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Chapter 18 - Chapter 18 Microbial Diseases of the Skin The...

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Chapter 18 Microbial Diseases of the Skin The Skin The largest organ Represents the boundary between the organism and the environment Sometimes shows symptoms of illness not associated with skin (rash Structure and Function of the Skin Epidermis Thin outer part, composed of layers of dead epithelial cells – stratum Replaced every 25-45 days Physical barrier against microbes Dermis Connective tissue; hair follicles, sweat glands, oil glands, neurons Sebaceus glands – softens and lubricate the skin Normal Microbiota of the Skin Normally harmless, able to survive on the skin resistant to drying and high concentration of salt These microbes cannot be completely removed through cleansing Those in hair follicles and sweat glands reestablish rapidly – responsible for body odor May be opportunistic pathogens Representatives: Gr+ cocci ( Staphlycococcus , Micrococcus) Gr + pleomorphic rods – diphteroids; Propionibacterium acnes – anaerobic bacteria, inhabit hair follicles, produce propionic acid (maintains acidity of the skin pH 3-5) The yeast, Malassezia – cause skin scaling dandruff Acne The most common skin disease caused by Propionibacterium acnes Normally, dead cells and sebum are discharged through pores to skin surface Overproduction of sebum leads to clogging the pores Propionobacterium acnes digests the oily material – fatty acids are produced Bacterial antigens + fatty acids causes local inflammation Bacterial Infections Staphylococcal skin infection Staphylococci – spherical Gr+ bacteria; form irregular clusters Two groups: coagulase “+” strains coagulase “–“ strains Coagulase positive strains coagulate (clot) fibrin in blood. Coagulase negative strains make up 90% of normal microbiota. Pathogenic only when the skin is broken. ( Staphylococcus epidermidis ) Staphylococcus aureus – the most pathogenic are coagulase positive (blood clot protects them against phagocytosis). Found in hospital environments, resistant to antibiotics, difficult to treat.
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