Less vs. Fewer: Less Plaque, Fewer Germs

I was delighted to get an e-mail from Mariah, an 8th grader from Connecticut, who writes:

Hi Snarky,
I think I just spotted a grammar mistake in this ad for Colgate Total toothpaste. The girl says, "You want to see a Colgate Total mouth? See? A lot less germs, and I brushed at 7 am." She should have said 'fewer germs.'

Excellent work, Mariah! Now go ask your ELA teacher to give you some extra credit. You deserve it! 

Where does Colgate go wrong? The copywriter should have used fewer with a countable noun.

Quick rules:

• Use fewer for countable nouns. Thanks to fluoridated water, kids get fewer cavities these days. We can count cavities.
• Use less for uncountable nouns. There is less plaque on your teeth after you brush. Plaque can be measured, but not counted.

Correction:

•  "You want to see a Colgate Total mouth? See? A lot fewer germs, and I brushed at 7am."
•  "You want to see a Colgate Total mouth? See? A lot less plaque, and I brushed at 7am."