That is a myth actually. Dogs mouth are not clean. They carry bacteria just like humans do except their strains of bacteria and different and they are deadly to birds, for instance pastuerella (more common in cats). Their licking wounds just stimulates tissue and provides debridement. Most bacteria in dogs mouth doesnt affect humans and that where the myths stem from.
Here are some sources on teh internet you can read up on.
Many people may have heard a myth that dogs' mouths are cleaner than the mouths of human beings. This rumor is likely to stem from the fact that dogs are often seen licking their wounds, which rarely get infected.
However, people who believe that dogs' mouths are cleaner than humans' mouths are greatly mistaken, according to veterinarians who have spoken on the subject. Most humans have a strong belief in good hygiene, and brush their teeth at least once or twice every day. Human mouths rarely come into contact with external bacteria. Dogs' mouths, in contrast, are rarely cleaned in most cases. Dogs also have a tendency to sniff around and eat bacteria-filled waste including rancid food and feces whenever they are given the opportunity to do so. Contrary to popular belief, dogs' mouths are far dirtier than the mouths of the typical human being.
The reason that dogs' mouths rarely infect the wounds that they lick is that their licking has the effect of clearing away the dead tissue, similar to the work a surgeon would do to clean out a wound. Even though dogs' mouths are full of bacteria, dog bites do not usually cause infections in humans because most bacteria in dogs is particular to their species. Therefore, even if the dog is infected with a disease, the human is unlikely to catch it. For this reason, it is not especially dangerous to share food with a dog or to kiss dogs' mouths, though neither option is exactly the most hygienic of decisions.
Here is some scientific infromation on pasteurella.
http://www.emedicine.com/MED/topic1764.htm
Here is some more info from the Louisiana School of Veterinary Medicine.
http://www.vetmed.lsu.edu/animal_bites.htm
Ive always heard that dogs mouths are cleaner than humans. Is this true?
Neither dogs nor cats nor humans have mouths that can even remotely be considered clean. All are filled with bacteria, many of which can cause disease if they enter broken skin. Over 130 disease-causing microbes have been isolated from dog and cat bite wounds.1 Animals saliva is also heavily contaminated with bacteria, so a bite may not even be necessary to cause infection; if you have a cut or scratch and allow a pet to lick it, you could be setting yourself up for trouble.
What are the particular dangers from animal bites?
Bites to the hand, whether from cats or dogs, are potentially dangerous because of the structure of the hand. There are many bones, tendons, and joints in the hand and there is less blood circulation in these areas. This makes it harder for the body to fight infection in the hand. Infections that develop in the hand may lead to severe complications, such as osteomyelitis or septic arthritis.
In small children, bites to the face, neck, or head are extremely hazardous. Because their small stature often puts their heads near dogs mouths, children are often bitten in these areas. Dog bites can cause fractures of the face and skull and lead to brain and nervous system infections. Dog bites cause, on average, about 15-20 fatalities a year in the United States. Most of these victims are infants and young children.
What kinds of infections can develop?
Many infection-causing bacteria have been isolated from dog and cat bite wounds. The four we discuss here are probably the most significant.
Pasteurellosis.
The most common bite-associated infection is caused by a bacterium called Pasteurella. Most cats and dogseven healthy onesnaturally carry this organism in their mouths. When an animal bites a person (or another animal), these bacteria can enter the wound and start an infection. The first signs of pasteurellosis usually occur within 2 to 12 hours of the bite and include pain, reddening, and swelling of the area around the site of the bite. Pasteurellosis can progress quickly, spreading toward the body from the bitten area. It is important that you seek medical care immediately if these symptoms occur. Untreated, this infection can lead to severe complications. Bites to the hand need special attention; if pasteurellosis develops in the tissues of the hand, the bacteria can infect tendons or even bones and sometimes cause permanent damage if appropriate medical care is not administered promptly.
Streptococcal and Staphylococcal Infections.
These bacteria can cause infections similar to those caused by Pasteurella. Redness and painful swelling occur at or near the site of the bite and progress toward the body. As with pasteurellosis, you should seek prompt medical care if these symptoms develop.
Capnocytophaga Infection.
This is a very rare infection, but we mention it here because it is so dangerous if it develops. There is no common name for this infection, which is caused by the bacterium Capnocytophaga canimorsus. Most of the people who have become infected were bitten by dogs; in many instances the bite wounds themselves were tiny and would not have ordinarily called for any special medical care. But Capnocytophaga can cause septicemia, or blood poisoning, particularly in people whose immune systems are compromised by some underlying condition (see box below). Up to 30 percent of people who have developed this septicemia have died. People who have had their spleens removed are at special risk for this infection. Early symptoms may include nausea, headache, muscle aches, and tiny reddened patches on the skin.
