Microbial diseases of the skin are usually transmitted by contact with an infected individual. Although the skin normally provides a barrier to infection, when it is penetrated by microorganisms, infection develops. Diseases of the eye are considered with the skin diseases because both occur at the surface of the body.
Staphylococcal infections. Staphylococci are Gram‐positive cocci occurring in clusters. The best known pathogen in this group is Staphylococcus aureus. This organism invades the hair follicles and causes folliculitis, also referred to as pustules. A deeper infection of the skin tissues is referred to as a boil, abscess, or furuncle. These lesions are usually filled with pus. A large lesion progressing from a boil is known as a carbuncle. Infections such as these are easily transmitted by skin contact as well as by fomites.
Toxin‐producing strains of S. aureus cause scalded skin syndrome. Usually found in young children and babies, this disease is characterized by vesicles on the body surface, which cause the skin to peel and give a scalded appearance. Penicillin or erythromycin antibiotics are used to treat this and other staphylococcal skin diseases.
Scarlet fever. Scarlet fever is caused by Streptococcus pyogenes, a Gram‐positive bacterium occurring in encapsulated chains. Most cases of scarlet fever begin as infections of the respiratory tract, followed by spread of the bacteria to the blood. The bacteria produce an erythrogenic toxin that causes the typical skin rash. Penicillin is used for therapy. Complications include damage to the heart valves known as rheumatic heart disease or damage to the joints, which is called rheumatic fever.
Erysipelas. Erysipelas is a skin disease caused by Streptococcus pyogenes and other pathogenic streptococci. Small, bright, raised lesions develop at the site of streptococcal entry to the skin and grow with sharply defined borders. Penicillin therapy is employed.
Impetigo contagiosum. Impetigo contagiosum is a contagious skin infection accompanied by pus. It is caused by species of streptococci, staphylococci, and others. The disease commonly occurs in children and is easily transmitted among them. Penicillin therapy is often recommended.
Madura foot. Madura foot is a general name for infections of the feet due to many microorganisms. Among the causes are species of soil bacteria belonging to the genera Nocardia, Actinomyces, and Streptomyces. These and other bacteria enter the tissues and cause granular lesions that spread and eventually invade the bone and muscle. Sulfurlike granules represent accumulations of microorganisms in the pus, and antibiotic therapy is necessary to prevent spread of the disease.
Gas gangrene. Gas gangrene is a disease of the deep skin and wounds as well as the blood. Several species of Clostridium cause gas gangrene, including Clostridium perfringens, C. novyi, and C. septicum. These anaerobic rods are transferred to the wound in their spore form. They germinate and grow in the dead, anaerobic tissue of a wound, putrefying the proteins and fermenting the carbohydrates to produce gas. The gas causes the tissue to expand, and as the cells die from lack of oxygen, gangrene begins. Bacterial toxins pass through the bloodstream to cause illness throughout the body, and degeneration of the muscle fibers occurs. Aggressive antibiotic therapy and removal of dead tissue are useful therapies.
Cat scratch fever. Cat scratch fever may accompany a skin wound following a cat scratch. Although the causative agent has not been isolated with certainty, it is believed to be a species of Rochalimaea or Afipia. Patients display a pustule at the skin site of entry and swollen lymph nodes on one side of the body. Treatment with antibiotics may or may not be successful. Mild fever and conjunctivitis often accompany the disease.
Rat bite fever. Rat bite fever may be caused by either Spirillum minor or by Streptobacillus moniliformis. The former is a flagellated spiral bacterium; the latter is a Gram‐negative rod in chains. Both species are transmitted during a bite by a rat, either wild or laboratory. Rat bite fever is associated with skin lesions, intermittent fever, and a skin rash. Arthritis may also be present.