Infectious Diseases: Skin & Eyes (Ch. 19)

The skin is inhospitable due to

Sebum (oil), high salt from sweat, dry keratinized cells.

Normal skin microbiota characteristics

Resistant to dry conditions + high salt; more abundant in moist regions.

Most common skin microbiota genera

Staphylococcus, Propionibacterium (Cutibacterium).

Transient microbiota

Temporary microbes acquired from environment.

Catalase test — Staph vs Strep

Staph = catalase+, Strep = catalase-.

S. epidermidis colony characteristics

Small, white.

S. epidermidis features

Coagulase-; does not ferment mannitol.

S. aureus colony characteristics

Golden yellow.

S. aureus features

Coagulase+; ferments mannitol.

Major virulence enzyme of S. aureus

Coagulase → clots fibrin; helps evade phagocytosis.

MRSA

Methicillin-resistant S. aureus.

How to differentiate MRSA from non-MRSA?

Kirby-Bauer → MRSA is resistant; RT-PCR detects mecA gene.

MSA selective for

Staphylococcus (salt-tolerant).

MSA differential for

Mannitol fermentation.

Fermentation on MSA →

Yellow (acid produced).

S. aureus on MSA

Yellow (positive).

S. epidermidis on MSA

Pink/red (negative).

Where does Propionibacterium live?

In hair follicles; feed on sebum.

Metabolic byproduct

Propionic acid.

Role in microbial antagonism

Acid lowers pH → inhibits pathogens.

Associated condition of Propionibacterium

Acne.

Folliculitis

Infection of hair follicles (commonly S. aureus).

Furuncle

Deeper infection; boil.

Carbuncle

Cluster of boils; more severe.

Impetigo

Highly contagious skin infection; S. aureus or S. pyogenes.

Cellulitis

Infection of deeper dermis/subcutaneous tissue.

Pathogen

S. aureus.

Toxin

Exfoliative toxin.

Toxin results in

Skin peeling (looks like burn).

Catalase test

Catalase-

Virulence enzyme

Hyaluronidase.

Skin diseases

Impetigo, cellulitis, erysipelas, necrotizing fasciitis.

Causative agent

GAS (S. pyogenes).

Key virulence factor

Exotoxins → rapid tissue destruction.

Pseudomonas Aeruginosa associated with

Burn victims.

Colony color of Pseudomonas Aeruginosa

Blue-green pigment (pyocyanin).

Main microbial cause

Cutibacterium acnes.

Pathogenesis

Sebum accumulation → bacterial growth → inflammation.

Smallpox (Variola)

• Eradicated; poxvirus. • Serious; lesions → scars.

Chickenpox (Varicella)

• Causes fever + rash; latent → shingles.

Shingles (Herpes Zoster)

• Reactivation of varicella-zoster; painful.

Measles (Rubeola)

• Highly contagious; Koplik spots; rash; can cause encephalitis.

Dermatophytes

• Ringworm (tinea) • Keratin-loving fungi • Scalp, foot, body

Type of dermatophyte Tinea pedis

athlete's foot

Type of dermatophyte Tinea corporis

body

Type of dermatophyte Tinea capitis

scalp

Why are eyes prone to infection?

Exposed surface; moist environment.

Protective factors of the eyes

Tears (lysozyme); blinking.

Conjunctivitis definition

Inflammation of conjunctiva.

Symptoms of conjunctivitis

Redness, discharge.

Viral conjunctivitis

Watery discharge.

Bacterial conjunctivitis

Thick/purulent discharge.

Causes of NEONATAL CONJUNCTIVITIS

Neisseria gonorrhoeae, Chlamydia trachomatis.

Prevention of NEONATAL CONJUNCTIVITIS

Prophylactic antibiotic eyedrops at birth.

Keratitis

Corneal infection; can cause blindness.

Common cause of keratitis

HSV (herpes).