Methods in Microbial Ecology (I)
1 sub-topics · Pages 559–571
1. Introduction
The 'great plate count anomaly' — the observation that 99% of environmental bacteria cannot be cultured on standard laboratory media — drove the development of culture-independent molecular methods that have transformed microbial ecology. Techniques such as FISH (fluorescence in situ hybridisation), 16S rRNA amplicon sequencing, metagenomics, and stable isotope probing allow us to identify who is present, what they are doing, and how diverse they are — without ever growing them in the laboratory. These methods have revealed entire domains of life previously unknown to science.
✏️ Fill in the Blank
1. The culture-independent technique that uses fluorescently labelled oligonucleotide probes targeting rRNA to identify microorganisms directly in environmental samples is called _______.
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FISH (Fluorescence In Situ Hybridisation)2. The genomic analysis of all genetic material recovered directly from an environmental sample, without prior cultivation, is called _______.
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Metagenomics3. The observation that only ~0.1–1% of bacteria visible by microscopy can be cultured on standard laboratory media is known as the great plate count _______.
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Anomaly4. The method that tracks incorporation of heavy isotope-labelled substrates (e.g., ¹³C-glucose) into specific community members to link identity with function is called stable isotope _______ (SIP).
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Probing5. The Shannon diversity index is symbolised as _______ and increases with both species richness and evenness.
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H'6. The technique that separates nucleic acid fragments by size using an electric field through a gel matrix is called gel _______.
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Electrophoresis7. The amplification and profiling of 16S rRNA gene fragments from environmental DNA without culturing is called _______.
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Amplicon sequencing (16S rRNA amplicon sequencing)8. The stable isotope ¹³C can be used in _______ experiments to trace carbon flow through microbial communities.
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Stable isotope probing (SIP)🔘 Multiple Choice
1. The 'great plate count anomaly' refers to the observation that:
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Correct: B) The number of bacteria that can be cultured on standard media is far lower than the total number visible by direct microscopy2. 16S rRNA amplicon sequencing of environmental samples allows researchers to:
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Correct: C) Identify and compare microbial community composition without culturing3. Denaturing Gradient Gel Electrophoresis (DGGE) of PCR-amplified 16S rRNA genes is used to:
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Correct: B) Obtain a community fingerprint showing changes in dominant populations over time or space4. Why is the 16S rRNA gene used as a phylogenetic marker for classifying prokaryotes?
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Correct: B) It is universally present, functionally conserved (resists HGT), contains both conserved and variable regions, and is easily amplified5. Rarefaction curves in microbial ecology are used to:
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Correct: B) Compare species richness across samples with different sequencing depths6. Stable isotope probing (SIP) with ¹³C-labelled substrates allows researchers to:
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Correct: B) Link metabolic activity with phylogenetic identity of active community members by labelling their DNA/RNA7. Metatranscriptomics differs from metagenomics in that it:
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Correct: B) Sequences mRNA to reveal which genes are actively transcribed at the time of sampling8. Terminal Restriction Fragment Length Polymorphism (T-RFLP) generates community fingerprints based on:
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Correct: B) Restriction enzyme cutting of fluorescently labelled PCR products at species-specific sites, generating unique fragment sizes9. Alpha diversity refers to:
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Correct: B) Diversity within a single habitat or sample10. Quantitative PCR (qPCR) of 16S rRNA genes in environmental samples provides:
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Correct: B) Absolute or relative quantification of target bacterial gene copies in the sample11. Fluorescence in situ hybridisation (FISH) allows researchers to:
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Correct: B) Visualise and identify specific microorganisms in situ using fluorescently labelled oligonucleotide probes12. Terminal restriction fragment length polymorphism (T-RFLP) is used to:
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Correct: B) Generate a community fingerprint by cutting labelled 16S amplicons with restriction enzymes13. Most Probable Number (MPN) estimation is most appropriate when:
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Correct: B) Samples contain cells that do not form discrete colonies on agar14. The Shannon diversity index (H') measures:
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Correct: B) Both species richness and evenness of abundance distribution15. Microautoradiography combined with FISH (MAR-FISH) can determine:
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Correct: B) Which specific cells in a community are actively taking up a radiolabelled substrate16. In shotgun metagenomics, sequence reads are assembled into longer contigs and then:
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Correct: B) Annotated for protein-coding genes using bioinformatic pipelines to infer taxonomic and functional composition💬 Open-Ended Questions
1. Why has the development of culture-independent molecular techniques revolutionised microbial ecology? What were the limitations of culture-based methods, and what major discoveries have been made possible?
