Human saliva contains a valuable source of genetic material, primarily from shed buccal (cheek) cells and white blood cells. Understanding how long DNA persists in a saliva sample is important for various applications, ranging from medical research to forensic investigations. This persistence is influenced by several factors, which determine the integrity and usability of the DNA over time.
Typical Duration of DNA in Saliva
The longevity of DNA in a saliva sample varies significantly depending on storage conditions. Unpreserved liquid saliva, especially at room temperature, experiences a relatively rapid reduction in DNA concentration, often within one month. If left to dry naturally, DNA in saliva stains on surfaces like fabric might remain detectable for weeks to several months, provided it is stored properly. However, for reliable genetic analysis, unpreserved samples are best processed quickly or stored at cold temperatures.
In controlled environments, DNA in saliva can remain suitable for genetic analysis for up to 18 months. Specialized collection kits with stabilizing buffers can preserve DNA for much longer, often allowing room temperature storage for several years, with some claiming stability for five years or more.
Factors Affecting DNA Stability
Several environmental and biological elements influence how long DNA remains stable in a saliva sample. Temperature plays a significant role, as elevated temperatures accelerate the chemical processes of oxidation and hydrolysis, which break down DNA molecules and lead to fragmentation. Conversely, lower temperatures effectively slow these degradation processes.
Humidity and moisture also contribute to DNA degradation by fostering the growth of microorganisms like bacteria and fungi within the sample. These microbes produce enzymes that actively break down DNA. High humidity can also promote hydrolysis, further fragmenting the DNA.
Exposure to light, particularly ultraviolet (UV) radiation, causes photodamage to DNA, resulting in strand breaks and cross-linking that compromise its integrity. Additionally, saliva naturally contains nucleases, which are enzymes capable of degrading DNA by cleaving its phosphodiester bonds.
Preserving Saliva DNA
Various methods can extend the viable lifespan of DNA in saliva samples. Refrigeration helps by slowing down enzymatic activity and microbial growth, allowing DNA to remain stable for several months. Freezing samples at -20°C or -80°C provides a more effective long-term preservation strategy. This significantly reduces biological activity, enabling DNA to be stored for years, potentially indefinitely. However, repeated thawing and refreezing cycles should be minimized, as they can contribute to DNA degradation.
Specialized DNA collection kits incorporate stabilizing buffers designed to counteract degradation. These buffers typically inactivate nucleases, inhibit microbial proliferation, and prevent chemical hydrolysis of the DNA. Many of these kits facilitate room temperature storage for extended periods, sometimes for five years or more, making sample collection and transport more convenient. Proper sample collection also impacts preservation; for instance, avoiding food, drink, or smoking for 30 minutes before collection helps maintain sample quality.
Practical Applications of Saliva DNA
Understanding DNA longevity in saliva is important across diverse fields. In forensics, saliva is a frequently encountered source of DNA evidence at crime scenes, found on items such as chewing gum, cigarette butts, or from bite marks. Knowledge of how long DNA persists in various conditions assists investigators in recovering viable genetic material from aged or compromised evidence.
Saliva has become a widely accepted source for genetic testing, offering a non-invasive and convenient alternative to blood samples for ancestry, paternity, and health predisposition analyses. In medical research, large-scale genetic studies and population-based investigations frequently rely on saliva samples due to their ease of collection and reduced logistical costs. The stability of DNA in these samples ensures the reliability of results when studying the genetic underpinnings of various diseases.