This episode of the 'This Week in Microbiology' podcast discusses two recent scientific papers, one exploring the distinct microbiome within living trees and the other detailing a search for novel biopesticides from Mediterranean bacteria.
The first study, published in Nature, reveals that trees host a vast and unique microbiome, with approximately one trillion prokaryotic cells per tree. This internal ecosystem is distinctly partitioned between the anaerobic heartwood (rich in Clostridia and methanogens) and the more aerobic sapwood. The research demonstrates that this wood microbiome is largely distinct from surrounding soil communities, varies by tree species, and shows metabolic activity consistent with its environment, suggesting a co-evolutionary relationship between the tree and its resident microbes.
The second paper, from Applied and Environmental Microbiology, details a systematic screening of 788 bacterial isolates from diverse habitats on the Greek island of Crete to find new biopesticides against mosquito larvae. The study identified 37 highly effective isolates from 20 different genera that caused 100% larval mortality within three days. Further analysis showed that the lethality was caused by produced metabolites rather than active infection, with most of the potent compounds being extracellular and non-polar, highlighting the potential for discovering environmentally friendly pest control agents from untapped microbial sources.
Microbes in Plants: From Tree Internals to Novel Biopesticides
0:24 Podcast Introduction: The hosts of "This Week in Microbiology" (TWiM) introduce episode 340, which will focus on two papers about the microbes found in and on plants.
4:18 Paper 1: Tree Microbiome Overview: The first paper discussed is a Nature article titled "A diverse and distinct microbiome inside living trees." It highlights that trees store over 300 gigatons of carbon and that the average tree contains one trillion prokaryotic cells in its woody tissues, an order of magnitude more than the average human.
5:55 Heartwood vs. Sapwood: The study surveyed the microbiomes of over 150 trees in the northeastern United States, comparing the innermost wood (heartwood) with the outermost wood (sapwood). They found distinct microbial communities in each.
9:09 Unique Microbial Communities: The wood microbiome contains unique bacteria not found in the surrounding soil, suggesting it's a distinct ecological niche. Wood has high abundances of Clostridia, Bacilli, and methanogenic archaea, which are rare in the neighboring soil. This indicates the microbes may have arrived with the seed or been established long ago.
13:58 Support for Plant Health: The study found taxa with metabolic capabilities relevant to plant function, such as nitrogen fixers, suggesting these microbes may contribute to the tree's nutrition and overall health.
16:09 Environmental Drivers of Diversity: The distinct microbial communities in heartwood and sapwood are driven by different environmental conditions. Heartwood is largely anoxic (low oxygen) and nutrient-poor, favoring anaerobic bacteria like Clostridia and methanogens. Sapwood is more aerobic, supporting different microbes.
20:13 Metabolic Activity Confirmed: Researchers confirmed the predicted metabolic activity by measuring gas concentrations in the trees. They found higher levels of anaerobic metabolites like methane and nitrous oxide in the heartwood. Furthermore, fresh tree core samples produced methane under anoxic lab conditions, confirming the activity of the resident microbes.
22:34 Minor Role of Soil: The study concludes that soil communities play a relatively minor role in shaping the wood microbiome, as only 3-5% of the community from mineral soils was found inside the trees.
29:46 Paper 2: Searching for Biopesticides: The second paper, from Applied and Environmental Microbiology, focuses on finding new biopesticides to combat mosquito-borne diseases and overcome insecticide resistance by screening bacteria from the Greek island of Crete.
32:27 High-Throughput Screening: Researchers collected and screened 788 bacterial isolates from 65 locations on Crete. They tested these isolates for their ability to kill mosquito larvae and identified 108 that showed larvicidal activity. From these, they selected 37 potent "hits" that caused 100% mortality in under three days.
35:23 Metabolites, Not Infection: To determine the killing mechanism, researchers used sonicated (lysed) bacterial cultures. The larvicidal activity persisted, proving that the killing was due to chemical metabolites produced by the bacteria, not a live infection.
36:58 Chemical Nature of Toxins: The study further characterized the active compounds by performing extractions with polar (ethyl acetate) and non-polar (hexane) solvents. This helps determine the chemical properties of the potential biopesticides.
39:30 Key Findings: The screening concluded that about 10% of environmental isolates produced compounds lethal to mosquito larvae. Most of the highly active compounds were extracellular (secreted by the bacteria) and non-polar. The majority of these potent bacteria were isolated from plants or the soil around their roots (the rhizosphere).
Summary of Provided References
A diverse and distinct microbiome inside living trees (Nature)
Focus: Explores the largely unstudied microbiome within the wood of living trees, which constitutes Earth's largest biomass reservoir.
Key Findings: A single tree can host approximately one trillion bacteria. These microbial communities are clearly separated between the inner heartwood and outer sapwood. The heartwood is a unique ecological niche dominated by anaerobic bacteria and archaea.
Implication: This research supports the "holobiont" concept (the host and its microorganisms as a single ecological unit) and establishes a new area for environmental microbiology research.
Bacteria isolated from biodiverse Mediterranean island habitats yield a large array of biopesticidal metabolites against mosquito larvae (Applied and Environmental Microbiology)
Focus: A search for new, environmentally friendly biopesticides to combat insecticide resistance in mosquitoes.
Methodology: Screened 788 bacterial isolates from various habitats on the island of Crete for their ability to kill mosquito larvae (Culex pipiens molestus).
Key Findings: Identified 108 larvicidal isolates, with 37 causing 100% mortality in less than 3 days. The killing activity is mediated by metabolites, not live bacterial infection. Non-polar extractions were generally more effective at isolating the active compounds.
Impact: Highlights the vast potential of bacteria as a source for novel insect control tools and lays the groundwork for developing new biopesticides.