BIOL 251 Chapter 21 Study Points

Immune defense mechanisms (skin)

• Physical barrier: Unbroken skin effectively blocks pathogen entry.

• Chemical defenses: Salt, a slightly acidic environment, dryness.

• Biological defenses: Dead skin cells shed regularly, making colonization hard; normal microbiota outcompete pathogens.

• SALT (Skin-Associated Lymphoid Tissue): Immune cells present to detect invaders.

Pathogens that cause skin infections

• Bacteria: Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus spp., Rickettsia, Bacillus anthracis

• Viruses: Varicella zoster virus (chickenpox, shingles), HSV, HPV, parvovirus B19

• Fungi: Candida albicans, dermatophytes (cause tineas/ringworm)

Factors that establish infection

• Skin barrier breaches: cuts, burns, insect bites, etc.

• Immune status of host

• Pathogen virulence

• Environmental exposure (e.g. hospital setting)

Characteristic of Staphylococcus aureus

• Gram-positive cocci in clusters

• Salt-tolerant, survives on skin and in nasal passages

• Spread easily via contact

• Often causes pus, inflammation, fever

• Can lead to toxic shock syndrome, scalded skin syndrome

Staphylococcus aureus

• Gram stain: Gram-positive cocci

• Catalase test: Positive (helps differentiate from Streptococcus)

• Mannitol salt agar: Ferments mannitol (turns media yellow)

• Coagulase test: Positive

• Antibiotic sensitivity testing (e.g., Kirby-Bauer)

Virulence factors of Staphylococcus aureus

• Capsule: Blocks phagocytosis

• Coagulase: Forms clots, slowing immune response

• Hyaluronidase, proteases, lipases: Break down host tissues

• Alpha-toxin, leukocidins, hemolysins: Kill cells

• Protein A: Binds antibodies inappropriately to avoid immune detection

MRSA

• Methicillin-Resistant Staphylococcus aureus

• Two types: HA-MRSA (hospital-acquired) & CA-MRSA (community-acquired)

• Resistant due to mecA gene

MecA gene

• Codes for PBP2a, a penicillin-binding protein

• PBP2a has low affinity for beta-lactam antibiotics (e.g., penicillin), making them ineffective

• Transferred via bacteriophage transduction

Staphylococcus aureus infections

• Hair follicle infections, wound infections

• Scalded skin syndrome (via exfoliatin toxin)

• Impetigo, cellulitis, erysipelas, necrotizing fasciitis

Staphylococcus or Streptococcus infections

• Impetigo

• Erysipelas (more often S. pyogenes)

• Cellulitis

• Necrotizing fasciitis (also Clostridium, E. coli, Aeromonas)

Characteristics of Pseudomonas aeruginosa

• Gram-negative rod, aerobic, has polar flagellum

• Forms biofilms, produces pigments (e.g., pyocyanin)

• Common in soil, water, hospital environments

• Highly opportunistic and resistant to many antibiotics

Pseudomonas infections

• Wound infections, especially in burn patients

• Lung infections, skin rashes, eye, ear, and foot infections

C. perfringens

Anaerobic, causes gas gangrene via alpha-toxin, collagenase, hyaluronidase

C. tetani

Produces tetanospasmin toxin (causes spastic paralysis/lockjaw), vaccine-preventable

Infections by Viral pathogens

• Varicella zoster: Chickenpox, shingles

• HSV-1: Cold sores

• HPV: Warts

• Roseola (HHV-6), Fifth disease (Parvovirus B19): Mild rashes

Fungal pathogens

• Dermatophytes (cause ringworm, tineas)

• Candida albicans: Causes cutaneous candidiasis

Varicella virus

• Enveloped dsDNA virus, part of Herpesviridae

• Chickenpox: Respiratory spread, lesions on skin

• Shingles: Reactivation of latent VZV in sensory ganglia

• Vaccine available; can be severe in adults and immunocompromised