YOUR mouth is like a tropical rainforest: It is hot, humid, and maintains a fairly constant temperature year round. And like a rainforest, it is teeming with life: Bacteria, viruses, fungi, and protozoa. Although you cannot see them, your mouth is home to billions of microorganisms (germs). By far the greatest number of organisms that populate your mouth are bacteria: Short ones, fat ones, skinny ones — they are all represented. You have so many bacteria living in your mouth that their population far exceeds the number of people living on earth.
Food for these little parasites is plentiful. What do they like to eat? They like to eat pizza, ice cream, and donuts! What you eat, they eat. They thrive on a diet of sugar and other carbohydrates — their preferred foods. They love the little tasty morsels that get caught between your teeth, or become wedged in the folds between your cheek and gums, where they can nibble on them happily for hours. It is no wonder that in such an ideal environment, our mouths are home to so many of these creatures. So, theoretically, a vicious bite from a person can easily be compared with a bite from a labaria snake, depending on the extant situation.
Your mouth, in essence, is a mini ecosystem. The weather forecast is the same every day: 95 F˚ (35 C) unless we are ill, with 100 per cent humidity. Microorganisms, as they are collectively called, are selective. They do not colonise your mouth in a random way; they form communities. Like an ecosystem in a tropical forest where some creatures prefer to live on the ground, others in the trees, or on water, the microorganisms in our mouth select their own living space. Some prefer life on the teeth; others the space between the gums and teeth or the roof of the mouth; and yet others prefer pockets in the front or back of the tongue. Although they may touch each other, each micro-community contains a distinct population.
Each person has a set of unique communities of microorganisms living in their mouths. A person living in London has different microorganisms from someone living in East Ruimveldt, in Georgetown, or in New York, who in turn has different groups of organisms than say people living in Ottawa, Canada. Even among family members, communities differ. Despite close contact, a husband and wife have their own distinct microbial populations.
The micro-communities in our mouths are unique for each individual, because the environment is a product of our diet, lifestyle, genetics, gender, etc. Stress, for example, can impact our immune system, which in turn affects the microbes in our mouths. Hormone levels also have an influence; certain hormones encourage the growth of specific organisms. People who are sub-clinically dehydrated most of the time have decreased saliva production; saliva contains buffers and enzymes that profoundly affect the environment and the microbe population. Smoking and alcohol consumption also have an impact. One of the biggest factors is diet. Sugar and other carbohydrates act like fertilizer in your garden; bacteria and yeasts proliferate in their presence.
Our health also affects the types of organisms that inhabit our mouths. High blood sugar, as seen in diabetics, encourages the growth of certain mouth-dwelling bacteria. Overweight individuals have different microbes in their mouths than normal-weight people. Medical researchers are even able to identify certain health conditions, based on the micro-communities in the mouth. So you can see there are many factors that influence the micro-populations in our mouths.
Microorganisms begin to inhabit the mouth as soon as we are born. While the mouth and digestive tract of newborns is sterile, they are quickly colonised by micro-flora in the air from contact with parents and siblings, and from things they randomly stick in their mouths.  We know now that people have an incredibly large amount of bacteria growing in their mouths. In fact, humans have more bacteria in their mouths than dogs do.
Considering all the disgusting places where dogs like to stick their muzzles, their mouths are amazingly clean. You would pick up more germs from kissing your spouse on the lips than you would from kissing the mouth of a drooling dog! Disgusting for sure, but true.
Dogs have antibodies in their saliva that are not found in the human mouth. These antibodies kill disease-causing germs. So, in summary, never underestimate the cleanliness or dirtiness of someone’s mouth.