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Plants serve as critical links between above- and below-ground microbial communitites, both influencing and being influenced by microbes in these two realms. Below-ground microbial communities are expected to respond to soil resource environments, which are mediated by the roots of plants that can, in turn, be influenced by the above-ground community of foliar endophytes. For instance, diverse plant communities deposit more, and more diverse, nutrients into the soil, and this deposition is often increased when foliar pathogens are removed. DiFFerences in soil resources can alter soil microbial composition and phenotypes, including inhibitory capacity, resource use, and antibiotic resistance. In this work, we consider plots differing in plant richness and application of foliar fungicide, evaluating consequences on soil resource levels and root-associated Streptomyces phenotypes. Soil carbon, nitrogen, phosphorus, potassium, and organic matter were greater in samples from polyculture than monoculture, yet this increase was surprisingly offset when foliar fungal communities were disrupted. We find that Streptomyces phenotypes varied more between richness plots--with the Streptomyces from polyculture showing lower inhibitory capacity, altered resource-use profiles, and greater antibiotic resistance--than between subplots with/without foliar fungicide. Where foliar fungicide affected phenotypes, it did so differently in polyculture than in monoculture, for instance decreasing niche width and overlap in monoculture while increasing them in polyculture. No diffserences in phenotype were correlated with soil nutrient levels, suggesting the need for further research looking more closely at soil resource diversity and particular compounds that were found to differ between treatments. [ABSTRACT FROM AUTHOR]

Carbon amendments are used in agriculture for increasing microbial activity and biomass in the soil. Changes in microbial community composition and function in response to carbon additions to soil have been associated with biological suppression of soilborne diseases. However, the specific selective impacts of carbon amendments on microbial antagonistic populations are not well understood. We investigated the effects of soil carbon amendments on nutrient use profiles, and antibiotic inhibitory and resistance phenotypes of Streptomyces populations from agricultural soils. Soil mesocosms were amended at intervals over 9 months with low or high dose solutions of glucose, fructose, a complex amendment, or water only (non-amendment control). Over 130 Streptomyces isolates were collected from amended and non-amended mesocosm soils, and nutrient utilization profiles on 95 different carbon substrates were determined. A subset of isolates (n = 40) was characterized for their ability to inhibit or resist one another. Carbon amendments resulted in Streptomyces populations with greater niche widths, and increased growth efficiencies as compared with Streptomyces in non-amended soils. Shifts in microbial nutrient use and growth capacities coincided with positive selection for Streptomyces antibiotic inhibitory phenotypes in carbon-amended soils, resulting in populations dominated by phenotypes that combine both antagonistic capacities and a generalist lifestyle. Carbon inputs resulted in populations that on average were more resistant to one another than populations in non-amended soils. Shifts in metabolic capacities and antagonistic activity indicate that carbon additions to soil may selectively enrich Streptomyces antagonistic phenotypes, that are rare under non-nutrient selection, but can inhibit more intensively nutrient competitors, and resist phenotypes with similar functional traits. These results shed light on the potential for using carbon amendments to strategically mediate soil microbial community assembly, and contribute to the establishment of pathogen-suppressive soils in agricultural systems. [ABSTRACT FROM AUTHOR]

Chemical communication among kin bacteria modulates diverse activities. Despite the general consensus that signaling among non-kin organisms is likely to influence microbial behavior, there is limited information on the potential for microbial interactions to alter microbial phenotypes in natural habitats. We explored patterns of interaction that alter inhibitory phenotypes among Streptomyces isolates from distinct communities. Shifts in inhibition in response to the presence of a partner were evaluated for 861 isolate combinations, and were considered in relation to nutrient use, 16S sequence, inhibition phenotype and community origin. The frequency of inhibition-shifting interactions was significantly higher among isolates from the same (0.40) than from different (0.33) communities, suggesting local selection for inhibition-shifting interactions. Communities varied in the frequency with which Streptomyces isolates responded to a partner but not in the frequency with which isolates induced changes in partners. Streptomyces isolates were more likely to exhibit increased inhibition of a target bacterium in response to isolates that compete for the same nutrients, are closely-related or are strongly inhibited by their antibiotics. This work documents a high frequency of interactions among Streptomyces that shift the capacity of Streptomyces to inhibit other microbes, and suggests significant potential for such interactions to shape microbial community dynamics. [ABSTRACT FROM AUTHOR]

