| No such thing.
Colloquially, By the skin of my teeth it is a metaphor for a very very narrow escape.
But if we don't have epithelium (the anatomical name for skin), what's that stuff I can scrape off with my fingernail?
Let's start off with a clean tooth surface. You have just finished brushing your teeth, hence facilitating the mechanical removal of any substance that's not tooth and not fastly adherent (of course, I'm referring to things like calculus and orthodontic braces). Within a few minutes, the surface is covered in a thin film of salivary glycoproteins.
This is called the acquired pellicle. It develops naturally and spontaneously, and is due to the attraction between proteins in saliva and the surface of the tooth enamel.
The acquired pellicle is the first stage in the formation of a biofilm on the tooth's surface. It allows for adhesion of microbes to the tooth and the start of bacterial colonisation. This is due to the interplay of attractive Van der Waal's forces and electrostatic repulsion and the interaction of specific adhesins and receptor between bacteria and the pellicle surface.
Up to 10 6 viable bacteria per mm 2 of tooth surface can be recovered 1 hours after cleaning. The early coloniser of the pellicle surface are gram positive cocci and Neisseria spp. The growth and metabolism of these pioneer species change local environmental conditions (eg redox potential, pH, coaggregation, substrate availability), and enable more fastidious organisms to colonise later.
The Streptococci secrete a substance "glucans" that form the matrix which houses and binds the bacteria. This extracellular matrix is the reason why this "skin" appears sticky when you scrape it off with your fingernail. This is dental plaque.
Within a few days, additional bacterial types such as Veillonella spp (gram negative anaerobe) , Actinomyces ( gram positive rod) and Capnocytophaga (gram negative rod) enter the region. Prevotella intermedia and filamentous fusobacterium species colonise the plaque between the first and the third weeks as an anaerobic environment becomes established.
Late colonisation with Porphyromonas gingivalis, motile rods and Treponema species (spirochaetes) grows during and after the third week if the plaque grows undisturbed.
So, what has formed is soft and white, of varying thickness. The exact composition of the microbial population within plaque varies based on site (e.g. smooth surface, pit and fissure, interproximal), available substrate, salivary components, duration and oral hygiene practices of YOU.
It is the dreaded dental plaque that is the root of most dental evils (decay and gum disease). It cannot be rinsed off, it can only be brushed off.
Here endth My Story of Dental Plaque, the skin on your teeth.
| 
