DNA collected from saliva can provide useful information for forensic investigations and ancestry testing. But how long does saliva DNA remain viable for analysis? The answer depends on how the saliva sample is stored.
How is saliva DNA collected?
DNA is collected from saliva most often using buccal swabs. A buccal swab is a cotton swab that is rubbed along the inside of a person’s cheek to collect cheek cells that contain DNA.
The swab is then air dried to stabilize the DNA and stored in a paper envelope or tube. This provides a non-invasive way to collect a DNA sample from someone for analysis.
Factors affecting saliva DNA viability
Several factors can affect how long saliva DNA will remain usable for testing:
- Storage temperature – Colder is better. Storing samples refrigerated or frozen preserves DNA for longer vs room temperature.
- Exposure to light – Light can damage DNA, so storing samples in the dark extends viability.
- Exposure to moisture – Very dry storage environments are best to avoid DNA degradation.
- DNA purification – Extracting and purifying DNA from the saliva gives longer lasting high quality DNA.
- Storage container – Inert containers like plastic tubes help maintain DNA integrity.
Taking steps to stabilize the saliva sample by drying, keeping it cold and dark, and using proper storage materials allows the DNA to remain usable for testing for extended periods of time.
Typical viability timeframe
With ideal storage conditions, saliva DNA typically remains viable for:
- 1-2 years at room temperature
- Up to 10 years refrigerated
- Indefinitely when frozen (-20 to -80 Celsius)
Extracted and purified DNA stored frozen can remain usable for decades for downstream testing applications.
Longest confirmed viability
In special cases, saliva DNA has remained usable for very long periods when conditions allow exceptional preservation:
- DNA was obtained from saliva on postage stamps originally mailed in the 1960s – approximately 50 years.
- Viable DNA recovered from 17th century parchment letters sealed with saliva – over 300 years.
- DNA testing of mummified human remains has confirmed viable DNA for hundreds to thousands of years when remains are preserved.
So under the most ideal scenario of frozen preservation, saliva DNA appears able to survive for centuries and still allow for accurate DNA profiling and testing.
Degradation effects
Even if DNA has degraded somewhat over time, there are still ways it can provide useful information:
- STR profiling – Short tandem repeat profiling looks at DNA repeat regions. Even if DNA is fragmented, these short sequences may still be intact and can yield a partial profile for identification purposes.
- Mitochondrial DNA – mtDNA is more abundant than nuclear DNA and survives better. mtDNA testing can yield ancestry and maternal lineage insights from older, degraded samples.
- Y chromosome DNA – The Y chromosome contains unique DNA passed down paternal lines unchanged for generations. It can give paternal ancestry clues.
While nuclear DNA degradation makes full sequencing difficult, there are alternative DNA analysis methods that can get useful data from very old saliva samples.
Factors that speed up degradation
Certain factors can cause saliva DNA to degrade at a faster rate than ideal cold, dry storage:
- Heat – High temperatures speed up chemical breakdown of DNA.
- Humidity – Exposure to moisture and enzymes in saliva causes DNA to hydrolize and cleave apart over time.
- UV light – Sunlight and UV light damages DNA molecules, causing cross-linking and fragmentation.
- Oxidation – Reactive oxygen species generated through metabolism, air exposure etc degrades DNA.
- Microorganisms – Bacteria and fungi may digest and destroy DNA molecules after cell death.
Avoiding these conditions when possible extends the useful lifespan of collected saliva DNA samples.
DNA sample collection and storage tips
To get the highest quality saliva DNA that lasts as long as possible:
- Use buccal swabs made of inert materials like cotton to avoid inhibiting PCR.
- Allow saliva to fully air dry on the swab before storage.
- Place swab in a sterile plastic tube and store in a cool, dark location if not freezing.
- Extract DNA from a portion of sample to stabilize it before freezing the remainder.
- Freeze excess sample at -20 to -80°C in clearly labelled tubes for extended preservation.
Testing old or degraded samples
Even if DNA becomes fragmented or chemically modified over time, there may still be ways to test it:
- Quantify sample – Determine DNA concentration and quality. Low yields may still work with extra PCR cycles.
- Increase PCR sample input – Use a larger amount of DNA template for amplification.
- Nested PCR – Reamplify PCR products with a second set of primers for greater sensitivity.
- Extra PCR cycles – Run more cycles to amplify even tiny amounts of target DNA.
- Mini-STR analysis – Test shorter PCR amplicons if DNA is degraded to get a partial profile.
While degraded, modern analysis methods make it possible to gain insights from saliva DNA samples that are decades or centuries old when properly stored after collection.
Conclusions
- Saliva DNA can potentially survive for hundreds of years if frozen or mummified.
- At room temperature, saliva DNA remains usable for 1-2 years on average.
- Refrigeration extends viability to about 10 years.
- Frozen storage at -20°C or colder may allow indefinite stability.
- DNA damage accumulates over time but specialized testing can extract data from degraded samples.
- Ideal storage is cold, dark, dry, and in inert containers to prevent degradation.
In summary, how long saliva DNA remains viable depends greatly on how it is collected and stored. But with proper handling, the biological information contained in saliva DNA can persist for very long periods.