Impetigo

Impetigo is a highly contagious skin infection that stays in the most superficial part of the epidermis, which is the top layer of the skin. It is caused mainly by gram-positive bacteria, a group defined by the way they stain in the laboratory.

The skin is a closed door: one scrape is all the bacteria need to take hold. Any break in that barrier exposes the fibronectin receptors, which are anchoring proteins. And it turns out that Staphylococcus aureus and group A streptococcus need exactly that in order to colonize, meaning to settle in and multiply.

Then there are two different forms. In the nonbullous form, the purulent exudate, the pus, forms the honey-colored crust, the classic picture of impetigo. In the bullous form the mechanism is different: S. aureus produces an exfoliative toxin that pries the cells of the superficial epidermis apart from one another.