Hint / Guidance
Culture-based limitations: biased toward fast-growing copiotrophs (R-strategists); VBNC not detected; obligate syntrophs cannot grow alone; <1% of environmental bacteria culturable (great plate count anomaly). Culture-independent discoveries: (1) Norman Pace 16S rRNA phylogenetic survey of Yellowstone hot springs → novel thermophiles never cultured; (2) SAR11 (most abundant ocean bacterium) uncultured for decades; (3) Ammonia-oxidising Thaumarchaeota (Crenarchaeota) discovered by 16S surveys, dominate deep ocean nitrification; (4) Candidate Phyla Radiation (CPR) — >50 phyla detected only by metagenomics; (5) Human microbiome project revealed 10,000+ species in gut. Revolution: we now estimate ~99% of microbial diversity was unknown before molecular methods.2. Describe the principle of FISH (Fluorescence In Situ Hybridisation). How are probes designed, what do they target, and what information can be obtained that is not possible with culture methods?
Hint / Guidance
Principle: fluorescently labelled oligonucleotide probes (15–25 nt) complementary to specific 16S rRNA sequences hybridise inside fixed, permeabilised cells; hybridised cells fluoresce under epifluorescence microscope. Probe design: conserved anchor regions + variable probe region; multiple probes can be multiplexed (different colours); universal (EUB338), domain-level (ARC915 for Archaea), genus/species-specific probes. Target: rRNA (104–105 copies/cell = amplified signal; ribosome abundance correlates with metabolic activity). Advantages over culture: visualise uncultured organisms; spatial localisation in biofilms/tissues; relative abundance estimates without cultivation; simultaneous identification of multiple taxa; FISH-microautoradiography links identity with substrate uptake.3. You are studying microbial community changes in a river receiving agricultural runoff over a season. Which combination of molecular methods would you use to assess (a) who is there, (b) what are they doing, and (c) how does diversity change over time? Justify each method choice.
Hint / Guidance
(a) Who: 16S amplicon sequencing (Illumina MiSeq/NextSeq) → taxonomic composition at OTU/ASV level; cost-effective, large sample throughput. FISH for validation of key taxa. (b) What: metatranscriptomics (mRNA sequencing) → active metabolic genes; ¹³C-SIP with specific substrates (e.g., ¹³C-nitrate) → link nitrogen cycling to active organisms; qPCR of functional genes (amoA for nitrification; nirS/nirK for denitrification). (c) Change over time: Shannon/Simpson indices from amplicon data; beta diversity (Bray-Curtis dissimilarity) over time-points; DGGE for rapid visualisation of shifts. Statistical: PERMANOVA for community composition changes with environmental variables (nutrients, flow rate, temperature).4. What is metagenomics and how does it differ from 16S amplicon sequencing? When would you choose each approach?
Hint / Guidance
16S amplicon sequencing: PCR amplification of specific 16S variable regions (V3-V4 most common); primer bias; taxonomic assignment only; cannot detect functional genes; cheap (£30–100/sample). Shotgun metagenomics: randomly shear all environmental DNA → sequence all fragments; no PCR bias; taxonomic AND functional gene information; detect novel genes; assemble near-complete genomes (MAGs — metagenome-assembled genomes); expensive (£200–1000/sample); computational challenge. Choose 16S when: large sample numbers, primarily taxonomic questions, limited budget. Choose metagenomics when: functional gene content important, novel organisms expected, genome-level insights needed, metabolic pathway reconstruction. Recent development: long-read metagenomics (Nanopore) allows direct sequencing without assembly.5. Describe what a rarefaction curve shows and how it is used to determine whether a sample has been sequenced deeply enough to capture community diversity.
Hint / Guidance
Rarefaction: subsample random sequences at increasing depth → calculate diversity metric (species richness, OTU count) at each depth → plot. Interpretation: steep initial slope = high diversity; plateau = sampling depth sufficient to capture most diversity (new sequences no longer discover new OTUs). If curve has not plateaued: undersampling — more sequencing would reveal more diversity. Comparison between samples: rarefy to equal depth (standardise) before comparing richness. Modern alternative: Chao1 estimator (non-parametric richness estimate accounting for singleton/doubleton OTUs) provides diversity estimate without losing data through rarefaction. Alpha diversity metrics requiring rarefaction: species richness, Shannon H', Faith's PD.6. Explain the principle of stable isotope probing (SIP) and how it can be used to identify which bacteria are actively degrading a specific pollutant in soil.
