The Gold Standard of Ecology | Nuclease Accessibility, Membrane Integrity and Fraction Cross-Contamination

1. Define accessibility rather than assuming location

Back to contents

A molecule degraded by DNase was accessible under the treatment conditions. A molecule that survives may be inside an intact membrane, inside a vesicle, adsorbed where the enzyme cannot reach, shielded by proteins or simply undigested because the treatment failed.

Use operational labels such as nuclease-accessible and nuclease-resistant until independent evidence supports a biological compartment.

2. Keep membrane integrity separate from viability

Back to contents

An intact membrane can protect DNA or RNA, but dormant, damaged or recently dead cells may retain membranes. Some viable cells have permeable states, and sample preservation can alter membranes. Protection therefore does not equal metabolic activity or recent shedding.

If the claim concerns viability or activity, combine integrity assays with RNA, cultivation, metabolic markers or time-series evidence appropriate to the organism.

3. Validate nuclease activity in the sample matrix

Back to contents

Salts, humic substances, particles, pH and chelators can reduce enzyme access or activity. Add a known accessible spike before treatment and measure its removal. Include a no-enzyme control and, where possible, a protected spike that should survive.

A clean buffer validation is not enough for sediment, turbid water or biofilm material. Report enzyme concentration, temperature, time, mixing and termination.

4. Measure lysis introduced by fractionation

Back to contents

Pressure, vortexing, freeze–thaw cycles, long storage and filtration can rupture cells and release DNA into an extracellular fraction. Conversely, free DNA can adsorb to filters or pellets and appear cell-associated.

Process matched controls in different orders and minimise delays. Compare an unfractionated aliquot with the sum of fractions to reveal losses and newly released material.

5. Quantify carryover between fractions

Back to contents

Use compartment-specific markers, microscopy, flow cytometry or fluorescent tracers where feasible. Estimate how much cellular material enters the supernatant and how much accessible DNA remains in the pellet. Treat carryover as a measurement-error problem rather than declaring fractions pure.

Report detection limits for contamination. A small absolute carryover can dominate a low-abundance fraction.

6. Worked example: DNase-treated stream eDNA

Back to contents

A study treats turbid stream water with DNase before cell lysis. Most target DNA survives, so the result is labelled intact cellular material from recent fish presence. The accessible spike also survives in storm samples, showing that clay and organic particles inhibited digestion. Microscopy finds few intact target-sized cells, and serial washing moves much of the signal from the pellet into the supernatant.

After optimising enzyme conditions and adding matrix controls, only part of the signal remains resistant. The conclusion changes to a mixture of protected and particle-shielded DNA whose recency is unresolved.

7. Use mass balance to challenge the separation

Back to contents

Measure target copies or mass in the original sample and every recovered fraction. Large deficits may reflect adsorption or extraction loss; totals above the original can reflect assay variability or lysis that improves recovery. Mass balance need not be perfect, but unexplained imbalance limits state claims.

Carry uncertainty from recovery and inhibition into fraction comparisons. Ratios built from two noisy fractions can look precise while both are biased.

8. Changed-context transfer check: wastewater resistance genes

Back to contents

Move from fish eDNA to intracellular and extracellular antibiotic-resistance genes in wastewater. The transferable method is matrix-validated nuclease treatment, lysis controls and carryover measurement. The changed context adds abundant cells, viruses, vesicles and solids.

A resistant gene signal may be protected and potentially mobile without proving that a viable pathogen carries it. Culture, host linkage or genomic context is needed for that stronger claim.

9. Distinguish particle shielding from membrane protection

Back to contents

Treat samples with staged washing, gentle desorption, size fractionation or microscopy before and after nuclease exposure. If a signal becomes digestible after particle removal without membrane lysis, shielding or adsorption is more plausible than a cellular compartment.

No treatment is perfectly selective. Present competing explanations and show which observations distinguish them.

10. Audit order, timing and blanks

Back to contents

Retain collection-to-treatment time, storage temperature, mixing, filter pressure, centrifugation force, wash volumes, enzyme lot, spike recovery, blanks and extraction controls. Randomise batch position and process negative controls through the full fractionation.

Cross-contamination can occur before PCR. A clean amplification blank does not test leakage between sample fractions.

11. Frequently asked questions

Back to contents

Does DNase resistance prove DNA is intracellular? No; shielding and incomplete digestion are alternatives. Does membrane integrity prove a living cell? No. Can fractions be corrected for carryover? Sometimes, if recovery and leakage are measured with suitable controls. Is serial filtration enough? It separates operational size classes, not pure biological states.

12. The gold standard makes protection testable

Back to contents

State-specific eDNA analysis becomes credible when accessibility, membrane integrity and fraction purity are measured rather than inferred from a label. Validate enzymes in the matrix, quantify lysis and carryover, inspect mass balance and test particle shielding.

The gold standard uses resistant and accessible fractions as conditional evidence. It does not turn a treatment outcome into a biological clock, a viability test or a clean compartment without the controls that make that interpretation possible.