Fusarium oxysporum. This study accounts to first report on biosynthesis of acinetobactin-like siderophore by the rhizospheric strain of A. calcoaceticus and its significance in inhibition of F. oxysporum.

Plasmid pUPI126-encoded pyrrolnitrin production by Acinetobacter haemolyticus A19 isolated from the rhizosphere of wheat

An Acinetobacter species identified as A. haemolyticus A19 produces an antibiotic and the enzyme chitinase. The antibiotic produced by A. haemolyticus A19 was extracellular and inducible by co-cultivation with Klebsiella pneumoniae in the optimum ratio 2:1, respectively. pH 7, temperature 28 °C, and addition of 2% (w/v) NaCl are the most suitable environmental conditions for production and activity of the antibiotic. The antibiotic was produced in the early stationary growth phase (48 h) of A. haemolyticus A19. It has a very broad spectrum of antimicrobial activity against plant and human pathogenic bacteria and fungi. The antibiotic was extracted with ethyl acetate and purified by column chromatography with further purification by preparative thin-layer chromatography. Yield of the antibiotic was 15 mg/l. The antibiotic was active at very low concentrations, for example 50 μg/ml, and was water-soluble. It was stable at room temperature for up to 7 days. (1)H NMR analysis revealed the antibiotic was a pyrrolnitrin. It was found that pyrrolnitrin production by A. haemolyticus A19 was encoded by plasmid pUPI126 of molecular weight 25.7 kb. Plasmid pUPI126 was transferred to E. coli HB101 at a frequency of 5 × 10(-5) per μg DNA. It was also conjugally transformed to E. coli HB101 rif (r) mutants at a frequency of 5.9 × 10(-8) per recipient cell. Plasmid pUPI126 was 100% stable in Acinetobacter and 95% stable in E. coli HB101. Transconjugants and transformants both produced the antibiotic. This is the first report of plasmid-mediated pyrrolnitrin production by A. haemolyticus A19 isolated from wheat rhizosphere.

Characterization of rhizobial isolates nodulating Millettia pinnata in India

Millettia pinnata (Synonym Pongamia pinnata) is a viable source of oil for the mushrooming biofuel industry, source for agroforestry, urban landscaping, and the bio-amelioration of degraded lands. It also helps in maintaining soil fertility through symbiotic nitrogen fixation. However, not much work is reported on classification and characterization of the rhizobia associated with this plant. In the present study, an attempt was made to isolate rhizobial strains nodulating Millettia from soils collected from southern regions of India. The isolates were characterized using numerical taxonomy, 16S rRNA gene sequencing, and cross nodulation ability. The results showed high phenotypic and genetic diversity among the rhizobia symbiotic with Millattia pinnata. The isolates formed five clusters at similarity level of 0.82 based on the results of numerical taxonomy. Results on 16S rRNA gene sequence analysis revealed that most microsymbionts of M. pinnata belonged to Rhizobium and Bradyrhizobium, which are closely related to Rhizobium sp., B. elkanii and B. yuanmingense. Among these isolates, some isolates could grow in a pH range of 4.0-10.0, some could tolerate a high salt concentration (3% NaCl) and could grow at a maximum temperature between 35 and 45 °C. M. pinnata formed nodules with diverse rhizobia in Indian soils. These results offered the first systematic information about the microsymbionts of M. pinnata grown in the soils from southern part of India.

Production and optimization of indole acetic acid by indigenous micro flora using agro waste as substrate

Indole Acetic Acid (IAA) producing bacterium was isolated from the Rhizosphere soil and identified as Rhizobium sp. and Bacillus sp., Optimization of Indole acetic acid production was carried out at different cultural conditions, such as pH, temperature and substrate with Rhizobium sp., Bacillus sp. and Rhizobium sp., produced higher amount of Indole acetic acid (6.1 mg mL(-1)) than the Bacillus sp., (4.4 mg mL(-1)) at pH 7 and 37 degrees C in the Bengal gram substrate. Partial purification of Indole acetic acid was done by Thin Layer Chromatography (TLC). In conclusion Rhizobium sp., appear to be a suitable soil microorganism for high level of IAA production.

Nitrogen-fixing chemo-organotrophic bacteria isolated from cyanobacteria-deprived lichens and their ability to solubilize phosphate and to release amino acids and phytohormones

AIMS

Cyanobacteria-deprived lichens of the species Canoparmelia caroliniana, Canoparmelia crozalsiana, Canoparmelia texana, Parmotrema sancti-angeli and Parmotrema tinctorum were screened for the presence of chemo-organotrophic nitrogen-fixing bacteria.

METHODS AND RESULTS

Fifty-three lichen samples subjected to enrichment selection using a nitrogen-free minimal medium were positive for acetylene reduction. Seventeen isolates, able to fix nitrogen, belonged to Gamma-proteobacteria group and were identified as: Acinetobacter sp., Pantoea sp., Pseudomonas sp., Pseudomonas stutzeri, Serratia marcescens and Stenotrophomonas maltophilia, according to 16S rRNA gene sequences and biochemical tests. The excretion of amino acid and phytohormone and the ability of mineral phosphate solubilization were determined in 14 isolates. All isolates were able to release amino acids and 3-indoleacetic acid. About 64% of the isolates solubilized phosphates and 30% released ethylene.

CONCLUSIONS

These data confirm sparse evidence from the literature on the occurrence of chemo-organotrophic nitrogen-fixing bacteria in cyanobacteria-deprived lichens; the isolates presented physiologic features which might benefit the host if they are expressed when the bacteria are harboured by lichens.

SIGNIFICANCE AND IMPACT OF THE STUDY

Chemo-organotrophic nitrogen-fixing bacteria were isolated from a high percentage (72.6%) of cyanobacteria-deprived lichens. All isolates presented important physiological characteristics, some of which are being described here for the first time.