Molecular Virology — Questions

a. The result is uninterpretable, not a true negative. The internal control is there to prove the reaction could amplify; because it failed, amplification was blocked, so a negative viral target cannot be trusted. That the positive and no-template controls behaved normally localises the problem to this sample, not the run.

b. The likely cause is inhibition by something in the specimen, commonly blood, heparin, excess salts or reagents carried over from extraction, or an extraction failure or a lost or mishandled sample.

c. The sample must be re-processed, not reported as negative: re-extract and repeat, and if inhibition persists dilute the sample to reduce the inhibitor and repeat. If it still fails, report the result as inhibited and request a fresh specimen rather than issue a false negative.

A false-negative CSF PCR arises before, during or after the assay.

• Timing. The specimen may be taken too early, before the target is detectable, or too late, after the virus has cleared from the cerebrospinal fluid.
• Low target. CSF volumes are small and viral loads in the central nervous system can be low, so the target may fall below the limit of detection.
• Inhibition. Substances in the sample, particularly blood from a traumatic tap, inhibit the polymerase; an internal control that fails alongside a negative target flags this rather than a true negative.
• Assay factors. Primer-site sequence variation in a divergent strain, RNA degradation, or extraction failure can each abolish amplification.

The TaqMan probe binds a sequence within the amplicon, between the two primers, and carries a reporter dye at one end and a quencher at the other. While the probe is intact the quencher sits close enough to the reporter to suppress its fluorescence.

During the extension step the 5’ exonuclease activity of Taq polymerase cleaves the bound probe, releasing the reporter dye away from the quencher so that it emits light on excitation. Because a probe is hydrolysed for each new amplicon, the fluorescent signal is proportional to the product made, and detection is sequence-specific because signal arises only when the probe has hybridised to its target.

• Raise the annealing temperature. A higher temperature allows only closely matched primer-template duplexes to remain bound, favouring specific over non-specific priming.
• Lower the salt (magnesium) concentration. Less magnesium destabilises mismatched binding, so the reaction tolerates less mismatch. Both changes trade some sensitivity for greater specificity.

• Gel electrophoresis. The amplicon is separated by size and visualised with an intercalating dye such as ethidium bromide that fluoresces under ultraviolet light.
• Real-time fluorescence. In real-time PCR the product is detected as it accumulates through a fluorescent dye or sequence-specific probe, with no separate post-amplification step.

CRISPR-based detection uses a programmable nuclease guided to a specific viral sequence, adding a layer of sequence recognition beyond the primers of an amplification reaction.

In practice it is coupled to an isothermal amplification step such as RT-LAMP: once the target is amplified and recognised by the guide, the activated nuclease shows collateral cleavage of a labelled reporter, generating a fluorescent or lateral-flow signal. The attractions for diagnostics are a rapid, low-cost, point-of-care format needing minimal instrumentation, and, because many CRISPR nucleases are available, the possibility of highly parallel genotyping in a single reaction. It confirms that the intended species was amplified, improving specificity over amplification alone.

A complete answer weighs the clinical gains against the interpretive and technical costs, then gives the practical ways a laboratory manages them.

Advantages. A single respiratory, meningitis-encephalitis or gastrointestinal cartridge tests one specimen for a dozen or more causes in about an hour. This gives broad, fast coverage when the differential is wide, supports empirical treatment and isolation decisions that cannot wait for sequential testing, and needs little hands-on time.

Challenges. Packing many reactions into one tube risks competition between targets and reduced sensitivity for any single one against a dedicated assay. Broad panels detect organisms of uncertain clinical significance, including prolonged shedding or incidental carriage, so a positive is not automatically the cause of the presentation. Cost per test is high, and a long result list can mislead if read uncritically.

Solutions. Match the panel to the clinical question rather than testing everything reflexively, confirm or quantify a key positive with a targeted assay where it changes management, and report with interpretative comment so the clinician reads each result against the clinical picture.

A complete answer groups the chemistries by what they multiply, target, signal or probe, then names the main examples of each with a line on how it works.

Target amplification makes more of the target sequence. PCR is the prototype, using flanking primers and a thermostable polymerase through cycles of denaturation, annealing and extension; an RT step (RT-PCR) is added for RNA. Variants include nested PCR (two primer rounds for sensitivity), multiplex PCR (several primer sets, several targets), and the isothermal methods that need no thermal cycler: NASBA and TMA (transcription-based, for RNA), SDA (strand displacement), and LAMP (loop-mediated, well suited to point-of-care).

Signal amplification leaves the target untouched and multiplies the detection signal. The branched DNA (bDNA) assay builds a tree of enzyme-labelled probes onto the captured target; hybrid capture detects RNA-DNA hybrids with an antibody and a chemiluminescent readout. Both avoid amplicon carryover and give stable quantification.

