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Erysipelas is a superficial bacterial skin infection that involves the upper dermis, whereas cellulitis is a deeper bacterial skin infection involving the deeper dermis and subcutaneous tissues.
Okay, let’s go over some physiology. Normally, the skin is divided into three layers, the epidermis, dermis, and hypodermis.
The hypodermis is made of fat and connective tissue that anchors the skin to the underlying muscle. Just above the hypodermis is the dermis, which contains hair follicles, nerve endings, glands, blood vessels, and lymphatics. And just above the dermis is the epidermis, which itself has multiple cell layers that form the external skin surface.
Now, normally, the skin surface is colonized by a huge number of microorganisms that make up the normal skin flora. This flora consists mostly of bacteria, such as Staphylococcus epidermidis, as well as certain fungi, such as Candida albicans. The normal skin flora is typically non-pathogenic, meaning that these microorganisms don’t cause any disease. In fact, they are beneficial, since they serve as a physical and competitive barrier that helps prevent pathogenic microorganisms from invading and infecting the skin.
Now, both erysipelas and cellulitis are typically caused by Gram positive pathogenic bacteria. For erysipelas, the most common causative bacterium is Streptococcus pyogenes, also called group A β-hemolytic Streptococcus; whereas cellulitis is usually caused by Streptococcus pyogenes or Staphylococcus aureus.
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