One of the most uncomplicated things to do besides washing your hands, is brushing your teeth. Saliva in your mouth keeps plaque from forming on your teeth but only for a short time. The enamel on your teeth absorb certain glycoprotein’s that are acidic to form this protective layer called an acquired enamel pellicle. This pellicle has a negative charge which repels most bacteria in the mouth because they are also negatively charged. This defence breaks down with the formation of dental plaque. Some gram positive bacteria such as Streptococcus gordonii, S. oralis, and S. mitis can attach to the pellicle and start colonization. These initial bacteria can coat the pellicle so that other bacteria normally not able to attach, can now attach to these bacteria that initially colonized the teeth. All these cells divide and grow to increase colony strength.
A matrix can now form in between the layers caused by the bacteria and dietary sucrose. Certain bacteria feed off of the dietary sucrose to produce "additional sticky microbial substances", which is where the other bacteria get their energy from. As this matrix of bacteria we call plaque thickens it can become more gram negative from all the other bacteria that is in the plaque. Because of the plaque the there is an absence of oxygen on the surface of the tooth which can lead to the growth of strict anaerobic bacteria, so the plaque can form between teeth. This formation of plaque can start to produce acids from the sucrose, that now cannot be neutralized, because the saliva cannot penetrate the biofilm. The acids can start to demineralise the teeth leading to tooth decay. It can happen fast, within days the biofilm formation can happen if you don’t brush or floss properly. So the thing is people need to brush, floss, and go to the dentist regularly because once this plaque forms only a trip to the dentist can get rid of it.
Prescott, Harley, & Klein's Microbiology Seventh Edition (page 991)