Probe amplification multiplies a probe rather than the target, the ligase chain reaction (LCR) joining adjacent probes only on a matching target.

Real-time detection chemistries sit alongside these: SYBR Green binds any double-stranded DNA and needs melting-curve analysis for specificity, while sequence-specific hydrolysis (TaqMan), hybridisation (FRET) and molecular-beacon probes confirm the amplicon’s sequence directly.

A complete answer covers the analytical, bioinformatic and interpretive problems that make an NGS resistance assay harder to validate than a single-target PCR.

The central analytical problem is the minority variant: NGS can report mutations present in a small fraction of the population, so validation must set a detection threshold and prove it separates true low-frequency variants from PCR and sequencing error. This means establishing a limit of detection and reproducibility across runs and controlling error rates, since a threshold set too low reports noise and one set too high misses real resistance.

The bioinformatic pipeline is part of the assay and must itself be validated and version-controlled, because a change in software or reference can change the result. Performance is anchored with reference standards, external quality assessment and concordance against Sanger sequencing as the established comparator.

Interpretation is unsettled: the clinical significance of small minority resistant populations is still uncertain, and sensitivity depends on the input viral load, so a low-load sample may not yield reliable variant calling.

Each cycle doubles the target, so a three-cycle-earlier threshold means about 2^3, or eightfold, more starting template. The cycle threshold falls as input rises.

Three cycles is not a threefold difference (a linear misreading), and 30 or 300 fold would correspond to far larger cycle gaps.

A mismatch at the 3’ end is poorly tolerated because that is where the polymerase begins extension, so a mismatched 3’ base blocks efficient synthesis. Mismatches nearer the 5’ end are better tolerated.

Extension begins at the 3’ end, not the 5’ (excluding A and E), and tolerance is not uniform along the primer (excluding B and C).

Clean DNA reads about 1.8; a lower ratio points to protein or phenol contamination, which absorb near 280 nm and depress the ratio. Such contaminants can also inhibit downstream PCR.

Pure DNA would sit near 1.8, RNA contamination would raise not lower the ratio, and the value reflects contamination rather than magnesium or an empty eluate.

Taq polymerase copies only DNA, so an RNA target must first be reverse-transcribed into complementary DNA (RT-PCR) before amplification.

Ligation, restriction digestion and alkaline denaturation are not part of RT-PCR, and uracil-N-glycosylase is a contamination-control measure, not a route to copying RNA.

Magnesium is the polymerase cofactor, and in excess it stabilises mismatched primer-template binding and undenatured double-stranded DNA, lowering specificity (and yield). Its concentration is therefore optimised carefully.

Too little magnesium starves the enzyme, but it does not degrade the polymerase, block reverse transcription, or govern the denaturation temperature.

A dideoxynucleotide lacks the 3’ hydroxyl group needed to form the next phosphodiester bond, so extension stops once it is incorporated. The labelled terminator marks the final base of each fragment.

The dideoxynucleotide keeps its 5’ phosphate and base, carries a fluorescent label, and chain termination is a covalent-extension problem, not a hydrogen-bonding one.

LAMP is isothermal, running at a single temperature, so it needs no thermal cycler and can be read simply by turbidity or a dye, which makes it attractive near the patient.

It does not need a thermal cycler or capillary electrophoresis, does not use radioactive labels, and can be adapted to RNA and crude samples.

Incorporating uracil in place of thymine and adding uracil-N-glycosylase destroys any carried-over uracil-containing amplicon before the next reaction, specifically countering carryover contamination.

Bovine serum albumin and extra polymerase counter inhibition, higher annealing temperature raises specificity, and neither magnesium nor sample volume addresses carryover.

The branched DNA assay leaves the target untouched and builds a tree of labelled probes onto it, amplifying the signal in proportion to target. This avoids amplicon carryover and gives stable quantification.

PCR, NASBA, LAMP and TMA are all target-amplification methods: they make more copies of the target sequence itself.

The copy-to-unit relationship differs between assays, so reporting against a WHO international standard in IU/mL lets a load be compared across laboratories and platforms and lets serial results be tracked meaningfully.

Copies can be measured, international units are not intrinsically more sensitive, the scale is not limited to DNA viruses, and quantification still uses a standard curve.

SYBR Green binds any double-stranded DNA, so it lights up primer dimers and off-target products as readily as the intended amplicon. Melting-curve analysis then resolves products by their distinct melting points, restoring specificity.

The dye is not sequence-specific, does not quench a probe, does not read RNA, and fluoresces when bound to double-stranded DNA rather than above the melting point.