Testing the efficacy of surface swab sampling to determine postmortem submersion interval (PMSI), using the microbiome colonization of skeletal remains

Postmortem interval (PMI) estimation contributes valuable information in the medicolegal investigation of decomposed human remains, and estimating the postmortem submersion interval (PMSI) can specifically aid investigations involving victims discovered in aquatic environments. Microbial succession-driven models in long-term decomposition studies have utilized the abundant colonizing bacterial community of skeletal remains to estimate the PMSI using bone powder. This study investigates the use of bone surface swabbing as an effective alternative method that minimizes time and resources required for bone sampling and also provides a highly replicable method for decomposition studies. Skeletal porcine (Sus scrofa) remains were caged and submerged in both lentic and lotic environments (Henley Lake in White Hall and James River at the Rice Rivers Center in Charles City, respectively) in Central Virginia from November 2017 to November 2018. Bone surface swabs and water samples were analyzed at 500 accumulated degree days (ADD) intervals, from baseline (0 ADD) to 4500 ADD. Variable region 4 (V4) of 16S rDNA was amplified and sequenced using the Illumina MiSeq Sequencing platform and analyzed using Mothur (v.1.39.5) and R (v.4.04). Analysis of Molecular Variance (AMOVA) indicated a significant difference in bacterial community structure among and between the swab, bone, and water samples (p < 0.001, F = 7.92331), and among and between lake and river samples (p < 0.001, F = 9.38829). PMSI models were constructed using random forest models for lake swabs (R² = 0.83, RMSE = 623.24) and river swabs (R² = 0.83, RMSE = 580.2). Swab samples from both aquatic environments predicted PMSI, albeit slightly less accurately than those previously reported from bone powder (lake: R² = 0.96, 334.1; river: R² = 0.94, 498.47).