[Dental Floss-derived Biological Sample Collection, DNA Extraction and STR Typing]

To evaluate the forensic application value of used dental floss as a source of biological evidence for individual identification by analyzing the effects of dental floss sample collection methods, DNA extraction methods, preservation conditions, and sampling sites on the success rate of STR typing.

Dental floss samples were collected using three techniques: direct cutting, cotton swab wiping, and flocked swab wiping, respectively. DNA was extracted respectively by the Chelex, spin column-based and magnetic bead-based methods. DNA quantification and STR typing were performed using the Qubit kit and FGI HumDNA Typing kit (Platinum), respectively. Storage environments (temperature and humidity, ultraviolet radiation) and sampling locations (the floss part, the handle part) on DNA quantity and STR typing were evaluated.

Through conducting a statistical analysis of three key indicators of average DNA mass concentration, STR locus detection rate, and typing accuracy rate, the direct cutting method demonstrated the highest efficacy, followed by cotton swab wiping mothed, and the flocked swab wiping method had the lowest efficacy. Direct cutting yielded an average DNA mass concentration greater than (4.94±1.87) ng/μL, with STR locus detection and accuracy rates of 100%. Bead-based DNA extraction method produced superior DNA concentration and quality compared to spin column-based and Chelex methods, regardless of whether the sampling technique used. Preservation conditions had a significant impact on the DNA analysis of samples. Particularly, the STR typing accuracy of samples preserved at 55 ℃/50%RH for 35 days dropped to (81.82±12.31)%, and that of samples exposed to ultraviolet radiation for 12 h dropped to (55.46±34.31)%. DNA concentration from the handle part of dental floss was extremely low, with an STR typing accuracy of only (30.91±27.35)%.

Using cotton swabs to wipe or directly cutting the thread of dental floss samples, and combining this approach with the magnetic bead method for DNA extraction, can best guarantee the concentration and quality of DNA. In addition, samples should be stored in low-temperature, low-humidity environment, protected from light and ultraviolet radiation.