The mouth contains a wide variety of oral bacteria, but only a few specific species of bacteria are believed to cause dental caries: Streptococcus mutans and Lactobacillus species among them. Streptococcus mutans are gram-positive bacteria which constitute biofilms on the surface of teeth. These organisms can produce high levels of lactic acid following fermentation of dietary sugars and are resistant to the adverse effects of low pH, properties essential for cariogenic bacteria. As the cementum of root surfaces is more easily demineralized than enamel surfaces, a wider variety of bacteria can cause root caries, including Lactobacillus acidophilus, Actinomyces spp. , Nocardia spp. , and Streptococcus mutans. Bacteria collect around the teeth and gums in a sticky, creamy-coloured mass called plaque, which serves as a biofilm. Some sites collect plaque more commonly than others, for example, sites with a low rate of salivary flow (molar fissures). Grooves on the occlusal surfaces of molar and premolar teeth provide microscopic retention sites for plaque bacteria, as do the interproximal sites. Plaque may also collect above or below the gingiva, where it is referred to as supra- or sub-gingival plaque, respectively.