CHAPTER 19 STUDY GUIDE

The skin acts as a physical barrier because its outer layers (stratum corneum) consist of dead cells that do not support microbial growth. The salt from sweat creates a hostile environment for many microbes by drawing out water through osmosis. Together, dead skin cells and salt create a protective barrier that prevents microbial invasion. While blood vessels help with immune responses, they do not directly act as a protective layer against microbes.

Normal skin microbiota are adapted to survive harsh conditions like high salt concentrations from sweat and sebum, an oily secretion that provides nutrients. These microbiota do not get washed away easily because sebum helps trap bacteria while salt inhibits the growth of many harmful microbes. Together, sebum and salt create an environment where only certain bacteria can thrive, contributing to microbial homeostasis on the skin.

A sty (hordeolum) is an infection of the sebaceous gland at the base of an eyelash or hair follicle on the eyelid. It is commonly caused by Staphylococcus aureus, which can block and inflame the gland, leading to a painful swelling. Furuncles and carbuncles are similar but occur elsewhere on the body rather than the eyelid. A pimple is a milder form of follicular inflammation, often linked to acne.

Diphtheroids are pleomorphic (having variable shapes) bacteria that are part of the normal skin microbiota. They include species like Corynebacterium and Propionibacterium, which play a role in skin health and acne formation. Staphylococci and Streptococci are also present on the skin, but they are not pleomorphic. Clostridia are anaerobic bacteria more commonly found in the gut and soil, rather than on the skin.

Virulent strains of Staphylococcus aureus produce coagulase, an enzyme that clots fibrin to form a protective barrier around bacterial colonies. This protects the bacteria from the immune system and antibiotics, including penicillin. Staphylokinase helps break down clots, lipase aids in fat metabolism, and a slime layer contributes to biofilm formation, but none of these directly resist penicillin.

Erysipelas is a severe skin infection caused by Streptococcus pyogenes, leading to red, swollen, and painful lesions, typically on the face or legs. It occurs when bacteria invade deeper skin layers, causing fever and systemic symptoms. Carbuncles and sties are related to Staphylococcus infections, while shingles is caused by varicella-zoster virus, not bacteria.

Impetigo is a highly contagious skin infection that can be caused by both Staphylococcus aureus and Streptococcus pyogenes. It results in red sores, blisters, and a honey-colored crust, often on the face of children. Staphylococcus epidermidis is a normal skin bacterium but not a common cause of impetigo. The infection spreads through direct contact, requiring prompt treatment with antibiotics.