It's a fact - bacteria like to eat. And we can benefit from their appetites for sugars and proteins. Some of the foods we eat and their unique characteristics are due to the action of bacteria that are NOT harmful to humans.
Bacteria are creatures of comfort and, unfortunately, there are harmful bacteria that find the human body to be the perfect host - it's warm and a constant source of nutrition.
Once inside the human body, pathogenic bacteria cause disease primarily through the production and secretion of toxins that interact negatively with its host. For example, cholera toxin works by disrupting the body's water balance, eventually leading to death from dehydration.
The human body is host to many bacteria that do not do any harm. For example, E. coli commonly inhabits the intestinal tract and is generally harmless. But there are some very dangerous strains of E. coli that can cause serious illness and even death if able to cross the body's natural lines of defense against infection.
One very common type of bacterial attack is tooth decay. Even though not immediately apparent, bacteria that have been allowed to accumulate on the surface of teeth metabolize the sugar in the foods we eat. The organic acids released by the bacteria have a corrosive affect on the enamel surface of our teeth. And several recent studies have shown a link between dental disease and coronary heart disease. A study published in J Am Dent Assoc 1998 Mar;129 (3):301-11 found a significant association between coronary heart disease and certain oral health indicators such as the number of missing teeth and amount of plaque.