Lecture #176: Microbiology: Skin and Mucus Membrane Infections

What are the predominant normal flora organisms of the skin?

Gram-positive staphylococci and streptococci predominate on normal skin flora.

What percentage of people carry Staphylococcus aureus on skin?

Approximately 30–60% of people carry S. aureus.

Which Staphylococcus species colonizes nearly all skin?

Staphylococcus epidermidis colonizes nearly 100% of skin.

What fungi are part of normal skin flora?

Candida albicans and dermatophytes.

What are the basic characteristics of staphylococci?

Gram-positive, catalase-positive, non-motile cocci that are facultative anaerobes.

What enzyme makes staphylococci catalase positive?

Catalase converts hydrogen peroxide into water and oxygen.

What culture conditions favor staphylococcal growth?

Blood agar, high NaCl concentrations, and temperatures between 18–40°C.

What is MRSA?

Methicillin-resistant Staphylococcus aureus.

What are the key identifying features of Staphylococcus aureus?

Coagulase-positive and beta-hemolytic on blood agar.

What does coagulase do in S. aureus infections?

Converts fibrinogen to fibrin creating a fibrin shield that inhibits phagocytosis.

How does the S. aureus capsule increase virulence?

It inhibits opsonization, phagocytosis, and complement-mediated leukocyte destruction.

What is the function of Protein A in S. aureus?

Binds Fc portions of IgG to inhibit opsonization and phagocytosis.

How does catalase benefit S. aureus?

Prevents oxidative killing by immune cells.

What enzymes contribute to S. aureus spread?

Hyaluronidase, fibrinolysin, lipase, and nuclease.

What does penicillinase do?

Opens the beta-lactam ring and inactivates penicillin.

What toxins are produced by S. aureus?

Cytolytic toxins, exfoliative toxins, TSST, and enterotoxins.

What is a furuncle?

An infected hair follicle commonly called a boil.

What is a carbuncle?

A coalesced cluster of furuncles.

What causes staphylococcal impetigo?

Toxin-mediated keratin separation causing flaccid blisters after skin injury.

What is staphylococcal scalded skin syndrome?

A toxin-mediated disease in children causing blistering and peeling of squamous epithelium.

What is the Nikolsky sign?

Horizontal pressure causing shearing off of superficial epidermis adjacent to lesions.

What severe systemic infections can S. aureus cause?

Sepsis, endocarditis, pneumonia, osteomyelitis, empyema, and septic arthritis.

How does toxic shock syndrome occur in S. aureus infection?

Superantigens trigger massive T-cell activation, cytokine release, capillary leak, and shock.

What are classic findings in staphylococcal toxic shock syndrome?

Strawberry tongue and diffuse erythematous rash.

What are the characteristics of Staphylococcus epidermidis?

Catalase-positive, coagulase-negative, novobiocin-sensitive, and non-hemolytic.

Why is S. epidermidis clinically important?

It forms slime layers that adhere to prosthetic devices and catheters.

What infection is associated with Staphylococcus saprophyticus?

UTIs in sexually active women.

What are the basic characteristics of streptococci?

Gram-positive, catalase-negative cocci arranged in pairs or chains.

What type of hemolysis is caused by Streptococcus pyogenes?

Beta hemolysis with complete clearing on blood agar.

What is alpha hemolysis?

Incomplete hemolysis causing green discoloration on blood agar.

What is gamma hemolysis?

No hemolysis on blood agar.

What is the role of the streptococcal capsule?

Antiphagocytic protection.

What is the function of M protein in GAS?

Antiphagocytic virulence factor and superantigen.

What toxins are produced by streptococci?

Pyrogenic exotoxins including erythrogenic toxin.

What causes the rash of scarlet fever?

Erythrogenic toxin encoded by a lysogenic bacteriophage.

What enzyme causes beta hemolysis in streptococci?

Streptolysin O.

What diagnostic tests are used for GAS pharyngitis?

Rapid antigen test, throat culture, and ASO antibodies.

What is first-line treatment for GAS pharyngitis?

Penicillin.

What does streptococcal impetigo look like?

Tense bullae with gold crusts.

What are classic findings in scarlet fever?

High fever, sore throat, and sandpaper rash.

What is erysipelas?

A rapidly progressive superficial skin infection with sharply demarcated raised borders.

What is cellulitis?

A rapidly spreading painful infection of skin and subcutaneous tissue.

What is necrotizing fasciitis?

A rapidly advancing, intensely painful soft tissue infection with bullae, crepitus, and sepsis.

Why is necrotizing fasciitis a surgical emergency?

It requires emergent debridement to prevent septic shock and death.

What post-streptococcal diseases can occur after GAS infection?

Rheumatic fever and post-streptococcal glomerulonephritis.

What infections are caused by Streptococcus agalactiae?

Neonatal sepsis, meningitis, and postpartum sepsis.

What diseases are associated with Streptococcus pneumoniae?

Pneumonia, sinusitis, otitis media, meningitis, and sepsis.

What are viridans streptococci associated with?

Endocarditis and dental infections.

What are the key features of Pseudomonas aeruginosa?

Gram-negative, motile, oxidase-positive, non-lactose fermenting bacillus.

Where is Pseudomonas commonly found?

Moist environments.

What toxin in Pseudomonas acts as an endotoxin?

Lipopolysaccharide (LPS).

What does Exotoxin A of Pseudomonas do?

Inhibits protein synthesis.

What infections are commonly caused by Pseudomonas aeruginosa?

Burn infections, surgical wound infections, cystic fibrosis infections, and ecthyma gangrenosum.

What organism is a major cause of acne in young adults?

Propionibacterium acnes.

How does Propionibacterium acnes contribute to acne?

Combines with sebum to stimulate inflammatory tissue damage.

What are the characteristics of Actinomyces israelii?

Gram-positive elongated rods that form filamentous structures.

What conditions are associated with Actinomyces israelii?

Lumpy jaw, granulomatous sinusitis, pelvic abscesses, and sulfur granules.

What organism causes Buruli ulcer?

Mycobacterium ulcerans.

What are features of Mycobacterium ulcerans infection?

Acid-fast bacillus causing large erythematous ulcers in tropical aquatic regions.

What virus causes fifth disease?

Parvovirus B19.

What are the characteristics of Parvovirus B19?

Non-enveloped, icosahedral, single-stranded linear DNA virus.

What is the classic rash of fifth disease?

Slapped-cheek rash.