Though often described as being "cleaner than those of humans," the mouths of dogs are veritable petri dishes, brimming to the top with bacteria. Many types of bacteria live in the mouths of dogs, and many are capable of causing disease in dogs and humans. Humans may acquire these bacteria through a friendly dog's licks or an aggressive dog's bite.
A 2003 study by S. Kasempimolporn and a team of reasearchers published in the "Journal of the Medical Association of Thailand," found a number of pathogenic bacteria in dog saliva samples. The study compared the bacteria cultured from oral swabs with the bacteria cultured from dog-bite victims, to determine which bacteria in the dog's saliva were responsible for causing infections. According to the study, Staphylococcus aureus, E. coli, Pasteurella multocida, Moraxella spp., Pasteurella canis, and Enterobacter cloacae were the most common bacteria that caused infections in humans. Despite these risks, a 1992 study by Ellie J. C. Goldstein, published in "Clinical Infectious Diseases," concluded that human bites are more likely to cause infection than animal bites.
Bacteria and Gum Disease
Some of the bacteria in the mouths of dogs can cause periodontal disease, and it appears that these bacteria can transfer to humans. In 2012, Y. Yamasaki and a research team investigated the occurrence of periodontopathic bacteria in the mouths of dogs and their owners. Publishing their results in "Archives of Oral Biology," the researchers found that while bacteria could transfer from one host to the other, the two groups normally have different bacterial flora in their mouths. However, researchers cultured Porphyromonas gulae from 13 human-dog pairs in the test group, and found Tannerella forsythia in over 77 percent of the dogs tested and nearly 31 percent of the humans.
Nurture, Not Nature
The type of care provided to a dog drastically affects the types of bacteria living in its mouth. In 2009, researchers at the Hebrew University of Jerusalem studied the oral bacteria of dogs that received differing levels of care. Those that exclusively consumed dry dog food had fewer bacteria than those that were fed a combination of dry food and human leftovers. Additionally, the researchers found that higher levels of care resulted in lower oral bacterial loads.
Because dog saliva can carry pathogenic bacteria, the Centers for Disease Control and Prevention advises pet owners to practice strict hygiene after handling or petting their dog. Always wash your hands with soap and hot water after touching any dog. Additionally, as dog bites can lead to serious infections, the CDC recommends obtaining medical attention when bitten by an unknown dog, or whenever a bite is serious or causes swelling or redness.