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Okay, let's dive in.
When most people think about forensic biology,
maybe even detectives, the first thing that comes to mind.
Yeah, it's almost always blood, semen, saliva, the usual suspects.
Exactly, the big three.
But what we're exploring today based on our source material is how crucial some other, let's say, less common fluids can be.
Definitely.
Things like urine, sweat,
fecal matter, even vomitus.
They might seem, well, messy or secondary.
Right, maybe overlooked.
But they can be absolutely critical for piecing together what happened at a crime scene.
Think about assault or certain homicides.
Strangulation cases, yeah.
The trauma can cause involuntary release of, say, urine.
Finding that stain tells you something very specific about the violence and potentially where it happened.
So that's really our mission here, isn't it?
How do forensic scientists take maybe an ambiguous stain and turn it into solid evidence?
That's the core challenge.
Because these fluids, chemically, they're complicated.
And worse, they often share some basic components.
Which makes telling them apart tricky.
Very tricky.
You can't just rely on a general test.
You need a strategy.
Start broad, maybe visual, then presumptive tests.
But quickly move to something really specific.
Exactly.
You need to find that unique biomarker, that sort of molecular signature that says definitively, yes, this is urine or this is sweat.
No doubt.
Okay, let's start with urine then.
It comes up in sexual assault investigations, harassment, those homicide cases we mentioned.
How do analysts first approach a potential urine stain?
Well, the initial screening is pretty straightforward.
You look for that yellowish color.
Sometimes there's a distinctive ammonia smell, especially as urea breaks down.
And of course, alternative light sources.
Many biological fluids fluoresce, so that helps locate stains on clothing or bedding.
And the first chemical check usually targets the most abundant waste product, right?
Urea.
Correct.
Urea is the big one.
Comes from breaking down amino acids.
The go -to in a cinnamaldehyde.
Quite a mouthful.
Oh, yeah.
So how does it work?
It's usually colorimetric.
You take a tiny cutting from the stain, extract it, add the DMA solution if urea is there.
You get a color change.
Yep.
A pretty distinct pink or magenta color usually pops up within about 30 minutes.
Pink means maybe positive for urine.
You mentioned shared components earlier.
Doesn't DM IZ has issues there?
Like isn't urea in other fluids too?
That's the crucial limitation, yes.
Saliva, sweat, they have low levels of urea.
So DM IZ is not specific.
So the potential false positive.
It can happen, yeah.
Especially if the DMAC solution isn't diluted correctly or if the stain is really concentrated.
That's why it's strictly a presumptive test.
Points you in a direction, but it's not proof.
You absolutely need confirmation.
Absolutely.
You'd never go to court based on just DMAC.
So what is the definitive proof?
What's that
biomarker for urine?
The gold standard is a protein called Tam -Horse -Fall protein, THP.
You might also see it called Uromodulin.
Tam -Horse -Fall protein.
Okay, what makes it so special?
Well, incredibly, it makes up something like 40 % of all the protein found in urine.
But the key thing is where it's made.
Where is that?
It's exclusively synthesized in the epithelial cells lining a specific part of the kidney nephron, the loop of Henle.
Only there, nowhere else in the body.
Nowhere else, which makes it a perfect target.
Your body just doesn't produce it anywhere else.
That sounds ideal for an antibody -based test.
Precisely.
And that's what the RSID urine test is.
It's an immunochermatographic strip test.
Works just like, say, a rapid strip test or a pregnancy test.
You add the sample, and if THP is there.
If THP is present, it binds to a labeled antibody on the strip.
This complex then travels up the strip and gets captured at the test line, creating a visible band.
Simple, fast, and very specific.
Critically, it doesn't react with plasma, saliva, semen, vaginal fluid, or sweat.
It only picks up THP.
That level of specificity is impressive.
But the source also mentions another layer of confirmation,
17 -ketosteroids.
Why bother with that if THP is so good?
That sounds more complicated, involving LCMS.
It is more complex, yes.
Liquid Chromatography Mass Spectrometry.
But it serves a couple of purposes.
One is confirming the human origin of the urine.
And it provides a really detailed molecular profile.
These 17 -ketosteroids, like endosterone and DHEA, are steroid hormone derivatives.
The liver modifies them conjugation, making them water soluble so they can be excreted in urine.
LCMS can pick those up.
