No Band From Soil DNA: Template, Inhibitors or Primers?
An undergraduate team is amplifying a 620 bp region from DNA extracted from a local soil sample. Each 25 µL reaction contains 1× Taq buffer, 1.5 mM MgCl₂, 0.2 mM dNTPs, 0.5 µM of each primer, 1 U Taq polymerase and 2 µL of template. The programme is 95 °C for 5 min, then 30 cycles of 95 °C 30 s, 58 °C 30 s, 72 °C 45 s, with a final 72 °C for 5 min. A pure-culture DNA positive control on the same plate gives a clean, strong band at 620 bp. The first soil extract gives no band at all. A second extract from a different site gives one faint band plus a bright smear below 200 bp. An annealing gradient from 55 to 65 °C produces the faint band only at 55 °C, and the smear worsens as temperature falls. The team meets in the evening to decide what to change before the next morning's run.
1. Rank the possible causes - bad primers, bad reagents, low template, PCR inhibitors - and justify the ranking from the evidence given.
Read the worked answer
Primers, dNTPs, polymerase, buffer and the thermal cycler are all exonerated: the positive control on the same plate, with the same master mix, produced a strong 620 bp product. Any reagent or primer-design fault would have hit the control too.
That leaves the template. The two soil samples behave differently, which is informative: sample 2 gives a faint specific band plus a low-molecular-weight smear, and the smear grows as annealing stringency drops. That pattern is characteristic of very little specific template competing against non-specific priming - with scarce target, the polymerase amplifies whatever partially matches, and lower temperature widens that window. Sample 1 gives nothing at all, which is the extreme of the same problem or a stronger inhibitor load.
So the ranking is: (1) low template yield plus co-extracted humic acids/polyphenols acting as inhibitors; (2) template degradation; (3) unlikely - cycling or reagent fault; (4) essentially excluded - primer design.
2. Design the single most decisive experiment for the next morning and state exactly what result would confirm inhibition.
Read the worked answer
Run a spike-in (mock-inhibition) test: prepare positive-control reactions containing pure-culture DNA, and add 2 µL of the failed soil extract to half of them, with 2 µL of water in the other half. Everything else stays identical.
Interpretation:
- Spiked reactions lose or strongly weaken the 620 bp band relative to the water-spiked controls → inhibitors carried over from the soil extract are confirmed, and the problem is the template, not the sample's DNA sequence.
- Spiked and unspiked reactions are equally strong → no inhibition; the soil extracts simply contain too little amplifiable DNA, and the fix is more template or a better extraction.
In parallel, run a 1:5, 1:10 and 1:50 dilution series of both extracts in the same block. A band appearing at 1:10 but not undiluted is a second, independent confirmation of inhibition, because dilution reduces inhibitor concentration faster than it reduces the already-scarce template. Together the two experiments take one plate and settle the question by morning.
3. Recommend the cheapest effective fix and say what evidence would show your diagnosis is wrong.
Read the worked answer
Cheapest first: dilute the template 1:10 in water or buffer and re-run, since inhibitors are diluted away while enough template usually survives. If that fails, add BSA at 0.2–0.4 µg/µL to the master mix - BSA sequesters humic substances and polyphenols and costs a few rupees per reaction - or add PVPP to the extraction. Only if both fail should the team spend on a clean-up column or a re-extraction, which costs hours and consumables.
Do not lower the annealing temperature to chase the faint band; the gradient already shows that 55 °C buys a little specific product at the price of a much brighter smear.
Falsification: if the spike-in shows no inhibition, if dilution and BSA change nothing, and if the extract's nucleic-acid quantification is near the detection limit, then the diagnosis is simply too little DNA - which is why quantifying the extracts before the next run is worth the ten minutes.