Actinomycetes of the Genus Micromonospora in Meadow Ecosystems

Investigations showed that micromonosporas, along with streptomycetes, are the major inhabitants of floodplain meadow ecosystems, where their population varies from tens of thousands to hundreds of thousands of CFU per g substrate. In spring, the population of micromonosporas in soil and on the plant roots was found to be denser than that of streptomycetes.     

Actinomycete complexes in the rhizosphere of winter rye on soddy podzolic soil

The structure of actinomycete complexes in the rhizosphere of winter rye (Secale cereale L.) varieties originating from different ecosystems and geographical zones was studied in field experiments on soddy podzolic soil. In addition to streptomycetes, the complexes studied contained actinomycetes of the genera Micromonospora and Streptosporangium, represented at high occurrence and comparable abundance rates. The rhizosphere of most of the studied rye varieties was dominated by micromonosporas. The antifungal potential of mycelial prokaryotes associated with winter rye was assessed. The taxonomic and functional structure of actinomycete complexes was shown to be similar in rye varieties originating from the nonchernozem zone of Russia. However, the actinomycete complex proved to be much different in the rye variety that developed in the steppe zone of the lower Volga region.     

Actinomycete complexes in the rhizosphere of winter rye on soddy podzolic soil

The structure of actinomycete complexes in the rhizosphere of winter rye (Secale cereale L.) varieties originating from different ecosystems and geographical zones was studied in field experiments on soddy podzolic soil. In addition to streptomycetes, the complexes studied contained actinomycetes of the genera Micromonospora and Streptosporangium, represented at high occurrence and comparable abundance rates. The rhizosphere of most of the studied rye varieties was dominated by micromonosporas. The antifungal potential of mycelial prokaryotes associated with winter rye was assessed. The taxonomic and functional structure of actinomycete complexes was shown to be similar in rye varieties originating from the nonchernozem zone of Russia. However, the actinomycete complex proved to be much different in the rye variety that developed in the steppe zone of the lower Volga region.__________Translated from Mikrobiologiya, Vol. 74, No. 2, 2005, pp. 271–277.Original Russian Text Copyright © 2005 by Shirokikh, Merzaeva.     

Importance of Micromonospora spp. as colonizers of cellulose in freshwater lakes as demonstrated by quantitative reverse transcriptase PCR of 16S rRNA

The relative abundance of micromonosporas in the bacterial communities inhabiting cellulose baits, water columns, and sediments of two freshwater lakes was determined by quantitative PCR (qPCR) of reverse-transcribed 16S rRNA. Micromonospora spp. were shown to be significant members of the active bacterial population colonizing cellulosic substrates in the lake sediment, and their increased prevalence with greater depth was confirmed by enumeration of CFU.     

Evidence for transfer of antibiotic-resistance genes in soil populations of streptomycetes

Phylogenetic analysis was used to evaluate the hypothesis of gene transfer in streptomycetes, many of which are antibiotic producers. The diversity and possible origins of streptomycin-resistance genes was investigated for a population of Streptomyces strains isolated from a site in Brazil where antibiotic production had previously been implicated. The analysis provides compelling evidence for the transfer of these genes. Examination of other Streptomyces -type strains also reveals a scattered distribution of streptomycin producers with respect to the overall phylogeny. These results suggest that horizontal gene transfer may be an important factor in the evolution of antibiotic genes in streptomycetes.     

Impact of the first <Emphasis Type="Italic">Streptomyces</Emphasis> genome sequence on the discovery and production of bioactive substances

An important addition to the field of bacterial genomics is the recent publication of the complete genome sequence of Streptomyces coelicolor. This strain has been for some decades the model organism for streptomycetes and other filamentous actinomycetes, Gram-positive bacteria highly valuable for their ability to produce thousands of bioactive metabolites, many of which have found important applications in medicine and agriculture. We discuss here the impacts that the S. coelicolor genome sequence is likely to have on the production of bioactive metabolites by current industrial strains, on the possible development of future superhost(s) for the production of valuable drugs, and on the search for new bioactive substances from microbial sources.     

