A cavity is not made of any single substance. It’s a hole in your tooth filled with a mix of softened, partially dissolved tooth mineral, bacteria, bacterial acids, and decomposing protein fibers. Understanding what’s actually inside that hole starts with knowing what healthy tooth material is made of, and what happens when bacteria break it down.
What Healthy Teeth Are Made Of
The outer shell of your teeth, called enamel, is the hardest tissue in the human body. It’s 96% mineral, 3% water, and 1% organic material. The mineral is hydroxyapatite, a crystalline form of calcium and phosphate. Think of it as tightly packed crystals that form a dense, protective shield.
Beneath the enamel sits dentin, which is softer and more porous. Dentin contains the same calcium-phosphate mineral but in lower concentrations, mixed with collagen (a structural protein) and tiny fluid-filled tubes that run from the outer surface toward the nerve inside the tooth. This layered architecture matters because a cavity changes composition as it eats through each layer.
How Bacteria Create the Cavity
Cavities start with bacteria. The primary culprit is Streptococcus mutans, the dominant species in dental plaque. These bacteria form a sticky biofilm on your teeth, and when you eat sugars or starches, they ferment those carbohydrates and produce organic acids, mainly lactic acid and some acetic acid. That acid sits against the tooth surface, trapped under the biofilm like a chemical compress.
Healthy enamel begins dissolving when the pH at the tooth surface drops below about 5.5. For reference, your mouth normally hovers around pH 7 (neutral). A sip of soda can drop it below 3. At pH 4.0, studies using imaging techniques show complete mineral loss in the affected area within weeks. Every time you eat something sugary, the bacteria produce a burst of acid that lasts roughly 20 to 30 minutes before your saliva can neutralize it. Frequent snacking means the tooth spends more total time under acid attack.
What’s Actually Inside a Cavity
Once you understand the process, the contents of a cavity make sense. A cavity is a zone of progressive destruction, and what’s “in” it depends on how far it has progressed.
In the earliest stage, before there’s even a visible hole, the enamel surface looks chalky white or yellowish. This is called a white spot lesion. The outer surface is still intact, but beneath it, acid has pulled calcium and phosphate ions out of the hydroxyapatite crystals, leaving microscopic voids in the enamel. At this point, the lesion contains partially dissolved mineral crystals, water that has seeped into the voids, and acid byproducts from bacteria on the surface. There’s no actual hole yet.
As the mineral loss continues, the weakened subsurface eventually collapses under the normal forces of chewing. Now you have a true cavity: a physical break in the tooth. Inside this hole you’ll find remnants of dissolved enamel crystite, colonies of living bacteria, the sticky polysaccharide matrix they use to anchor themselves, lactic and acetic acid, food debris, and dead bacterial cells. The material is soft and often discolored brown or black. That dark color comes from a combination of bacterial pigments, stained proteins, and mineral breakdown products.
When the cavity reaches the dentin layer, the composition shifts. Dentin has collagen fibers woven through its mineral structure, and as the mineral dissolves, those collagen fibers are left behind like a rotting scaffold. Bacteria invade the tiny tubules in the dentin, traveling as deep as 1,480 micrometers (about 1.5 millimeters) into the tissue. The material inside a dentin cavity is notably softer and wetter than an enamel cavity. It contains degraded collagen, bacterial colonies living inside the tubules, dissolved calcium and phosphate, and a mushy layer of partially broken-down tooth tissue that dentists describe as “infected dentin.” Microscopy of this tissue shows enlarged tubules packed with bacteria and a disorganized structure compared to healthy dentin.
Why Cavities Look and Feel Different at Each Stage
A shallow enamel cavity is often hard to notice. Enamel has no nerve endings, so you won’t feel pain. The spot may appear as a small white, brown, or black mark on the tooth surface. The material in this cavity is mostly demineralized enamel: still somewhat crystalline but porous and fragile.
Once the cavity penetrates into dentin, things change quickly. Dentin’s tubules connect to the nerve inside the tooth, so you start feeling sensitivity to hot, cold, or sweet foods. The cavity material at this stage is a wet, soft mix of bacterial biofilm, degraded collagen, dissolved minerals, and organic acids. If you’ve ever had a dentist scrape out a cavity before filling it, that soft, scoopable material is the decayed dentin. It’s structurally nothing like the hard, glassy enamel that was there before.
If the cavity reaches the pulp (the nerve and blood supply at the tooth’s center), bacteria and their acid byproducts cause infection and inflammation. The contents now include pus, dead tissue, and a thriving bacterial population that can spread beyond the tooth into the surrounding bone.
Your Mouth’s Built-In Repair System
Saliva is not just a lubricant. It’s a mineral-rich solution containing dissolved calcium and phosphate ions that can actually redeposit into the damaged crystal structure of enamel. This process, called remineralization, is your body’s natural defense against early cavities. After every acid attack, saliva gradually raises the pH back to neutral and supplies the raw materials to patch those microscopic voids.
This repair works only on early-stage damage, specifically the white spot lesions where the surface hasn’t yet collapsed into a hole. Fluoride accelerates this process by helping calcium and phosphate ions integrate into the crystal structure more efficiently, forming a slightly different mineral that’s actually more acid-resistant than the original enamel. Once a cavity has physically broken through the tooth surface, no amount of saliva or fluoride can rebuild it. That’s the threshold where a filling becomes necessary.
The Short Answer
A cavity is made of what’s left when bacteria dissolve your tooth. Its contents are a mixture of partially dissolved calcium-phosphate crystals, degraded collagen (in dentin), living and dead bacteria, the sticky biofilm matrix they produce, organic acids (primarily lactic acid), trapped food particles, and water. The deeper the cavity, the more biological material and the less mineral structure remains. It’s essentially a pocket of ongoing chemical destruction, filled with both the agents causing the damage and the wreckage they leave behind.