Estimating the time since deposition (TsD) in saliva stains using temporal changes in microbial markers

Area of Science:

• Microbiology
• Forensic Science
• Bioinformatics

Background:

• Accurate time since deposition (TsD) estimation is crucial for forensic investigations.
• Current methods for determining short-term TsD of biological traces are limited.
• Human salivary microbial communities exhibit dynamic changes over time.

Purpose of the Study:

• To investigate the potential of salivary microbial community succession as a predictive tool for TsD.
• To develop and validate models for estimating TsD in dried saliva stains.
• To establish microbial biomarkers for forensic applications.

Main Methods:

• Collection and analysis of human saliva stains under indoor conditions for up to 20 days.
• 16S rRNA profiling using high-throughput sequencing to characterize microbial composition.
• Development of regression models (random forest and qPCR-based) using TsD-dependent bacterial indicators.

Main Results:

• Significant differences in microbial composition were observed across different time points.
• Indoor exposure time of saliva stains was inversely correlated with alpha diversity.
• A random forest model achieved a mean absolute deviation (MAD) of 1.41 days; a simplified qPCR model using specific bacteria had a MAD of 1.32 days.
• Mock sample assessment yielded a MAD of 3.53 days.

Conclusions:

• Salivary microbial abundance significantly changes with prolonged TsD.
• Microbial biomarkers in dried saliva stains can serve as a reliable "clock" for TsD estimation.
• This approach offers a novel and efficient tool for forensic casework resolution.