Nontraditional microbial bioactive metabolites

Microorganisms produce low-molar-mass secondary metabolites exhibiting different biological activities, which are used.e.g., in medicine as antimicrobial and antifungal agents, alkaloids and toxins. Some of these substances have highly diverse biological activities and unusual structures. They are produced by streptomycetes, fungi, and bacilli, but interesting products have also been obtained from microorganisms growing in extreme conditions. Several thousands of microbial products have so far been discovered and many other, which can be potentially useful, and/or prospective for human use, can still be in the offing.     

Analysis of genomic similarity in Streptomycetes belonging to the fluorescent subgroup as a confirmation of their taxonomic revision using a population method

Genomic similarity was analysed in streptomycetes belonging to the fluorescent subgroup: Streptomyces chrysomallus, S. fluorescens, S. galbofluorescens and S. citreofluorescens. The degree of reference S. chrysomalius DNA hybridization with S. fluorescens and S. galbofluorescens DNAs was 75 and 82%, respectively, thus being within the limits of the intraspecial hybridization level. S. citreofluorescens DNA showed a 55% homology with reference S. chrysomallus DNA, which corresponded to the range of interspecies hybridization. These conclusions were confirmed by the results obtained in analysing the thermostability of hybrid duplexes. Therefore, these findings are consistent with the data of revising the species taxonomy of this streptomycetes subgroup which was done using the method of comparative population analysis. The population model proposed by one of the authors can be used to assess the intraspecies level of DNA-DNA hybridization.     

Distribution of actinomycetes in near-shore tropical marine sediments

Actinomycetes were isolated from near-shore marine sediments collected at 15 island locations throughout the Bahamas. A total of 289 actinomycete colonies were observed, and all but 6 could be assigned to the suprageneric groups actinoplanetes and streptomycetes. A bimodal distribution in the actinomycete population in relation to depth was recorded, with the maximum numbers occurring in the shallow and deep sampling sites. This distribution can be accounted for by a rapid decrease in streptomycetes and an increase in actinoplanetes with increasing depth and does not conform to the theory that actinomycetes isolated from marine sources are of terrestrial origin. Sixty-three of the isolated actinomycetes were tested for the effects of seawater on growth. Streptomycete growth in nonsaline media was reduced by 39% compared with that in seawater. The actinoplanetes had a near obligate requirement of seawater for growth, and this is presented as evidence that actinomycetes can be physiologically active in the marine environment. Problems encountered with the enumeration of actinomycetes in marine sediments are also discussed.     

Distribution of actinomycetes in near-shore tropical marine sediments.

Actinomycetes were isolated from near-shore marine sediments collected at 15 island locations throughout the Bahamas. A total of 289 actinomycete colonies were observed, and all but 6 could be assigned to the suprageneric groups actinoplanetes and streptomycetes. A bimodal distribution in the actinomycete population in relation to depth was recorded, with the maximum numbers occurring in the shallow and deep sampling sites. This distribution can be accounted for by a rapid decrease in streptomycetes and an increase in actinoplanetes with increasing depth and does not conform to the theory that actinomycetes isolated from marine sources are of terrestrial origin. Sixty-three of the isolated actinomycetes were tested for the effects of seawater on growth. Streptomycete growth in nonsaline media was reduced by 39% compared with that in seawater. The actinoplanetes had a near obligate requirement of seawater for growth, and this is presented as evidence that actinomycetes can be physiologically active in the marine environment. Problems encountered with the enumeration of actinomycetes in marine sediments are also discussed.     

Experimental studies on the ecological role of antibiotic production in bacteria

Summary Genetically well-characterized strains of antibiotic-producing soil bacteria (Streptomyces griseus andStreptomyces coelicolor) were used to examine the ecological role of antibiotic production. Streptomycetes were competed against sensitive and resistantBacillus subtilis, another soil bacterium, on surface (agar) culture. The ecological role of antibiotics was examined in three levels of competition. (1) Capacity of antibiotics to allow invasion of producing organisms (B. subtilis established and streptomycetes added later). (2) Capacity of antibiotics to mediate competition between established populations (B. subtilis and streptomycetes co-inoculated). (3) Capacity of antibiotics to prevent invasion by competitors (streptomycetes established andB. subtilis added later). Antibiotic production was found to play a significant role in preventing the invasion of competitors in these experiments. Antibiotic production did not improve the ability of producers to invade a population of sensitive cells nor did it play a strong role in mediating competition between established populations. Antibiotic production also selected for antibiotic-resistant bacteria among invading competitors.     