Although recent molecular techniques have greatly expanded our knowledge of microbial biogeography, the functional biogeography of soil microorganisms remains poorly understood. In this work, we explore geographic variation in Streptomyces phenotypes that are critical to species interactions. Specifically, we characterize Streptomyces from different locations from multiple continents for antibiotic inhibition, resistance, and resource use phenotypes. Streptomyces from different locations varied significantly in antibiotic inhibition, resistance, and resource use indicating that communities vary in functional potential. Among all isolates, there were substantial differences in antibiotic inhibition, resistance, and resource use within and among and within Streptomyces species. Moreover, Streptomyces with near-identical 16S rRNA gene sequences from different locations sometimes differed significantly in inhibition, resistance, and resource use phenotypes, suggesting that these phenotypes may be locally adapted. Thus, in addition to a likely role of environmental filtering, variation in Streptomyces inhibitory, resistance, and resource use phenotypes among locations is likely to be a consequence of local selection mediated by species interactions. [ABSTRACT FROM AUTHOR]

Though traditionally perceived as weapons, antibiotics are also hypothesized to act as microbial signals in natural habitats. However, while subinhibitory concentrations of antibiotics (SICA) are known to shift bacterial gene expression, specific hypotheses as to how SICA influence the ecology of natural populations are scarce. We explored whether antibiotic ‘signals’, or SICA, have the potential to alter nutrient utilization, niche overlap, and competitive species interactions among Streptomyces populations in soil. For nine diverse Streptomyces isolates, we evaluated nutrient utilization patterns on 95 different nutrient sources in the presence and absence of subinhibitory concentrations of five antibiotics. There were significant changes in nutrient use among Streptomyces isolates, including both increases and decreases in the capacity to use individual nutrients in the presence vs. in the absence of SICA. Isolates varied in their responses to SICA and antibiotics varied in their effects on isolates. Furthermore, for some isolate-isolate-antibiotic combinations, competition-free growth (growth for an isolate on all nutrients that were not utilized by a competing isolate), was increased in the presence of SICA, reducing the potential fitness cost of nutrient competition among those competitors. This suggests that antibiotics may provide a mechanism for bacteria to actively minimize niche overlap among competitors in soil. Thus, in contrast to antagonistic coevolutionary dynamics, antibiotics as signals may mediate coevolutionary displacement among coexisting Streptomyces, thereby hindering the emergence of antibiotic resistant phenotypes. These results contribute to our broad understanding of the ecology and evolutionary biology of antibiotics and microbial signals in nature. [ABSTRACT FROM AUTHOR]

Correspondence between two distinct genetic traits, 16S rRNA gene sequences and repetitive element-sequence-based BOX-PCR DNA fingerprints, and antibiotic inhibition and resistance phenotypes was explored for a spatially explicit sample of Streptomyces from a prairie soil. There was no correspondence between 16S rRNA gene sequence groups and antibiotic phenotypes. However, 16S rRNA gene sequence groups differed significantly in mean inhibition zone sizes. Specific antibiotic phenotypes may reflect local selection pressures, as suggested by the significant differences in mean inhibition zone sizes against specific test isolates by Streptomyces from the same 16S rRNA gene sequence group but from different locations in soil. Significant correlations between antibiotic phenotypes and BOX-PCR fingerprints were found, but were small ( r=0.19–0.22). Although genetic characterizations alone were not predictive of specific antibiotic phenotypes, 16S rRNA gene sequence analyses may identify isolates that are most or least likely to possess substantial inhibitory potential, providing insight into the broad ecological strategy for individual isolates. [ABSTRACT FROM AUTHOR]

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Though traditionally perceived as weapons, antibiotics are also hypothesized to act as microbial signals in natural habitats. However, while subinhibitory concentrations of antibiotics (SICA) are known to shift bacterial gene expression, specific hypotheses as to how SICA influence the ecology of natural populations are scarce. We explored whether antibiotic ‘signals’, or SICA, have the potential to alter nutrient utilization, niche overlap, and competitive species interactions among Streptomyces populations in soil. For nine diverse Streptomyces isolates, we evaluated nutrient utilization patterns on 95 different nutrient sources in the presence and absence of subinhibitory concentrations of five antibiotics. There were significant changes in nutrient use among Streptomyces isolates, including both increases and decreases in the capacity to use individual nutrients in the presence vs. in the absence of SICA. Isolates varied in their responses to SICA and antibiotics varied in their effects on isolates. Furthermore, for some isolate-isolate-antibiotic combinations, competition-free growth (growth for an isolate on all nutrients that were not utilized by a competing isolate), was increased in the presence of SICA, reducing the potential fitness cost of nutrient competition among those competitors. This suggests that antibiotics may provide a mechanism for bacteria to actively minimize niche overlap among competitors in soil. Thus, in contrast to antagonistic coevolutionary dynamics, antibiotics as signals may mediate coevolutionary displacement among coexisting Streptomyces, thereby hindering the emergence of antibiotic resistant phenotypes. These results contribute to our broad understanding of the ecology and evolutionary biology of antibiotics and microbial signals in nature. [ABSTRACT FROM AUTHOR]