Our body harbors trillions of bacteria in “peaceful” symbiosis, mostly in the gut, on the skin and other mucous membranes. The normal resident microflora of the skin depends very much on the body area. Most microorganisms live in the superficial layers of the stratum corneum and in the sebaceous gland and hair follicles. Staphylococcus epidermidis is a major inhabitant (up to 90% of the resident aerobic flora) and Cutibacterium acnes. Staphylococcus aureus colonizes in particular the nose and the perineum and is increased in eczematous skin, like in atopic dermatitis. Other bacteria found on normal skin are Micrococci and Corynebacteria. Streptococci and Gram-negative bacteria are rarely found on normal skin. The microflora protects the skin against overgrowth of pathogen bacteria, which especially in persons with compromised immunity can lead to serious illnesses and disturbances of wound healing. Whereas most infections stay superficially on the surface of the skin, some may involve deeper structures of the connective tissue and even spread into the peripheral blood, inducing sepsis or produce toxins leading to life-threatening complications. The ways of contamination may be the penetration of bacteria through open skin fissures or injuries (pyoderma, erysipelas) or transmissions in close body contact, through fluids or lubrication contacts (syphilis, leprosy, or Lyme disease through tick bites). The sexually transmitted bacterial infections (STI) will be discussed in a separate chapter (Chap. 10 ).