Introduction The Skin Anatomy Cellulitis is an acute bacterial infection of the deep dermis and subcutaneous tissue in both humans and companion animals. It is a painful, spreading inflammation of the tissues under the skin  (the subcutis). The most common causative agents in humans are Streptococcus species and Staphylococcus aureus, while in pets, the most frequent species is Staphylococcus pseudointermedius , which is rarely pathogenic to humans (Bannoehr & Guardabassi,