Yeah.
It can separate and identify them very precisely.
The key is detecting the profile of all five major conjugated 17 -ketosteroids.
Finding that specific pattern is considered definitive proof.
This is human urine.
So multiple layers of certainty.
THP for urine specificity, 17 -ketosteroids for human origin confirmation.
Okay.
Sweat itself doesn't usually form large stains like urine might, but the residue left behind, especially in a fingerprint, is a prime source of DNA.
So identifying the sweat helps contextualize that DNA.
Right.
And we need to know which type of sweat we're likely dealing with.
There are two main glands.
Acritrine and apocrine.
Correct.
Acrine glands are everywhere, all over your body.
They produce that watery sweat, mainly for cooling you down thermoregulation.
That's usually what we find in forensic contexts.
And a concrete?
Those are mostly in the underarm and genital areas.
They produce a thicker, oilier sweat, more linked to emotional stress.
And they only really kick in around puberty.
Okay.
So acrine is the main target.
But didn't we just say sweat contains urea?
Could that cause issues, maybe confuse things if there's a mixed stain?
It could potentially interfere with the presumptive DMA test.
Yeah.
But for confirmation of sweat, we ignore the urea entirely.
We look for something unique to sweat.
Another specific biomarker.
Yep.
For acrine sweat, the key player is an antimicrobial peptide called Dermcidin.
Dermcidin.
Okay.
And that's only in sweat.
Specifically, it's expressed in acrine sweat glands, not in apocrine glands, not in semen, saliva, or urine.
It's our unique marker for acrine sweat.
Fantastic.
So how do labs find it, especially if it's just trace amounts, like from a fingerprint?
We need really sensitive methods.
Elyse assays, that's an immunological technique, are incredibly sensitive.
They can apparently detect Dermcidin even if the sample's diluted 10 ,000 times.
Wow.
Or if you have enough cellular material, you could use RT -PCR to look for the messenger RNA,
the genetic instructions for the DCD gene, which is the gene that codes for Dermcidin.
So Dermcidin cuts through the noise, confirms it's acrine sweat.
Yeah.
Great.
Okay.
Let's move on to part three.
Fecal matter.
Forensically, when does this become evidence?
Well, it can be relevant in cases of sodomy, unfortunately.
Sometimes in assaults where it's used as a weapon or vandalism.
Or maybe burglary, if someone defecates at the scene.
It happens, yeah.
Provides a direct link if you can get DNA.
But first, you need to confirm it is fecal matter.
How did that start?
Visuals again.
Pretty much.
Color of the brown comes from urobilinoids, which are breakdown products of heme from red blood cells.
And while the odor is pretty characteristic, caused by bacterial byproducts like indole and scatol.
Not pleasant, but distinctive.
What about under the microscope?
Microscopically, you're looking for undigested food.
Plant fibers, vegetable fragments,
sometimes even animal meat fibers, which can have telltale striations.
These point towards it being feces.
Interesting clues.
But again, presumptive.
Right.
Need chemistry.
You mentioned urobilinoids giving the color.
Can we test for those?
Yes.
There are classic chemical tests like the Schlesinger test or the Edelman test.
They try to detect urobilin and stercobalin.
Those are the main urobilinoids.
How do they work?
You mix the sample extract with zinc acetate solution.
If those urobilinoids are present, the zinc forms a complex with them, and that complex glows.
It gives off a characteristic green fluorescence when you shine UV light on it.
Green fluorescence sounds like a good indicator.
But.
I sense a but coming.
Specificity issues again.
Big time.
These tests are notoriously non -specific.
Firstly, they often can't tell human feces from animal feces.
Okay, that's a problem.
And secondly, high levels of urobilin can sometimes be found in urine, especially if someone has certain liver conditions.
So these tests can give a false positive, making you think it's feces when it might just be urine.
So the old chemical tests are a bit shaky.
Where do we turn for certainty?
Modern methods.
Modern methods go straight to the source of what makes feces unique.
The bacteria.
Specifically, the gut microbiota.
Our gut bacteria.
Exactly.
A huge proportion of fecal mass is actually bacteria, and a major group in the human gut is the genus Bacteroids.
Makes up maybe 30 % of the bacteria there.
So we target their DNA.
Precisely.
Forensic scientists use techniques like RT -PCR to look for specific DNA sequences found only in certain bacteroid species.
