skin functions, microbiota and infections

Physiology of the skin

• mechanical barrier to heat, injury UV light

• Regulation of body temperature

• Retention/ excretion of water

• Sensory detection

• Synthesis of key molecule

Structure of the skin

• epidermis

• Dermis

Epidermis

• varied thickness

• Composed mainly of keratinocytes

Dermis

• fibroblasts producing extracellular matrix

• Elastin and collagen fibres

• Blood and lymphatic vessels

• Hair follicles and sebaceous or sweat glands

Structure of the epidermis

• stratus corneum

• Stratum lucidum

• Stratum granulosum

• Stratum spinosum

• Stratum basal

Main cells of the epidermis

• keratinocytes

• Melanocytes

• Merkel cells

• Lamngerhans cells

Dermis structure

• hair

• Sebaceous glands

• Sweat glands

• Blood and lymphatic vessels

• Nervous termination

Hair

• made of keratin

• Consisting of the hair follicle, root, and shaft

• Provides protection, insulation and sensory

Sebaceous glands

• associated to hair follicles

• Secrete sebum

Sweat glands

• produces sweat

• Thermoregulation and waste excretion

Skin microbiota

• despite being a harsh environment, stratum corneum support diverse skin microbiota categorised into micro environments:

  • Oily

  • Moist

  • Dry

  • Foot

Skin resident species

• permanent inhabitants of the skin

• Crucial roles in maintaing skin health

• Establish stable population over time

• Unlikely to. Cause opportunistic infections

Skin transient species

• short term colonisers

• Their contribution is not key

• Can be easily removed by routine hygiene

• Can cause opportunistic infections

Bacteria

• may cause acne

• Responsible for body odour

Yeasts

• Harmless in healthy individuals

• Causing dandruff or interfere with pigment

Viruses

• unclear contribution

Skin as a barrier for infection

• physical barrier of corneocytes in the corneum stratus

• Low pH

• Low moisture

• Low levels of nutrients

• defence mechanism

• Constant exfoliation of cells

• Pathogenic colonisation prevent s

Defence mechanisms

• antimicrobial peptides release

  • Produced by sweat and sebaceous glands

  • Directly kill or inhibit the microbial growth

• Lysozyme release

  • Breaking it down the peptidoglycan of bacterial cell wall

Mechanism of skin opportunistic infections

• cuts, lesions, changes in barrier permeability

• Microbiota altercation

• Immunosuppression

S. Aureus virulence factors

• proteolytic enzymes to invade tissue

• Factors to evade host defences

• Cause host damage by producing different exotoxins

Distinct strep A strains with different virulence factors

• proteolytic enzyme to invade tissue

• Factors to evade host defence

• Cause host damage by producing different exotoxins

Impetigo

• affect superficial layer of the skin

• Rash not painful, but may be itchy

• Typcial in children

• Highly contagious

Non bullous impetigo

• thin walled vesicles or pustules that rupture quickly

• Leaking pus forming brownish/ honey crust

Bullous impetigo

• larger and fluid filled blisters

• On the trunk, buttocks, extremities

• Their rupture leaves a flat, yellow/ varnish crust