Dental caries, commonly called tooth decay or cavities, is caused by acid-producing bacteria in your mouth dissolving the mineral structure of your teeth. It is the most common health condition globally, according to the Global Burden of Disease 2021. The process isn’t sudden. It’s a slow tug-of-war between acid attack and your body’s natural repair system, and cavities form when that balance tips in the wrong direction for too long.
How Acid Dissolves Tooth Enamel
Your teeth are made primarily of hydroxyapatite, a crystalline mineral built from calcium, phosphate, and hydroxyl ions packed tightly together. When acid lowers the pH at the tooth surface below a critical threshold, those ions begin to break free and dissolve into the surrounding fluid. For enamel, that critical pH is about 5.5. For dentin, the softer layer beneath enamel, it’s higher: around 6.2 to 6.4, which is why exposed root surfaces decay more easily.
The most common form of acid attack in everyday life comes from weak acids in the pH range of 4.5 to 6.9. These don’t etch the surface dramatically. Instead, they cause subsurface dissolution, pulling minerals out from beneath an intact-looking outer layer. This is why early decay often appears as a chalky white spot on the tooth rather than an obvious hole. The surface is still there, but the mineral scaffold underneath is quietly crumbling.
At the chemical level, hydrogen ions from the acid bind with phosphate in the tooth crystal, forming complexes that pull calcium out of the surrounding mineral lattice. This cascading loss of calcium and phosphate is what dentists call demineralization. If acid exposure is brief and infrequent, your saliva can push those minerals back in. If it’s prolonged or repeated, the damage outpaces the repair, and a cavity eventually forms.
The Bacteria Behind the Acid
Your mouth is home to hundreds of bacterial species living in a thin, sticky film on your teeth called dental plaque, or biofilm. Most of these bacteria are harmless. The trouble starts when certain acid-producing species gain a foothold, and the most well-studied of these is Streptococcus mutans. This bacterium thrives in acidic conditions, produces large amounts of acid from sugar, and builds a sticky structural matrix that helps the biofilm cling to tooth surfaces.
What makes S. mutans especially effective at causing decay is a combination of traits. It is acidogenic, meaning it generates acid as a metabolic byproduct. It is also aciduric, meaning it tolerates and continues to function in the low-pH environment it creates. And it produces extracellular polysaccharides, a glue-like substance that forms the structural backbone of plaque. Other bacteria, including various Lactobacillus species, contribute to acid production as well. Even certain fungi, particularly Candida albicans, have been found at high levels in children with early childhood caries.
The key insight from modern research is that cavities aren’t caused by the presence of any single “bad” bacterium. They result from a shift in the overall microbial community. When frequent sugar exposure keeps plaque pH low, acid-tolerant species outcompete others, and the biofilm becomes increasingly acidic. It’s the sustained low pH that breaks down the tooth, not the bacteria alone.
Why Sugar Is the Primary Fuel
Bacteria need fermentable carbohydrates to produce acid, and sucrose (table sugar) is the most cariogenic of all dietary sugars. It is uniquely damaging for two reasons. First, bacteria ferment it rapidly, dropping plaque pH from a neutral 7 to 5.0 or below within minutes of eating something sweet. Second, sucrose serves as the raw material for building extracellular polysaccharides, the sticky matrix that holds plaque together and makes it harder to remove.
Those polysaccharides do more than just anchor bacteria to the tooth. They increase the bulk and porosity of the plaque, allowing sugar to diffuse more easily through the biofilm and reach deeper layers. The result is that even bacteria sitting right next to the enamel surface are bathed in sugar and producing acid directly against the tooth. Glucose and fructose also fuel acid production, but they lack sucrose’s ability to drive polysaccharide synthesis, which is why sucrose is considered the worst offender.
Bacteria have another trick that extends the damage. They can store excess sugar internally as intracellular polysaccharides and metabolize those reserves later, during periods when you’re not eating. This means acid production can continue between meals, prolonging the time your teeth spend below that critical pH of 5.5 and keeping the plaque environment hostile even during fasting.
How Saliva Fights Back
Saliva is your body’s primary defense against tooth decay, and it works through several mechanisms at once. The most important is acid neutralization. Saliva contains three buffer systems: a bicarbonate system (the most powerful), a phosphate system, and a protein-based system. Together, these chemical buffers absorb excess hydrogen ions and raise the pH in your mouth back toward neutral after eating. Bacteria in plaque also break down urea into ammonia, which contributes to maintaining a more neutral pH in the oral environment.