Hint / Guidance
Principle: add isotopically-labelled substrate (e.g., ¹³C-naphthalene) to soil; organisms that actively use it incorporate ¹³C into macromolecules (DNA-SIP: ¹³C-DNA is denser → separates from ¹²C-DNA by ultracentrifugation in CsCl density gradient); extract ¹³C-'heavy' fraction → 16S amplicon sequencing reveals identity of active degraders. Application: for PAH degradation — soil incubated with ¹³C-phenanthrene; heavy DNA extracted → Sphingomonas, Mycobacterium spp. identified as primary degraders. Advantages: culture-independent; links function directly to identity; works with any C-containing substrate. Limitations: minimum labelling required (~15–30 days); cross-feeding (secondary consumers incorporate ¹³C from labelled cells); time-lag bias.7. What are OTUs (Operational Taxonomic Units) and ASVs (Amplicon Sequence Variants)? What are the advantages and disadvantages of each?
Hint / Guidance
OTU: cluster 16S sequences at 97% similarity threshold → represent 'species-level' groupings. Method: UCLUST, CD-HIT; traditional approach. Problems: 97% threshold arbitrary; merges distinct species; masks within-species diversity; computationally intensive for large datasets; different clustering algorithms give different results. ASV (denoising approach: DADA2, Deblur): infer exact biological sequences correcting for sequencing errors; single-nucleotide resolution; reproducible across studies; can compare datasets without re-clustering. Advantages of ASVs: higher resolution (distinguish strains); stable identifiers (exact sequence); databases can be shared between studies; detects rarer taxa. Disadvantages: may split PCR chimeras into false positives if not denoised properly; single errors can create false novel ASVs. Modern standard: ASVs preferred over OTUs.8. Explain the concept of beta diversity and how it is used to compare microbial communities. What is the Bray-Curtis dissimilarity and when would UniFrac be more appropriate?
Hint / Guidance
Beta diversity: dissimilarity between communities (samples). Methods: (1) Bray-Curtis dissimilarity: based on abundances of shared/unshared OTUs; 0=identical, 1=completely different; quantitative (accounts for abundance differences); does not use phylogeny. (2) Jaccard distance: presence/absence only; ignores abundance. (3) UniFrac: phylogenetic dissimilarity — incorporates branch lengths on phylogenetic tree; unweighted (presence/absence + phylogeny); weighted (abundance + phylogeny). Use Bray-Curtis when: phylogenetic relationships unknown/unimportant; quantitative community composition. Use UniFrac when: evolutionary divergence matters; comparing communities with many novel taxa; gut microbiome studies where phylum-level shifts important (Firmicutes/Bacteroidetes ratio meaningful). Ordination: PCoA of Bray-Curtis/UniFrac matrices visualises community relationships; PERMANOVA tests significance.9. Why is the 16S rRNA gene used as a 'universal phylogenetic clock' for prokaryotes? Describe three advantages and two limitations of this approach for microbial diversity studies.
Hint / Guidance
Advantages: (1) Universally present in all prokaryotes — universal primers (515F/806R) amplify broad range; (2) Functionally constrained — essential ribosome component cannot tolerate most mutations → slow, clock-like evolution; (3) Contains 9 variable regions (V1–V9) flanked by conserved regions — allows both universal amplification and species discrimination; (4) Extensive reference databases (SILVA, RDP, Greengenes) enable taxonomic assignment. Limitations: (1) Multiple 16S copies per genome (up to 15 in Bacillus) → overestimates richness; (2) PCR bias: some taxa poorly amplified by universal primers (low GC, secondary structure); (3) 97% species boundary arbitrary — does not map cleanly to biological species; (4) Horizontal transfer of 16S genes can mislead phylogeny (rare but documented); (5) Functional information not available — taxonomy alone cannot predict metabolic capabilities.10. Describe the use of functional gene markers in environmental microbiology. Give examples of three functional genes used as biomarkers for specific microbial processes.
