Dental plaque is a thin, pale yellow layer that develops naturally on the teeth. It is caused by bacteria sticking to the tooth's smooth surface.
Dental plaque is made up of about a thousand species of bacteria. The layer has so many species because the body has no way of getting rid of them. This is unlike skin, for example, which is shed all the time, removing bacteria as well. It is rather like a meadow, allowed to grow as it will.
When it is first formed, it is soft, and can be brushed off, or scraped away with a fingernail. But after about two days the bacterial material hardens and becomes very difficult to remove except with special dental equipment.
This is the reasoning behind brushing your teeth at least once a day. The hardest place to get to is between the teeth, and this is why the area between teeth on many people is a yellow colour. That is plaque they have not removed. These areas are most easily removed by brushing up and down, which is why dentists recommend brushing up and down, rather than side to side.
Why do dentists worry about dental plaque, other than it may look unsightly? The problem is that, as the bacteria feed, so they release acid, and this can dissolve the teeth, leading to tooth decay. They can also affect the gums, leading to them becoming weaker and teeth loosening and falling out.
As none of these effects are good news, and can be reduced by brushing teeth regularly, it makes sense to brush teeth up and down at least once a day using toothpaste.
Why should you use toothpaste, and what did people use in the past?
Toothpaste is a mixture of chemicals and fine gritty powder. The gritty powder helps to rub away the plaque. The chemicals are mostly alkaline, and they balance out the acid produced by the plaque. Most toothpastes also contain fluoride, a chemical which sinks into the surface of the teeth and makes them harder and so resists tooth decay.
In the past people rubbed their teeth with their finger coated in ash from the fire. This was, in fact, very sensible. Fire ash is alkaline and contains gritty bits. It just doesn't taste very nice! Without toothbrushes, the plaque between the teeth cannot be removed.
