Compost spatial heterogeneity promotes evolutionary diversification of a bacterium

Evolving a plant-beneficial bacterium in soil vs. nutrient-rich liquid culture has contrasting effects on in-soil fitness

Inoculation of plant-beneficial microbes into agricultural soils can improve crop growth, but such outcomes depend on microbial survival. Here, we assessed how exposure to prior environmental conditions impacts microbial in-soil fitness, particularly focusing on incubation in liquid culture as an unavoidable phase of inoculant production and on pre-incubation in target soils as a potential method to improve performance. We conducted experimental evolution on a phosphorus-solubilizing bacterial species, Priestia megaterium, in (i) soil only, (ii) liquid media only, and (iii) soil followed by liquid media, using population metagenomic sequencing to track mutations over time. Several typical in vitro evolutionary phenomena were observed in liquid media-incubated populations, including clonal interference, genetic hitchhiking, and mutation parallelism between replicate populations, particularly in the sporulation transcription factor spo0A. Liquid media-incubated populations also developed a clear fitness reduction in soil compared to the ancestral isolate. However, soil-incubated populations grew slowly, experienced far fewer generations despite longer absolute time, and accumulated minimal mutational changes. Correspondingly, soil-incubated populations did not display improved survival compared to the ancestral isolate in their target soils, though there did appear to be minor fitness reductions in unfamiliar soil. This work demonstrates that adaptation to liquid media and/or a native soil can impact bacterial fitness in new soil and that bacterial evolution in more complex real-world habitats does not closely resemble bacterial evolution in liquid media.

IMPORTANCE

Innovative solutions are needed to address emerging challenges in agriculture while reducing its environmental footprint. Management of soil microbiomes could contribute to this effort, as plant growth-promoting microorganisms provide key ecosystem services that support crops. Yet, inoculating beneficial microbes into farm soils yields unreliable results. We require a greater knowledge of the ecology of these taxa to improve their functioning in sustainable agroecosystems. In this report, we demonstrate that exposure to laboratory media and lingering adaptation to another soil can negatively impact the in-soil survival of a phosphorus-solubilizing bacterial species. We go further to highlight the underlying mutations that give rise to these patterns. These insights can be leveraged to improve our understanding of how soil-dwelling beneficial microorganisms adapt to different evolutionary pressures.

Rapid decline of adaptation of Pseudomonas fluorescens to soil biotic environment

Interactions between microbes can both constrain and enhance their adaptation to the environment. However, most studies to date have employed simplified microbial communities and environmental conditions. We determined how the presence of a commercial potting compost microbial community affected adaptation of the soil bacterium Pseudomonas fluorescens SBW25 in potting compost. Pseudomonas fluorescens clones isolated from populations evolved in both the presence and absence of the community showed similar fitness increases when measured in the absence of the community. This suggests the presence of the community did not constrain adaptation. By contrast, fitness measured in the presence of the community increased for community-evolved populations, but decreased below the ancestral state for populations evolved in the absence of the community. This suggests some, but not all, mutations that were beneficial with respect to the abiotic environment were costly in the presence of the community, with the former selected against in the presence of the community. Whole-genome sequencing supports this interpretation: most mutations underpinning fitness changes were clone-specific, suggesting multiple genetic pathways to adaptation. Such extreme mutational effects have not been observed in comparable in vitro studies, suggesting that caution is needed when extrapolating results from simplified in vitro systems to natural contexts.

