Visualising latent DNA on swabs

Abstract

Collection for touch DNA either at scenes or on items submitted to a forensic laboratory is based on assumptions as to where a person made direct contact. In many instances a swab may be applied to an area where no contact has been made. Many swabs may therefore be submitted for DNA profiling on which no DNA is present, resulting in the loss of both time and resources by analysing such swabs. This study has developed a simple, fast, DNA-staining and fluorescence microscopy-based screening method for swabs to indicate if there is any DNA from which to generate a profile. Ten different types of swabs were tested covering the major types used (foam, cotton and nylon). Each swab was treated by: no addition of dye or DNA, addition of dye only, addition of known DNA and addition of dye and DNA. The stain used was Diamond™ Nucleic Acid Dye (DD) and fluorescence microscopy was achieved with a digital microscope equipped with a blue LED light source (480nm) for excitation and an emission filter of 510nm. Two types of samples were tested, either buccal swabs or swabs collected from areas touched by volunteers and all analyses were performed in triplicate. The samples were collected and retained at room temperature with time intervals of 0 day, 7 days, 14 days, 21 days, and 28 days before detection using DD staining and fluorescence microscopy. Seven of the swab types used were found to be unsuitable due to the lack of any difference in the fluorescence detected when no DNA, or only the dye, or a combination of DNA and dye were added. Three swab types (black cotton swab, Ultrafine dental applicator, and Cylinder dental applicator) were found to be much more effective for collection of DNA. Further, stained cellular material retained its fluorescence for up to 4 weeks and swabs containing cellular material that had been stored for four weeks could be stained and visualised. Additionally, DD did not affect DNA profiling. This screening method has the potential to be a routine step in a forensic laboratory to save costs of processing samples where swabs are devoid of any DNA. This technique is rapid, easy, cheap, non-destructive and safe.

Analysis of rapid HIT application to touch DNA samples

“…The tapelifted fabric also produced informative profiles in 8/10 samples, while the electrical tape and ziplock bag samples resulted in 4/10 and 3/10 samples, respectively; the wire produced only one informative profile (24 alleles), and the twine produced none. The reduction in alleles observed in DNA profiles for the other sample types, excluding the fabric, may be due to ineffective recovery of cellular and cell-free DNA by the swab [31,32] or a stunted release of collected material into the extraction solution [33]. Additional sample types that are either swabbed, tapelifted, or placed directly into the cartridge would be worth investigating to explore whether there is any relationship between sample treatment prior to cartridge use and allelic yield.…”

Evaluation of a fluorescent dye to visualize touch DNA on various substrates

“…The cigarette, human tooth, and wooden matchstick were held between the index finger and thumb for 15 s with medium pressure. The contact duration of 15 s was applied to be consistent with similar studies [13,16,18]. After the donor touched one of the eight substrates for 15 s, they then washed their hands, underwent the 15 m intervening period, and then touched another of the eight substrates.…”

“…A number of previous studies have used different dye solutions using DD diluted with either 75% ethanol (EtOH) in sterile water for non-absorbent substrates such as glass slide, ziplock bag, and cartridge case [9,[13][14][15], or the dye diluted with water for staining DNA transferred to swabs, electric tape or PVC [16,17]. The comparison of the two different DD solutions on pre-enhanced fingermarks on non-absorbent substrates has been reported [18].…”

DNA Transfer in Forensic Science: Recent Progress towards Meeting Challenges

“…DD to detect DNA on used swabs to potentially assess if further processing for DNA profiling is warranted [43].…”