It probably is not true. I read an article in a veterinary journal several years ago that detailed a study in which bacteria were cultured from the mouths of cats, dogs and humans. The article reported that the mouths of dogs yielded the greatest number of bacterial colonies. Cats were second, and humans third.
However, a little research on google determined that the study in question (link is PDF format) was not published in a peer-reviewed journal. In fact, the study was submitted to the California State Science Fair by E. Jayne Gustafson. Ms. Gustafson’s methodology looks good, but I cannot vouch for the rigorousness of her technique or statistical analysis.
All animals have bacteria in their mouths. A cat’s mouth probably has no more bacteria than the mouth of any creature that does not brush its teeth. However, cats have unique teeth. Their (inappropriately named) canine teeth, or fangs, are extremely long and sharp. When cats bite people or other cats (they tend to use their claws, rather than their teeth, in altercations with dogs), their canine teeth puncture deep into tissues and introduce bacteria. Because the teeth have small diameters, the seemingly minor wounds heal rapidly.
Cats’ mouths harbor a type of bacteria called Pasteurella multocida. Unfortunately, this bacteria thrives when trapped in healed-over bite wounds. Severe infections often result.
Therefore, cat bites are more likely to lead to serious infections than dog bites. I believe this is the origin of the myth that cats’ mouths contain more bacteria than dogs’.
About the photo: Curtis demonstrates feline canine teeth. I have no reason to believe that he’s ever bitten anyone.