Streptococcus mutans
Streptococcus mutans is a facultatively aerobic
Facultative anaerobic organism
A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...

, Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 coccus-shaped bacterium
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 commonly found in the human
Humans are the only living species in the Homo genus...

 oral cavity and is a significant contributor to tooth decay
Dental caries
Dental caries, also known as tooth decay or a cavity, is an irreversible infection usually bacterial in origin that causes demineralization of the hard tissues and destruction of the organic matter of the tooth, usually by production of acid by hydrolysis of the food debris accumulated on the...

The microbe was first described by J Kilian Clarke in 1924.
Streptococcus mutans is a gram-positive organism that is the primary causative agent in the formation of dental cavities in humans. Gram-positive bacteria are those that are stained dark-blue or violet by Gram staining.