The smaller, not-so-golden snitches- how microbiology helps link crime scenes.

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While most people strive to be unique, none are ready to face the double-edged sword that comes with it, that every step, conversation and interaction imprints identity, one certainly specific to each individual. This, in the realm of forensics, extends far beyond dusting fingerprints to find the culprit; rather it’s analysing every shift in lifeform, studying the world concealed to most, to reveal even the smallest of changes arising from a shift in environment, for even the smallest of changes could make a world of difference at a crime scene.

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One of the most important cases in microbial forensics is that of the Amerithrax Case in 2001, wherein victims received mail containing anthrax spores in the United States, an infamous crime which followed the 9/11 attacks. The white powder in the mail, upon microscopic and morphological examination, was found to be a member of the Bacillus genus, which was the first indication that the specimen was weaponised. Running culture-based assays was used, since only certain media would be suitable for its growth and revealed the colony morphology of the specimen. The genetic identity, however, was confirmed using real-time PCR, which revealed the virulent nature and presence of specific anthrax genes, to confirm that it was indeed anthrax. The FBI, partnered with TIGR (The Institute for Genomic Research), to sequence the anthrax genome, and managed to identify 4 unique mutations which allowed them to trace it back to a single flask at the USAMRIID (U.S. Army Medical Research Institute of Infectious Diseases), a sample intended to be used for vaccine challenge studies. This was only one of the numerous cases, wherein analysing microbes helped in identifying the exact origin of the crime. A homicide case in Argentina in 2023 is a prime example of the same, but it takes a slightly different approach. A man who had gone missing in Buenos Aires was found 25 days later near the coastline. One of the steps taken was running a comparative analysis between the body site, surroundings, as well personal effects of the victim. A high concentration of marine pollen and coastal fungi was found both in the suspect's belongings and the crime scene, but not in his home environment. Forensic evidence of marine palynomorphs and fungi linked the suspect’s items specifically to the coastal site, which, coupled with further investigation, allowed them to find the culprit. The two cases mentioned take vastly different approaches but work on the same principle: every step leaves a trace, and microorganisms are selective enough to use as reliable markers at a crime scene. This is something universal, regardless of the scene of the crime, be it land, water or air.

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DNA barcoding of soil bacteria gives an overview of the exact microbial composition of the soil profile, serving as a good tool for comparison. Waterbodies consist of a distinct community of diatoms, cyanobacteria, and protists, which in turn act as good markers. This is especially useful since diatoms from water only enter the system when the victim is alive, which can, in turn, pinpoint exactly where the crime took place, distinguishing post-mortem disposal from incidents of drowning. A Similar approach is used to analyse air samples for specific, uncommon microbes which may usually be absent in the environment, which is achieved using metagenomic sequencing from air filters along with microbiome databases to match locations. While these techniques are based on the surrounding environment, the Personal Microbiome Transfer utilises each person’s unique microbe environment. Corynebacterium species, Streptococcus mutans and Firmicutes are present in a wide range of strains, and relative abundances, to the extent that the lack of fingerprints at a crime scene, isn’t as much of a hindrance as most would expect.

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Another key application of microbial forensics is to determine the time since death or the postmortem interval. When a person dies, their body undergoes an almost predictable microbial succession. For instance, the first 48 hours are characterised by the proliferation of anaerobic gut bacteria since immune defence is absent. The days that follow see a shift toward facultative anaerobes, and noticing spore forms or Candida can out the body at 2 weeks or more. By creating a predictive model, it’s possible to accurately pinpoint when, putting a date and time to the crime scene.

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In a world where humans use ingenuity to cover up their crimes lie microbes, a silent witness to the best and worst humanity has to offer. The entire ecosystem harboured in the human body is living proof that every contact leaves a trace, a ripple effect felt on a microscopic level, revealing that there’s always more than meets the eye.