Understanding Tooth Decay

Scanning electron microscope image of bacteria and red blood cells on a tooth

Scanning electron microscope image of bacteria and red blood cells on a tooth

Let's start fresh

Want a fresh start?  Everyone likes a fresh start.  Slates wiped clean.

Let’s do that now with your knowledge about tooth decay (AKA dental caries or cavities).

You have probably been lectured on end about brushing and flossing in order to prevent cavities.  You may have heard that sugar causes cavities (sugar was my husband’s answer when I asked him what he thought causes cavities).  You may be aware that bacteria cause cavities.

What is the basis for all of this, and what is true?  We will begin to address all of this in this post.  It will be our fresh start together.  A new understanding to set us up for success in the future.

I say “begin” and “start” because dental caries is such a complex infectious disease.  There are many factors that come into play and many bacterial species are involved.  

Defining Tooth Decay

With regards to tooth decay, in the interest of starting out simply, I will say that:

Bacteria as a whole get a bad reputation because of the ones that cause disease. However, there are many bacteria in on our skin, in our gut, and in our mouths that do not cause disease and are considered healthy. In fact, the normal, healthy bacteria can help keep the disease-causing bacteria at bay.

Tooth decay is ultimately the result of an imbalance between healthy and decay-causing bacteria.  In other words, when the environment in our mouths selects for cavity-causing bacteria and discourages the growth of healthy bacteria, you are at risk for tooth decay.

Let’s break this down.

Imagine a fish tank that has gone a little too long without a good cleaning.  There is a green film on the glass, right?  This is a biofilm, a complex community of microorganisms that accumulates and develops when given the time.  This same process is constantly occurring on our teeth!  Except instead of being green, it is most often white (fortunately).  Have your teeth ever felt fuzzy?  You were feeling an exceptionally thick biofilm.  

The key with biofilm is to acknowledge that it is always present on our teeth.  We call it dental plaque.  It can be a healthy biofilm that is host to a diverse group of overall “well-behaved” microbials, or one dominated by cavity-causing bacteria.  What determines which you have?  Well, that is the key question, so I’m glad you asked.

Dental Plaque (biofilm) being removed from teeth with a dental instrument

Dental Plaque (biofilm) being removed from teeth with a dental instrument

The role of pH and sugar

As I said before, there are several factors, but it all comes down to pH.  I’ll explain.

Do you remember learning about the mineral content of teeth in the post about tooth structure? Although our teeth are strong, they are vulnerable to mineral loss in an acidic (low pH) environment, at pH of 5.5 or below, to be exact. Mineral loss is the start of tooth decay.  And guess what?  Unlike healthy bacteria, cavity-causing bacteria are both aciduric (acid-tolerant) and acidogenic (acid-producing).

Because they are acid-tolerant, any prolonged period of low pH in the mouth provides an environment that specifically favors their growth.  They then further contribute to the acidic environment because they are also acid-producing.  Specifically, when fermentable carbohydrates, like sucrose, glucose and other short-chain sugars, are available to them, the unhealthy bacteria digest them and produce acids as by-products.  

Unfortunately, healthy bacteria cannot live in acidic conditions.  As long as the pH remains low, the unhealthy bacteria thrive while healthy bacteria dwindle away, and all the while minerals are lost from the teeth and cavities may start to form.

Seem like a vicious cycle?

Do not dismay.  There are known factors that can lead to a drop in pH in our mouths, as well as several factors that protect our teeth from decay.  We will address those in a separate post on risk factors.

For now, we’ll stop here because this was already a lot of information.

Here’s a summary:

  1. Biofilm, a complex and dynamic community of microoganisms like bacteria, is always present on our teeth. It consists of both healthy and unhealthy bacteria, and environmental factors affect which bacteria thrive.
  2. Low (acidic) pH selects for cariogenic (cavity-causing) bacteria
  3. Dental caries (AKA tooth decay or cavities) occurs when unhealthy bacteria dominate. This is because:
    • Minerals dissolve out of teeth at a pH of 5.5 (this is the start of a cavity)
    • Unhealthy bacteria both thrive in acid and produce acid, while healthy bacteria cannot survive in acidic environments