BACTERIAL DISEASES Bacterial infections of the skin usually develop as a result of previous damage to the skin by a physical agent or disease process (e.g. atopic dermatitis), which allows a portal of entry for the microorganism. Most infections are due to organisms which are not normally part of the resident flora, particularly Streptococcus pyogenes (group A streptococcus) and Staphylococcus aureus, the clinical features of the infection depending on the depth of skin inflammation.