It More than Adds Up: Interaction of Antibiotic Mixing and Temperature

Use of antibiotics for the treatment and prevention of bacterial infections in humans, agri- and aquaculture as well as livestock rearing leads to antibiotic pollution of fresh water and these antibiotics have an impact on free-living bacteria. While we know which antibiotics are most common in natural environments such as rivers and streams, there is considerable uncertainty regarding antibiotics’ interactions with one another and the effect of abiotic factors such as temperature. Here, we used an experimental approach to explore the effects of antibiotic identity, concentration, mixing and water temperature on the growth of Pseudomonas fluorescens, a common, ubiquitous bacterium. We exposed P. fluorescens to the four antibiotics most commonly found in surface waters (ciprofloxacin, ofloxacin, sulfamethoxazole and sulfapyridine) and investigated antibiotic interactions for single and mixed treatments at different, field-realistic temperatures. We observed an overall dependence of antibiotic potency on temperature, as temperature increased efficacy of ciprofloxacin and ofloxacin with their EC₅₀ lowered by >75% with a 10 °C temperature increase. Further, we show that mixtures of ciprofloxacin and ofloxacin, despite both belonging to the fluoroquinolone class, exhibit low-temperature-dependent synergistic effects in inhibiting bacterial growth. These findings highlight the context dependency of antibiotic efficacy. They further suggest antibiotic-specific off-target effects that only affect the bacteria once they enter a certain temperature range. This has important implications as freshwater systems already contain multi-drug antibiotic cocktails and are changing temperature due to environmental warming. These factors will interact and affect aquatic food webs, and hence this creates an urgent need to adapt and improve laboratory testing conditions to closer reflect natural environments.

Rapid evolution of bacterial mutualism in the plant rhizosphere

While beneficial plant-microbe interactions are common in nature, direct evidence for the evolution of bacterial mutualism is scarce. Here we use experimental evolution to causally show that initially plant-antagonistic Pseudomonas protegens bacteria evolve into mutualists in the rhizosphere of Arabidopsis thaliana within six plant growth cycles (6 months). This evolutionary transition is accompanied with increased mutualist fitness via two mechanisms: (i) improved competitiveness for root exudates and (ii) enhanced tolerance to the plant-secreted antimicrobial scopoletin whose production is regulated by transcription factor MYB72. Crucially, these mutualistic adaptations are coupled with reduced phytotoxicity, enhanced transcription of MYB72 in roots, and a positive effect on plant growth. Genetically, mutualism is associated with diverse mutations in the GacS/GacA two-component regulator system, which confers high fitness benefits only in the presence of plants. Together, our results show that rhizosphere bacteria can rapidly evolve along the parasitism-mutualism continuum at an agriculturally relevant evolutionary timescale.

Rapid evolution of bacterial mutualism in the plant rhizosphere

While beneficial plant-microbe interactions are common in nature, direct evidence for the evolution of bacterial mutualism is scarce. Here we use experimental evolution to causally show that initially plant-antagonistic Pseudomonas protegens bacteria evolve into mutualists in the rhizosphere of Arabidopsis thaliana within six plant growth cycles (6 months). This evolutionary transition is accompanied with increased mutualist fitness via two mechanisms: (i) improved competitiveness for root exudates and (ii) enhanced tolerance to the plant-secreted antimicrobial scopoletin whose production is regulated by transcription factor MYB72. Crucially, these mutualistic adaptations are coupled with reduced phytotoxicity, enhanced transcription of MYB72 in roots, and a positive effect on plant growth. Genetically, mutualism is associated with diverse mutations in the GacS/GacA two-component regulator system, which confers high fitness benefits only in the presence of plants. Together, our results show that rhizosphere bacteria can rapidly evolve along the parasitism-mutualism continuum at an agriculturally relevant evolutionary timescale.

Compost spatial heterogeneity promotes evolutionary diversification of a bacterium

Spatial resource heterogeneity is expected to be a key driver for the evolution of diversity. However, direct empirical support for this prediction is limited to studies carried out in simplified laboratory environments. Here, we investigate how altering spatial heterogeneity of potting compost-by the addition of water and mixing-affects the evolutionary diversification of a bacterial species, Pseudomonas fluorescens, that is naturally found in the environment. There was a greater propensity of resource specialists to evolve in the unmanipulated compost, while more generalist phenotypes dominated the compost-water mix. Genomic data were consistent with these phenotypic findings. Competition experiments strongly suggest these results are due to diversifying selection as a result of resource heterogeneity, as opposed to other covariables. Overall, our findings corroborate theoretical and in vitro findings, but in semi-natural, more realistic conditions.