Bad breath is usually caused by oral bacteria

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The most common cause of halitosis is the anaerobic bacteria that live in a person's mouth.

For the majority of people (85 to 90% of those who have bad breath [halitosis]), the single most frequent cause of breath odor problems is bacteria that live in their mouth. Bacteria, just like humans, go through their lives consuming foods and excreting wastes. The waste products produced by some oral bacteria are sulfur compounds and it is this type of odoriferous waste product that usually lies at the root of a person's breath problems.

Have you ever smelled a rotten egg? The stench associated with rotten eggs is caused by the sulfur compound hydrogen sulfide. The stinky smell emanating from feed lots and barnyards is one created by the sulfur compound methyl mercaptan. The odor you associate with the ocean is in part due to the presence of dimethyl sulfide. And each of these types of sulfur compounds is also excreted as a waste product by the bacteria that live in our mouths.

As a class, dentists refer to these compounds as "volatile sulfur compounds" (VSC's). The term "volatile" simply describes the fact that these compounds evaporate readily, even at normal temperatures. The extreme volatility of VSC's explains how these compounds have the ability to offend those around us, instantly.

While volatile sulfur compounds are the principle causative agents of bad breath, the bacteria that live in our mouth produce other waste products too. Some of these have their own unique and unpleasant smell. A few of these waste byproducts are:

Cadaverine - the smell we associate with corpses.
Putrescine - the compound responsible for much of the foul odor produced by decaying meat.
Skatole - the characteristic smell of human fecal matter.
Isovaleric Acid - the smell of sweaty feet.

How surprised you must be to discover that this wonderful mix of compounds emanates from the mouths of humans. And no one is an exception. Everyone has some amount of these unpleasant compounds in their breath. Fortunately, however, at low levels these odorous compounds cannot be detected by the human nose. It's only when the amount of these compounds becomes elevated that others can detect our "bad" breath.

Bad breath is caused by waste products produced by anaerobic oral bacteria.
Most of the compounds that cause bad breath (hydrogen sulfide, methyl mercaptan, cadaverine, putrescine, skatole) are the waste products of anaerobic bacteria (more specifically Gram-negative anaerobic bacteria). The term "anaerobic" describes the fact that these types of bacteria grow best in environments that are devoid of oxygen.

A person's mouth can be home for hundreds of different species of bacteria. And on going in our mouth, at all times, is a constant battle for living space between the types of bacteria which do create waste products that cause bad breath and those that don't. And it is the precise balance between the relative numbers of these two different types of bacteria that will ultimately determine the quality of a person's breath.

Dental plaque accumulation (the whitish film that forms on teeth both above and below the gum line and also on the tongue) can tip the scales in favor of the odor causing bacteria. You might be surprised to learn that a layer of plaque as thin as 0.1 to 0.2 millimeters (about the same thickness as a dollar bill) can be oxygen depleted, thus creating the precise type of environment in which anaerobic bacteria will flourish.

So, as more and more plaque builds up in a person's mouth the bacteria that cause bad breath gain available living space and proliferate, thus increasing the level of odor causing compounds that escape with each breath that is exhaled.

What is the food source for the anaerobic bacteria that cause bad breath?
Most of the volitile sulfur compounds that cause bad breath are waste products created by anaerobic bacteria as they digest proteins. This means that as we consume food items such as meat and fish, the bacteria that live in our mouths are getting a meal too and subsequently producing the waste products that cause our bad breath.

Even without an obvious protein source, like just having eaten a cheeseburger, it's not hard for the anaerobic bacteria that live in our mouths to find a meal. There are always protein food sources floating around in our mouths such as dead skin cells or one of the many protein compounds found in saliva. And then also, especially for those who are not diligent with brushing and flossing, there is always the leftover food debris from yesterday's meal, and the meal before that, and the one before that....

High protein foods.
Meat, fish and seafood, eggs, and dairy foods (milk, cheeses, and yogurt) are all obvious examples of foods high in protein. Most of us get about two thirds of our needed protein from eating them. Other protein sources include cereal grains (and cereal grain products), nuts, and the seeds from pod bearing plants (peas, beans, and lentils). The ingredients found in many of our favorite deserts (especially cakes and pies) can make these food items surprisingly high protein sources.

Where do the bacteria that cause bad breath live?
For most of us the bacteria that cause our bad breath live on the surface of our tongue. There can be, however, other locations (which are typically secondary in nature) that harbor these culprit bacteria as well.

Bad breath source: Bacteria that live on a person's tongue.
Think back to the breath testing experiments described at the beginning of this topic. While the smell emanating from the anterior portion of a person's tongue can be unpleasant, it usually it isn't the source of their breath problems. The most common odor-producing region of the tongue is the posterior (back) portion.

Go to a mirror, stick your tongue out and take a look at it. With many people, you can actually see a whitish coating on the tongue's surface. The further back (toward the throat) you look, the whiter this layer usually appears. This is dental plaque and it is chocked full of the types of anaerobic bacteria that cause bad breath.

The type of surface texture found on a person's tongue will influence the amount of coating that it tends to accumulate. People whose tongues are deeply grooved or furrowed will be more likely to accumulate a bacterial laden coating, as opposed to those who have a smoother tongue surface.

It only takes a coating that has a thickness of 0.1 to 0.2 millimeters (about the same thickness as a dollar bill) to provide an environment depleted of oxygen. This type of environment is referred to as "anaerobic" and it is precisely the type of environment in which the bacteria that cause bad breath will flourish.

Research has shown that there is a direct correlation between the amount of coating that exists on a person's tongue and the total number of anaerobic bacteria that are present. And as you can probably guess, when the anaerobic bacterial count on a person's tongue is reduced there is usually a direct correlation, an improvement, in the odor emanating from their mouth.

Bad breath source: Bacteria that live at and below a person's gum line.
The type of bacteria that cause bad breath can find a suitable home in places other than just on the surface of a person's tongue. When you floss you may have noticed that at times you have dredged up a foul taste or odor. Maybe this odor is more noticeable as you floss between the teeth that are relatively further toward the back of your mouth. These locations are places where the bacteria that cause bad breath often find a cozy home. The taste and smell you get when flossing these locations is evidence of this.

Even in a relatively healthy mouth, bacteria can and do find anaerobic (oxygen deprived) environments in which to live underneath the gum line, both around and in between teeth. These anaerobic environments are, however, even more numerous and available in the mouths of people who have experienced periodontal disease ("gum disease").

This is because periodontal disease can cause damage to the bone that surrounds a person's teeth. This bone destruction leads to the formation of deep spaces that lie between the teeth and gums (termed "periodontal pockets"). The deep recesses of periodontal pockets can be difficult, if not impossible, to clean effectively. This makes them an ideal anaerobic environment for the types of bacteria that cause bad breath.