Beyond the Standard: How We Prepare Unusual Sample Types for Microbiome Sequencing
At Microbiome Insights, we receive a wide range of sample types. While stool, soil, and swabs are familiar territory, some of our most interesting projects arrive as insects, tree roots, litres of wastewater or ocean water, and decades-old preserved tissue. The common thread is that no single extraction protocol fits every sample—much of the real work happens before extraction even begins, during sample pre-preparation. Here are the key considerations that guide how we approach these unique sample matrices.
Insects and Other Hard-Bodied Samples
Ants, bees, beetles and similar insects have tough exoskeletons that can resist lysis, so they may need additional mechanical disruption before extraction. The right approach depends on the specimen, the extraction method, and whether the study targets the whole-body microbiome or a specific compartment such as the gut.
Depending on the sample, options include:
- Small insects: Homogenize using micro-pestles or bead-beating, as appropriate for the extraction method. Chilling or freezing before grinding may help with handling and sample preservation.
- Larger insects: Cut the specimen into smaller pieces with sterile scissors and forceps before grinding or homogenizing.
- Cryogenic homogenization: For particularly tough specimens, freezing in liquid nitrogen with a larger grinding ball, followed by homogenization, can shatter the material.
The goal is sufficient disruption while preserving the biological information relevant to your study. If targeting internal endosymbionts or gut communities, rigorous surface sterilization (e.g., ethanol/bleach rinses) is essential, and the final rinse buffer should be sequenced as a control.
Tough Plant Tissue
Woody tissues such as bark, roots, and fibrous leaves require intensive mechanical disruption. Freezing in liquid nitrogen followed by bead-milling breaks down tough cell walls and yields a fine powder, allowing lysis buffers to penetrate the matrix effectively.
Plant chemistry also introduces challenges: secondary metabolites such as polyphenols and polysaccharides are released during grinding and can severely inhibit PCR amplification or downstream sequencing if carried over. While pre-preparation focuses on physical breakdown, it is critical to pair these samples with a DNA extraction protocol that features dedicated inhibitor-removal steps (such as specialized precipitation or magnetic-bead cleanup) to ensure clean, amplifiable DNA.
Water Samples and Filtration
Water samples vary dramatically in biomass and turbidity depending on the source. These differences affect how much water can be processed and which microorganisms are recovered.
For many microbial community studies, membrane filtration concentrates microbial biomass before extraction, and the filter itself is used for DNA extraction. A 0.2 micron filter is a common choice, but the best pore size, membrane material and volume depend on the water source and research goals.
A few practical considerations:
- Sediment-heavy or turbid water: Suspended particles can clog filters and limit the volume that can be processed. Options include a coarse prefilter mesh, multiple filters, or a smaller processed volume. Prefiltration can also remove particle-associated microbes, so keep that in mind when interpreting results.
- Filter format: Syringe filters suit smaller volumes and vacuum filter units suit larger ones.
- Consistency: Keep collection, filtration and preservation steps consistent across comparable samples, and record the volume processed.
Low-Biomass and High-Host Samples
Samples with little microbial biomass are especially vulnerable to contamination from collection materials, handling and reagents. In host-associated samples, host DNA can also make up much of what is recovered, particularly in shotgun metagenomic studies. These are related but distinct challenges.
Negative controls show where contamination may be coming from and give essential context for interpreting results. Depending on the study, useful controls include:
- Collection blanks: For example, an unused swab or filter exposed to the relevant collection and handling procedures, as appropriate for the study.
- Extraction blanks: Buffer-only tubes with no sample that pass through the full extraction workflow.
- Other process controls: PCR or library preparation negatives, where appropriate.
Controls don’t eliminate contamination or automatically identify every contaminant, but the background level of microbial detection is valuable data and strengthens how reliably results can be evaluated. For low-biomass projects, we recommend discussing control design with the lab before collection begins.
FFPE and Museum Samples: A Special Challenge
Formalin-fixed paraffin-embedded (FFPE) tissues and archived museum specimens are among the most demanding inputs in molecular biology. Formaldehyde fixation causes severe nucleic acid cross-linking, extreme fragmentation, and chemical modifications (such as cytosine deamination).
For FFPE tissue, preparation commonly includes:
- Complete Deparaffinization: Organic solvents or heat-mineral oil protocols must fully strip paraffin wax so lysis buffers can reach target cells.
- Optimized De-crosslinking: Extended high-temperature incubations break covalent formaldehyde bonds to make nucleic acids accessible for amplification.
- Adapted Library Prep: Because DNA from these sources is highly fragmented, library preparation workflows optimized for short-fragment inputs or enzymatic repair steps are required to generate representative sequencing libraries.
The appropriate workflow depends on the specimen and extraction method, and successful preparation does not guarantee sufficient DNA quality or microbial signal for sequencing.
The Takeaway
Good microbiome data starts well before the extraction kit comes out. Matching preparation to the sample type and including appropriate controls is what separates reliable results from noise. If you’re planning a project with an unusual sample type, get in touch early. We’re happy to talk through the best approach for your samples.