Influences of soil acidity on Streptomyces populations inhabiting forest soils.

The Streptomyces populations inhabiting five acidic forest soils were examined. It was found that lowering the pH of a medium selective for streptomycetes (starch-casein agar) to the pH of the particular soil horizon being plated influenced both the total numbers and types of streptomycetes that were isolated from the soils examined in this study. On the acidified medium both the numbers of streptomycetes and the percentage of total bacteria on the plates represented by streptomycetes increased (as compared with the same medium with a pH of 7.2). These differences were greatest on the isolations from the most acid soils. The largest concentrations of streptomycetes were found in the surface horizon (0 to 15 cm) and the litter layer immediately over the surface mineral horizon. Acidity tolerance tests demonstrated that random samplings of isolates contained acidophilic, neutrophilic, and acidoduric strains, with the largest numbers of acidophiles being found on the acidified media from the most acid soils. There were no differences between overall utilization of selected carbohydrates among the isolates taken from either the neutral or acidic media, although a larger proportion of the acid media isolates produced acid from the carbohydrates. Evidence is presented which indicates that different types of streptomycetes were isolated on the acid media, and possible reasons for the presence of these acid-tolerant populations are discussed.     

Influences of soil acidity on Streptomyces populations inhabiting forest soils.

The Streptomyces populations inhabiting five acidic forest soils were examined. It was found that lowering the pH of a medium selective for streptomycetes (starch-casein agar) to the pH of the particular soil horizon being plated influenced both the total numbers and types of streptomycetes that were isolated from the soils examined in this study. On the acidified medium both the numbers of streptomycetes and the percentage of total bacteria on the plates represented by streptomycetes increased (as compared with the same medium with a pH of 7.2). These differences were greatest on the isolations from the most acid soils. The largest concentrations of streptomycetes were found in the surface horizon (0 to 15 cm) and the litter layer immediately over the surface mineral horizon. Acidity tolerance tests demonstrated that random samplings of isolates contained acidophilic, neutrophilic, and acidoduric strains, with the largest numbers of acidophiles being found on the acidified media from the most acid soils. There were no differences between overall utilization of selected carbohydrates among the isolates taken from either the neutral or acidic media, although a larger proportion of the acid media isolates produced acid from the carbohydrates. Evidence is presented which indicates that different types of streptomycetes were isolated on the acid media, and possible reasons for the presence of these acid-tolerant populations are discussed.     

Molecular detection of streptomycin-producing streptomycetes in Brazilian soils.

Actinomycetes were isolated from soybean rhizosphere soil collected as two field sites in Brazil. All the isolates were identified as Streptomyces species and were screened for streptomycin production and the presence of two genes, strA and strB1, known to be involved in streptomycin biosynthesis in Streptomyces griseus. Antibiotic resistance profiles were determined for 53 isolates from cultivated and uncultivated sites, and approximately half the strains were streptomycin resistance. Clustering by the unweighted pair group method with averages indicated the presence of two major clusters, with the majority of resistant strains from cultivated sites being placed in cluster 1. Only representatives from this cluster contained strA. Streptomycetes containing strA and strB1 were phenotypically diverse, and only half could be assigned to known species. Sequence comparison of 16S rRNA and trpBA (tryptophan synthetase) genes revealed that streptomycin- producing streptomycetes were phylogenetically diverse. It appeared that a population of streptomycetes had colonized the rhizosphere and that a proportion of these were capable of streptomycin production.     

Method for isolating cyanobacteria-lysing streptomycetes from soil

A new technique is described for the isolation and enumeration of cyanobacteria-lysing Streptomyces spp from soil or water. Two cyanobacteria, Anabaena cylindrica (ACTT 27899) and Tolypothrix tenuis (ATCC 27914) were used as the test organisms. Ten-day-old cyanobacterial cultures were vacuum-filtered to form a lawn on a 7 cm Whatman No. 1 filter paper which was then supported on Allen's agar. When the lawn was inoculated with dilutions of a heavy clay prairie soil and incubated at 27 PT 1dEC under constant illumination, white or grey colonies of streptomycetes grew. These colonies were surrounded by zones where a yellowing and lysis of the algal cells occurred. Streptomyces achromogenes was identified as a major lytic species within a population of 5 times 10₃ plaque-producing streptomycetes/g (dry weight) soil.     

