CHAPTER 21 Microbial Diseases of the Skin and Eyes I II Structure and function of the skin Epidermis thin outer portion composed of several layers of epithelial cells outermost layer containing a waterproofing protein called keratin When unbroken the epidermis is an effective physical barrier against microbes Dermis inner relatively thick portion of the skin composed mainly of connective tissue Hair follicles sweat gland ducts and oil gland ducts provide pathways for entry Perspiration provides moisture and nutrients for microbial growth It contains salt inhibits microbe growth lysozyme breaks down cell walls of certain bacteria and antimicrobial peptides Defensins Sebum secreted by the oil glands Mixture of lipids proteins and salts that prevents the skin and hair from drying out and inhibits the growth of some pathogens a Mucous membranes linings of body cavities consisting of sheets of tightly packed epithelial cells that secrete mucous Some systems have cilia such as the respiratory system functioning to sweep particles upward out of the body Usually acidic limiting microbial populations Folded to maximize surface area Normal microbiota of the skin more likely to be gram positive a On superficial skin surfaces certain aerobic bacteria produce fatty acids from sebum which inhibit many microbes and allow better adapted flora to flourish b Microbes are resistant to drying low pH fatty acids and to relatively high salt concentrations a Normally contains large numbers of gram positive bacteria i Staphylococcus Streptococus low GC count ii Diptheroids coryneforms high GC gram iii Some yeasts c Areas of the body with more moisture such as the armpits and between the legs have higher populations of microbes and are the main contributors of body odor a Diptheroids anaerobic and inhabit the hair follicles Their growth is supported by oil secretions making them a factor in acne b Malasseia furfur yeast capable of growing on oily skin secretions thought to be responsible for dandruff i Shampoos treating dandruff contain the antibiotic ketoconazole or zinc pyrithione or selenium sulfide III Microbial diseases of the skin a Rashes and lesions i Vesicles small fluid filled lesions ii Bullae larger vesicles iii Macules flat reddened lesions iv Papules raised lesions v Pustules papules containing pus vi Exanthema skin rash that arises from disease conditions vii Enanthem exanthema that occurs on mucous membranes b Bacterial diseases of the skin staphylococcus and streptococcus have adapted fairly well to the physiological conditions in the skin i Staphylococcal skin infections 1 Coagulase negative strains 2 Coagulase positive strains a S epidermis is very common to the skin representing 90 of the normal flora They are generally pathogenic only when the skin barrier is broken or is invaded by medical procedures such as the insertion and removal of catheters They produce a slime layer that protects the bacteria from disinfectants and desiccation a S aureus low GC most pathogenic of the staphylococci and is a permanent resident in the nasal passages of 20 of the population S aureus has 300 000 more base pairs in its genome allowing it to have an impressive array of virulence factors and means of evading host defenses High correlation with the bacterium s ability to form coagulase and its production of damaging toxins Once it infects the skin it secretes a protein that blocks chemotaxis of neutrophils to the infection site Toxemias result from toxins entering the blood It is also resistant to opsonization and phagocytosis in addition to containing a lysozyme resistant cell wall Folliculitis occurs as pimples when the organism enters through hair follicles i Sty infected follicle of an eyelash ii Furuncle boil type of abscess a localized region of pus surrounded by inflamed tissue More difficult to treat because antibiotics don t penetrate abscesses Draining the pus is the first step iii Carbuncle when body fails to wall off a furuncle neighboring tissue can be invaded Causes formation of a hard round deep inflammation of tissue under the skin Patient exhibits symptoms of generalized illness with fever Impetigo highly contagious skin infection affecting children 2 5 years spread by direct contact 1 Nonbullous impetigo pathogen enters through iv some minor break in the skin Lesions eventually rupture and form light colored crusts Topical antibiotics are applied but lesions can heal without treatment and scarring 2 Bollous impetigo caused by a staphylococcal toxin Toxin A remains localized causes bullous impetigo and toxin B circulates to distant areas causing scalded skin syndrome a Pemphigus neonatorum impetigo of the infant common in hospital nurseries v Toxemias when toxins enter bloodstream 1 Toxic shock syndrome scalded skin syndrome is a characteristic of the late stages of TSS Condition includes life threatening fever vomiting and a sunburnlike rash followed by shock and organ failure This occurs because a novel staphylococcal toxin TSST 1 is formed at the growth site and circulates in the bloodstream when a vaginal tampon stays in place too long ii Streptococcal skin infections grow in chains gram low GC 1 Strep case a wide range of disease conditions including meningitis pneumonia sore throats otitis media endocarditis puerperal fever and dental caries As they grow they secrete toxins including hemolysins which lyse red blood cells Depending on the type they produce they are categorized as alpha beta or gama hemolytic a hemolytic streptococci associated with human disease i Group A GAS S pyogenes most important and are responsible for a number of human diseases The M protein of this group prevents the activation of complement and allows the microbe to evade phagocytosis and killing by neutrophils It also helps the bacteria adhere to and colonize mucous membranes Virulent strains have a mucoid appearance on blood agar plates from heavy encapsulation Produce substances that promote the rapid spread of infection through tissue and by liquefying pus Streptokinases hyaluronidase deoxyribonucleases and streptolysins are some of these substances Exotoxin A produced by the M protein acts as a super antigen which causes the immune system to contribute to the damage Broad spectrum antibiotics are usually prescribed because of the possibility that multiple pathogens are present Impetigo isolated postulates 1 2 Erysipelas when S pyogenes infects the dermal layer of the skin causing the skin to erupt
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