Hint / Guidance
Functional gene markers: instead of taxonomic marker (16S), amplify genes encoding enzymes of specific biogeochemical processes → assess presence/abundance/activity. (1) amoA (ammonia monooxygenase subunit A): marker for ammonia-oxidising bacteria (amoA-bacteria) and archaea (amoA-Thaumarchaeota); qPCR quantifies nitrification potential; revealed archaea dominate ocean nitrification. (2) nifH (nitrogenase reductase): marker for N₂ fixation; high diversity in environment; includes non-cultured diazotrophs; used to identify active N-fixers in soil/ocean. (3) dsrA/dsrB (dissimilatory sulfite reductase): marker for sulfate-reducing organisms; identifies SRB diversity in anoxic sediments/oil reservoirs. (4) pmoA (particulate methane monooxygenase): marker for methanotrophs in wetlands/lake sediments. Combined with SIP: qPCR on ¹³C-DNA fraction identifies active organisms expressing specific functions.11. Compare 16S rRNA amplicon sequencing with shotgun metagenomics for microbial community analysis. When would you choose each approach?
Hint / Guidance
16S amplicon: PCR-amplify 16S variable region (V3–V4); sequence millions of reads; taxonomic profiling; cheap; limited to bacteria/archaea; primer bias; no functional information; good for large survey studies, clinical diagnostics, time-series. Shotgun metagenomics: sequence all DNA; taxonomic + functional (KEGG, COG, CAZy); detect viruses, eukaryotes, AMR genes; reconstructs MAGs (metagenome-assembled genomes); no primer bias; expensive; requires deep sequencing to detect rare taxa; complex bioinformatics. Choose 16S: large-n taxonomic survey, low budget. Choose shotgun: functional characterisation, virus detection, novel organisms, clinical resistance gene surveillance.12. What is stable isotope probing (SIP) and how is it used to link microbial identity with metabolic function in environmental samples?
Hint / Guidance
SIP: incubate environmental sample with ¹³C (or ¹⁵N, ¹⁸O)-labelled substrate; active metabolisers incorporate heavy isotope into biomass (DNA, RNA, PLFA); density-gradient ultracentrifugation separates ¹³C-labelled 'heavy' DNA from unlabelled 'light' DNA; 16S rRNA amplicon sequencing or metagenomics of heavy fraction identifies organisms that were actively metabolising the substrate. Example: ¹³C-methanol SIP in soil → identifies methylotrophic bacteria (Methylobacterium, Hyphomicrobium). Advantage: links 'who is there' (taxonomy) with 'who is doing what' (function) without cultivation. RNA-SIP more sensitive (less incubation time needed); FISH-MAR also links activity and identity.13. Describe the process of constructing a metagenome-assembled genome (MAG) and discuss the limitations of this approach.
Hint / Guidance
Process: (1) shotgun metagenomics → paired-end reads; (2) Assembly: SPAdes/MEGAHIT assembles reads into contigs; (3) Binning: group contigs into bins representing single genomes using tetranucleotide frequency + differential coverage across samples (MetaBat2, CONCOCT, MaxBin2); (4) Bin quality assessment: CheckM estimates completeness and contamination using single-copy core genes; high quality: >90% complete, <5% contamination. (5) Taxonomic classification: GTDB-Tk; functional annotation: Prokka. Limitations: incomplete assemblies (repetitive regions, strain heterogeneity); chimeric bins (two organisms mixed); underrepresented low-abundance organisms; cannot confirm all genes are from same cell; no phenotypic validation.14. How is quantitative PCR (qPCR) used to quantify specific microbial populations in environmental samples? What are its key controls and limitations?
Hint / Guidance
Principle: real-time fluorescence detection (SYBR Green or TaqMan probe) during PCR amplification; fluorescence crosses threshold at cycle threshold (Ct); standard curve (serial dilutions of known copy number) converts Ct to gene copy number (e.g., 16S rRNA copies/g soil). Controls: positive control (known template); negative control (no template); inhibition control (internal standard to detect PCR inhibitors in environmental extract); efficiency check (95–105% acceptable). Limitations: primer/probe specificity (cross-react with non-target organisms); PCR inhibitors in soil/sediment (humic acids, iron); rRNA gene copy number varies (1–15 copies/genome); no viability information; relative abundance only unless paired with total cell counts; cannot discover unknown organisms.15. What is network analysis in microbial ecology and what ecological insights can it provide about community structure?