Streptomycetes: a new model to study cell death.

Colonies of streptomycetes are now viewed as multicellular entities containing morphologically and biochemically differentiated cell types which have specific functions and precise spatial relationships to one another. Like multicellular organisms, colony development in streptomycetes is also maintained by a tight balance between cell proliferation and cell death processes. This review describes the current state of knowledge concerning cell death in streptomycetes.     

Low-molecular-weight thiols in streptomycetes and their potential role as antioxidants.

The intracellular low-molecular-weight thiols present in five gram-positive Streptomyces species and one Flavobacterium species were analyzed by high-performance liquid chromatography after fluorescence labeling with monobromobimane. Bacteria were chosen to include penicillin and cephalosporin beta-lactam producers and nonproducers. No significant amount of glutathione was found in any of the streptomycetes. Major intracellular thiols in all strains examined were cysteine, coenzyme A, sulfide, thiosulfate, and an unknown thiol designated U17. Those streptomycetes that make beta-lactam antibiotics also produce significant amounts of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine (ACV), a key intermediate in their biosynthesis. In Streptomyces clavuligerus, a potent producer of beta-lactams, the level of ACV was low during the early phase of growth and increased rapidly toward the end of exponential growth, paralleling that of antibiotic production. These and other observations indicate that ACV does not function as a protective thiol in streptomycetes. U17 may have this role since it was the major thiol in all streptomycetes and appeared to occur at levels about 10-fold higher than those of the other thiols measured, including ACV. Purification and amino acid analysis of U17 indicated that it contains cysteine and an unusual amine that is not one of the common amino acids. This thiol is identical to an unknown thiol found previously in Micrococcus roseus and Streptomyces griseus. A high level of ergothioneine was found in Streptomyces lactamdurans, and several unidentified thiols were detected in this and other streptomycetes.     

Alkaliphilic soil actinomycetes

The actinomycete complex of alkaline soils was found to be dominated by alkaliphilic streptomycetes, which showed maximal radial rates of colony growth at pH 8. At pH values of 7 and 10, the growth of these streptomycetes was poor. Alkaliphilic streptomycetes can be morphologically differentiated from other actinomycetes based on their high radial rates of colony growth and increased spore formation in alkaline media as compared to neutral media.     

Alkaliphilic Soil Actinomycetes

The actinomycete complex of alkaline soils was found to be dominated by alkaliphilic streptomycetes, which showed maximal radial rates of colony growth at pH 8. At pH values of 7 and 10, the growth of these streptomycetes was poor. Alkaliphilic streptomycetes can be morphologically differentiated from other actinomycetes based on their high radial rates of colony growth and increased spore formation in alkaline media as compared to neutral media.     

Friends and foes: streptomycetes as modulators of plant disease and symbiosis

The ecological role of soil streptomycetes within the plant root environment is currently gaining increased attention. This review describes our recent advances in elucidating the complex interactions between streptomycetes, plants, pathogenic and symbiotic microorganisms. Streptomycetes play diverse roles in plant-associated microbial communities. Some act as biocontrol agents, inhibiting plant interactions with pathogenic organisms. Owing to the antagonistic properties of streptomycetes, they exert a selective pressure on soil microbes, which may not always be for plant benefit. Others promote the formation of symbioses between plant roots and microbes, and this is in part due to their direct positive influence on the symbiotic partner, expressed as, e.g., promotion of hyphal elongation of symbiotic fungi. Recently, streptomycetes have been identified as modulators of plant defence. By repressing plant responses to pathogens they facilitate root colonisation with pathogenic fungi. In contrast, other strains induce local and systemic resistance against pathogens or enhance plant growth. In conclusion, while streptomycetes have a clear potential of acting as biocontrol agents, care has to be taken to avoid strains that select for virulent pathogens or enhance disease development. We argue towards the use of an integrated screening approach in the search for efficient biocontrol agents, including assays on in vitro antagonism, plant growth, and disease suppression.