They often target a gene called RPB.
Is there one particular species that's the gold standard indicator?
Bacteroids uniformus is generally considered the key indicator bacterium for forensic fecal ID.
Studies show it's pretty undetectable in other human fluids like blood, saliva, or semen.
So finding B.
uniformus DNA is strong evidence.
Very strong.
It's our definitive bacterial marker.
Although it's worth remembering that diet can influence which bacteria dominate.
Sometimes it's Bacteroids, sometimes maybe Prevotella.
But B.
uniformus is still a key target.
Okay, last one.
Vomitus.
Finding this at a scene sounds like it could point towards poisoning or maybe severe trauma.
Absolutely.
It can be really critical evidence in those scenarios.
Vomitus is essentially the forceful expulsion of stomach contents, the gastric fluid.
And that fluid is famously acidic.
Extremely acidic, yes, due to hydrochloric acid, HCl.
But the athlete itself isn't what we test for directly.
What is it then?
What's the unique component?
It's an enzyme, or rather, it's precursor.
The stomach secretes inactive enzyme precursors called pepsinogens.
The HCl in the stomach activates these pepsinogens, turning them into the active enzyme, pepsin.
And pepsin digests protein, right?
That's its job.
It's the main protein digesting enzyme in the stomach.
So if we can detect active pepsin, we know we're dealing with gastric fluid.
How do you test for an enzyme's activity like that?
Sounds complicated.
Like you said, it's designed to break things down.
It's actually quite an elegant test called the pepsin proteolytic assay.
You're essentially using the enzyme in the to do its job on a test plate.
They prepare an agarose gel plate that contains a special substrate called fibrin blue.
Fibrin blue is an insoluble complex protein linked to a blue dye, but it's colorless when bound together like this.
Insoluble, colorless, got it.
So you put the suspected vomitous sample onto this gel.
Exactly.
You load the sample onto the gel plate with the fibrin blue, then you incubate it.
And if pepsin is present in the sample?
The pepsin gets to work, it digests or cleaves the protein part of the fibrin blue complex.
When it does that, it releases the water soluble blue dye.
So you'd see blue color spreading out?
Precisely.
You get a visible blue ring forming around the sample spot on the gel.
It's a direct result of pepsin activity.
That's a really neat visual confirmation, very specific to gastric fluid.
Highly specific.
Other body fluids don't contain active pepsin, so they won't produce that blue ring.
It confirms gastric origin.
But like some of the older tests, maybe not species specific.
That's the one main limitation, yeah.
It confirms the presence of gastric fluid because pepsin is common to vertebrates, but the test itself can't tell you if it's human vomitous or from, say, a dog.
Okay, so wrapping this all up, this whole deep dive really highlights the journey forensic scientists take from these sometimes ambiguous stains.
Yeah, starting with maybe just a visual check or a simple presumptive test like DMAC for urea.
And then methodically moving towards these highly specific confirmatory tests, finding that one unique molecule.
Whether it's a protein made only in the kidney like THP or a peptide specific to skin glands like Dermcidin.
Or even targeting the DNA of specific gut bacteria like Bacteroids uniformus, or harnessing the unique enzymatic power of pepsin.
It really underscores how forensic identification depends entirely on that specificity.
Finding that unique biomarker protein, steroid profile via LC -MS, DNA sequence, enzyme activity that lets you say, beyond reasonable doubt, this came from this type of fluid.
It's about locking down the source, moving from a general clue to absolute molecular proof.
And often finding these fluids links directly to the event itself.
Right, the violence might cause involuntary excretion or forceful expulsion like vomiting.
It's a physiological echo of the crime.
Which leads to a really interesting final thought, building on what you said about fecal matter analysis.
The undigested food aspect.
Exactly.
If you can identify specific food stuffs in feces, or maybe even vomitous, and you know roughly how long digestion takes, you mentioned 12 to 24 hours in the large intestine.
Right, the timing varies, but there's a general window.
So what could analyzing the content of that last meal tell us?
Could it provide temporal clues?
Maybe about where the victim or perpetrator was or what they were doing in the hours before the crime.
That's the potential, isn't it?
Connecting the biological trace not just to the person, but potentially to their timeline leading up to the event.
It shows just how much information can be hidden in these often overlooked types of evidence.
That's the power we're talking about.