What Are the Different Types of Bacteria Found in the Human Mouth?

Bacteria reside in great numbers in many places, both inside and on the surface of the human body. In many cases, like in the gut, the presence of bacteria is beneficial. Bacteria in the mouth causes the formation of cavities, and can increase the chance of other disease and illness. Many different types of bacteria live in the mouth, and some types simply pass through depending on the types of food consumed.


The species of Staphylococcus most often found in the mouth include Staphylococcus epidermidis and Staphylococcus aureus. These bacteria have a thick cell wall, known as gram-positive, and are oval in shape. These organisms are opportunistic pathogens, and can cause infection in humans, given the optimal set of circumstances. An illness in another part of the body may cause reduced immune function, resulting in a secondary infection from Staphylococcus.


In the mouth, the genus Streptococcus represents a large portion of the total organism count. Streptococcus mutans, Streptococcus mitis, Streptococcus salivarius, Streptococcus pneumoniae and Streptococcus pyogenes all live in the mouth. The gram-positive Streptococci are found in nature in oval-shaped chains of bacterial cells. Streptococcus mutans is not only a potential pathogen, but also has a role in causing cavities in the teeth. This organism converts the sugar sucrose into lactic acid. The formation of lactic acid begins to dissolve the enamel of the teeth. Once the enamel is dissolved, the lactic acid can continue its path into the tooth and down to the gum tissue.


The lactic acid produced by organisms of the genus Streptococcus is not the only source of lactic acid wreaking havoc in the mouth. Members of the genus Lactobacillus convert lactose into lactic acid, adding more cavity-causing acid. Lactobacillus bacteria have a long rod-shape morphology, while also exhibiting a thick cell wall.

E. Coli

While most of the rod-shaped E. coli in the body is located in the intestines, a small amount of the bacteria is present in the mouth. With a thin cell wall, E. coli is one of the few gram-negative organisms present in the mouth. Foods like ground beef, leafy greens and other fresh foods have a contamination potential from the natural environment. While not all E. coli is as dangerous as the well-publicized O157:H7 variety, the E. coli found in the environment where food is grown should be viewed as a potential pathogen.

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