Skin

"The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.
Skin has three layers: The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.
The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis."

Ecthyma

Ecthyma is an ulcerative pyoderma of the skin caused by bacteria such as Pseudomonas (the most common isolate), Streptococcus pyogenes, and Staphylococcus aureus. Because ecthyma extends into the dermis, it is often referred to as a deeper form of impetigo. Causes include insect bites and an ignored minor trauma. Wound cultures usually reveal that the lesions are teeming with bacteria. Ecthyma describes ulcers forming under a crusted surface infection. The site may have been that of an insect bite or of neglected minor trauma. It is treated by antibiotics like cloxacillin, erythromycin, and cefalexin. Psuedomonas infections are often treated with two antibiotics due to frequent resistance.