113lecture2906 - BIOLOGY 113 MICROBIOLOGY Lecture 29...

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BIOLOGY 113 - MICROBIOLOGY Lecture 29: Microbial Diseases of the Skin and Eyes The skin serves as an important physical barrier in nonspecific resistance (Tortora et al., Figure 21.1) - The outermost layer of the epidermis , the stratum corneum , is essentially impenetrable - The dermis is a layer of connective tissue that contains numerous blood vessels, lymph vessels, nerves, hair follicles, and sweat and oil glands = Sweat glands convey perspiration, a mixture of water, salts and small organic molecules, to the skin surface; it creates a hypertonic environment that inhibits most microorganisms, but provides nutrients for the skin's native flora organisms = Oil glands secrete sebum , a mixture of lipids and proteins, into the hair follicles · Presumably, sebum helps control drying of hairs and the stratum corneum · Like perspiration, sebum inhibits growth of many microorganisms and provides nutrients for a few - As an environment for microorganisms, skin is acidic and hypertonic ; as a result, its native flora consists of organisms that favor these conditions = Most of the skin is also an aerobic environment; although anaerobic "microhabitats" are found in the hair follicles, allowing growth of some strict anaerobes = Composition of the normal flora of the skin varies among locations · Cells of species of Micrococcus and Staphylococcus are most abundant · Gram-positive rods, some belonging to the genus Corynebacterium , are also very common; these are sometimes referred to as diphtheroids · Propionibacterium acnes is an anaerobe found in hair follicles - Infectious diseases of the skin involve not only infections in which the pathogen colonizes the skin, but also a number of systemic infections in which symptoms are manifest on the skin (Tortora et al. Figure 21.2) Bacterial skin infections vary from superficial to severe - Staphylococcus aureus is a common cause of skin infections = S . aureus possesses several virulence factors, icluding coagulase , which may protect them from phagocytosis by shielding them with a fibrin clot = Most strains of S . aureus seen today are penicillinase producers, the so-called MRSA = S . aureus often enters the body through a break in the skin or hair follicle · Pimples may be a manifestation of folliculitis , infection of the hair follicle · Furuncles are instances of abscess (localized region of pus surrounded by inflamed tissue) · Carbuncles are areas of extensive damage, frequently involving S . aureus = Impetigo of the newborn is often due to S . aureus
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