Always an interesting queston for a science fair...

Dog's saliva is less acid than man's one with a pH of ~9 whereas a human has a saliva pH range of between 6.5 to 7.5. This high pH would prevent cavities in a dog, but it would promote the precipitation of calcium phosphate, then tartar formation from the plague on the dog's teeth [http://www.petngarden.co m/dogs/dogs14.html].

Although there is some evidence to suggest that Canine saliva can inhibit the growth of certain E. coli and Streptococcus strains, particularly those found in nasal mucous, they are by no means an effective antiseptic as they contain many species of their own bacteria that are potentially harmful to man (Hart & Powell, 1990). A website here too: [http://www.gi .alaska.edu/ScienceForum/ASF12/1234.html].

An important consideration is the idea of biofilms on teeth. There is an amazing lattice framework of bacteria (and some fungi also) on teeth that both core anaerobes and some moderate aerobes. They exist in a polysaccharide "glue" that protects them from the environment. For more on biofilms, try googling for "bacterial biofilm" of "Dental Biofilm" and variations thereof. Biofilms can contain upwards of a hundred bacterial species, many of which are harmless and many of which can't be cultured or are as yet unidentified.

So it is likely that the different pH of dog saliva may not necessarily reduce bacterial numbers, but may consist of a different makeup of mouth flora than a humans, i.e. bacterial strains suited to the slight increase in pH. Of course, there are only so many bacterial cultures that can be cultured at all (and safely), and for most purposes a commonly available nutrient agar that may show a disparity between bacterial numbers in human and dog saliva, may be selectively excluding certain fastidous strains of bacteria (see another MadSci question I answered here : Growing Colonies of Bacteria and Why is it hard to culture most bacteria).