According to the University of Illinois at Chicago College of Dentistry, there are over 300 specific types of bacteria living in your mouth. When you don't brush, these bacteria get to work ingesting food particles and sugary debris that ultimately cause tooth decay. If you miss just one time brushing your teeth, the interaction between these particles and bacteria will likely create a bad taste and a bad smell in your mouth. What's more, this buildup of bacteria can be spread through direct contact, such as kissing.
If you fall short of brushing one night, the consequences aren't necessarily severe (though you should aim to get back on track by flossing and brushing right away). But if you've missed brushing and flossing more frequently, or if your attention to thorough and proper brushing has waned, you can increase your risk of bone and tissue loss around the teeth, according to the University of Illinois at Chicago College of Dentistry.