Beyond buffering, saliva is saturated with calcium and phosphate ions, the same building blocks that make up tooth enamel. When the pH rises above the critical threshold after an acid attack, these minerals can redeposit into damaged areas of enamel, essentially patching the early damage. This process, called remineralization, is why not every acid exposure leads to a cavity. If fluoride ions are present in saliva (from toothpaste or fluoridated water), they get incorporated too, creating a more acid-resistant form of the mineral.
Saliva also contains antimicrobial proteins. Lactoferrin starves bacteria by binding iron they need for metabolism. Lysozyme breaks apart bacterial cell walls and disrupts their ability to clump together. Histatins have direct antibacterial effects, and salivary peroxidase prevents the buildup of harmful byproducts. This is why people with dry mouth, whether from medication, radiation therapy, or autoimmune conditions, face dramatically higher rates of tooth decay. Without adequate saliva flow, both the chemical buffering and the biological defenses are compromised.
Genetics and Enamel Quality
Not everyone’s teeth are equally resistant to decay, and part of this comes down to genetics. Several genes involved in enamel formation influence how strong and well-mineralized your enamel turns out to be. The gene AMELX, located on the X chromosome, encodes amelogenin, a major structural protein in the enamel matrix. Variations in this gene have been linked to a roughly 40% increase in caries risk. Other genes, including ENAM (which produces enamelin, critical for enamel crystal growth) and a family of genes that produce matrix metalloproteinases involved in tooth mineralization, also show significant associations with cavity risk.
A large meta-analysis found that when researchers looked at clusters of these enamel-formation genes together, the combined association with caries risk was strong. This helps explain why some people who brush diligently still get cavities, while others with mediocre hygiene habits seem resistant. Genetics don’t override behavior, but they do set the baseline. Thinner or less perfectly mineralized enamel dissolves faster at the same pH, giving saliva less time to mount a defense.
How Fluoride Strengthens Teeth
Fluoride protects teeth by modifying the mineral crystal itself. When fluoride ions replace hydroxyl ions in the hydroxyapatite lattice, they form fluorapatite. Because fluoride ions are physically smaller than the hydroxyl ions they replace, the crystal structure packs more tightly, increasing the forces of attraction between ions and making the whole structure more stable.
This tighter crystal has a practical consequence: it requires a lower pH to begin dissolving. Hydroxyapatite starts breaking down at pH 5.5, but fluorapatite resists dissolution until the pH drops further. The reason is that when acid lowers the surrounding pH, it depletes hydroxyl ions much faster than it depletes fluoride ions. So even at the same acid strength, fluorapatite holds together while regular enamel is losing minerals. This is why consistent, low-level fluoride exposure from toothpaste or drinking water is so effective. It doesn’t just coat the tooth surface temporarily. It becomes part of the mineral structure, making every remineralization cycle produce slightly stronger enamel than what was lost.
What Tips the Balance Toward Decay
Dental caries is ultimately a disease of imbalance. On one side, you have acid production from bacterial metabolism of sugars, the stickiness and acidity of plaque biofilm, and any genetic vulnerabilities in enamel quality. On the other, you have saliva flow and buffering, mineral availability for remineralization, fluoride exposure, and oral hygiene that physically disrupts the biofilm.
The factors that most reliably shift this balance toward decay are frequent sugar consumption (especially between meals, when each exposure resets the acid clock), reduced saliva flow, and inadequate fluoride exposure. The frequency of sugar intake matters more than the total amount. Sipping a sugary drink over two hours causes far more acid exposure than drinking the same amount in five minutes, because each sip restarts the pH drop. Sticky foods that cling to teeth extend contact time in the same way.
Physical disruption of plaque through brushing and flossing is effective precisely because it breaks up the biofilm before it matures. Young, thin plaque is less acidic and easier to neutralize. Older, thicker biofilm traps acid against the tooth, shields bacteria from saliva, and allows the internal sugar-storage mechanism to keep acid production going around the clock. The combination of frequent sugar, undisturbed plaque, low fluoride, and reduced saliva is what creates the conditions for a white spot lesion to progress into a full cavity.