A Staff Report from the Straight Dope Science Advisory Board

Are dog mouths cleaner than human mouths?

Dear Straight Dope:

I was reading Marilyn vos Savant's article in Parade today when I came across a question about whether dogs' mouths are cleaner than those of humans. Marilyn said it was a myth, but provided no information whatsoever to back up her claim. (This is typical of her, I have found.) Is she wrong again?

We got this response from the Straight Dope Message Board and Chat Room's resident vet (as in animal doc, not as in ex-marine), DrMat:

Here's all I know on the subject. It scares me to think I've got this much gray matter tied up with this. It explains the missing car keys, anyway.

When people "have always heard" that dogs' mouths are cleaner than humans', they're thinking of two things. One is that supposedly bites by a human upon a human are more likely to get infected than bites by a dog. The other is that if a dog licks a wound, it is less likely to get infected. There are multiple variables to consider here, including: the immune status of the victim, the degree of penetration of the bite, the location of the bite, and probably most importantly, the type of bacteria involved. Notice I did not say the amount of bacteria. Would you rather your child be cared for by a group of grungy bikers or one sex offender? Bad example. But there are fairly low-key staph bacteria that really don't want to offend anyone and then there are the flesh-eating types with a real grudge. One well placed deep bite near a joint with a nasty Klebsiella or Pseudomonas involved will do more damage than a dozen superficial chomps. As far as dogs licking things, the latest recommendations on wound treatment is to flush the wound thoroughly daily, even with plain tap water, and then change the bandage frequently. No antibiotic dressing helps better than frequent hydrotherapy. I think it is the mechanical action of the dog's licking that helps rather than any antibacterial properties possessed by dog spit.

But don't take my word for it! A small, independent study was conducted to determine the relative nastiness of canine and human mouths. For the purposes of our research, "nastiness" was defined as number of bacterial colonies per square centimeter, as determined by placement on a scale of 1 to 4 (as in "yeah, that looks like a 3." You want accuracy, call the Mayo Clinic.) The researcher decided to use gross numbers as a measure of dirtiness instead of determining the actual species of bacteria involved because most people would define "dirtiness" that way and because it was cheaper. Ten randomized humans were selected (by previously hiring them to work at the practice) and were required to submit sample of their oral flora. These were plated on Mueller-Hinton agar, along with ten samples of oral flora from random dogs, who just happened to be boarding over the July 4th weekend. The dogs were a cross section of typical canine mouths, which is to say funky, ranging from young to old, clean teeth to pyorrhea. The humans included males and females, smokers and nonsmokers, all after eating and before brushing teeth. After 48 hours incubation:

Results: Amount of bacteria

1+ 2+ 3+ 4+

Dogs 1 0 3 6

People 7 2 1 0

In other words, the majority of the human cultures had pinpoint colonies in light growth. The majority of the canine plates looked (and smelled) like that gunk that gets down in your garbage can after the raw chicken got left in there. One was actually green and I swear I heard it chuckling to itself. Science ain't for sissies.

There you have it. Have fun. Email me the royalties.


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