CHAPTER 21 — Microbial Diseases of the Skin

Microbial Diseases of the Skin

Chapter Overview

  • Title: A Handbook of Clinical Signs in Black and Brown Skin

  • Authors: Mukwende M, Tamony P, Turner M

  • Edition: First Edition

  • Institution: St George's University of London

Skin Defenses

  • Natural Defenses of the Skin:

    • Natural shedding of surface cells provides a physical barrier against pathogens.

    • Salts present on the skin surface inhibit microbial growth.

    • Lysozyme enzyme hydrolyzes peptidoglycan, a component of bacterial cell walls, leading to cell lysis.

    • Fatty acids present in sebum inhibit the growth of certain pathogens.

Normal Microbiota of the Skin

  • Diversity by Region:

    • The normal microbiota varies significantly by region—particularly between dry and moist areas of the skin.

    • Individuals harbor different major organisms on their skin and external mucosa.

    • The composition of the normal microbiota can vary significantly among individuals.

    • (Referenced image source: National Human Genome Research Institute)

Terminology of Skin Lesions

  • Crust: Dried fluids such as sebum, pus, or blood (cells + debris).

  • Cyst: An encapsulated sac filled with fluid, pus, or gas.

  • Macule: Smooth red spots that do not contain fluid.

  • Papule: Small raised bumps on the skin.

  • Pustule: Small lesions filled with pus.

  • Ulcer: An open sore on the skin.

  • Vesicle: A small fluid-filled lesion less than 1 cm in size.

  • Wheal: Swollen, inflamed skin that itches or burns.

Types of Skin Lesions (Visual Reference)

  • Visual descriptions include:

    • Crust

    • Cyst

    • Macule

    • Papule

    • Pustule

    • Ulcer

    • Vesicle

    • Wheal

Staphylococcal Skin Infections

  • Staphylococcus epidermidis:

    • Gram-positive cocci, categorized as coagulase-negative bacteria.

  • Staphylococcus aureus:

    • Gram-positive cocci, categorized as coagulase-positive.

    • Leukocidin: An enzyme that targets and kills white blood cells.

    • Exfoliative toxin: Causes skin peeling.

Types of Infections

  1. Folliculitis: Infection of hair follicles.

  2. Sty: Inflammation of the eyelash follicle.

  3. Furuncle (boil): A painful, pus-filled bump on the skin.

    • Carbuncle: A cluster of boils.

More Staphylococcus aureus Infections

  • Impetigo of the Newborn:

    • Characterized by pustular rash that ruptures and becomes crusted.

  • Scalded Skin Syndrome:

    • Caused by exfoliative toxin leading to skin peeling in sheets.

  • Toxic Shock Syndrome:

    • A serious condition caused by toxin in the bloodstream.

Streptococcal Skin Infections

  • Streptococcus pyogenes:

    • Beta-hemolytic streptococci associated with various infections.

    • Divided into serotypes A-T, with type A being the most pathogenic.

    • 80 different immunological types based on M protein antigenic properties.

    • M proteins help prevent the activation of the complement system, which is part of the immune response.

Types of Infections

  • Erysipelas: Infects the dermal layer of the skin.

  • Impetigo: Can also be caused by Streptococcal infections, characterized by pustules that erupt and crust.

Invasive Group A Strep Infections

  • Pathogenesis Factors:

    • Streptokinases: Enzymes that break down blood clots.

    • Hyaluronidase: Allows rapid spread of bacteria through connective tissue.

    • Exotoxin A: A potent toxin contributing to disease severity.

Forms of Infections

  1. Cellulitis: Attack on solid tissue.

  2. Myositis: Infection of muscle tissue.

  3. Necrotizing Fasciitis: Most severe, involving the tissue covering muscle.

Pseudomonas Infections

  • Pseudomonas aeruginosa:

    • Gram-negative, aerobic rod that can produce pyocyanin, resulting in blue-green pus.

  • Associated Conditions:

    • Pseudomonas dermatitis.

    • Hot tub folliculitis.

    • Otitis externa (ear infection).

    • Post-burn infections.

Acne

  • Types of Acne:

    • Comedonal Acne: Blockage of sebum channels by shed cells.

    • Inflammatory Acne: Caused by Propionibacterium acnes, a Gram-positive, anaerobic rod.

    • Treatment Options: Antibiotics, peroxide, visible blue light therapy.

    • Nodular Cystic Acne: Involves deeper scarring, treated with isotretinoin.

Anthrax

  • Types:

    • Cutaneous (most common), ingestion, and inhalation.

  • Causative Agent: Bacillus anthracis, a Gram-positive endospore-forming facultative anaerobe.

  • Prevention: Animals routinely vaccinated against anthrax.

Conjunctivitis

  • Overview:

    • Caused by various communicable bacterial and viral species.

    • Generally resolves within a few days.

Warts

  • Causative Agent: Human Papilloma Virus (HPV).

  • Characteristics: Can vary in shape and location.

Cold Sores

  • Causative Agent: Herpes Simplex Virus-1 (HSV-1).

    • Characterized by the outbreak of painful sores.

Mycoses - Fungal Diseases

  • Types of Reactions:

    • Allergic reactions to spores.

    • Ingestion of fungal toxins (e.g., from poisonous mushrooms).

  • Types of Fungal Infections:

    • Classified into three categories:

    • Cutaneous Mycoses: Affect the epidermis only.

    • Subcutaneous Mycoses: Affect the epidermis and dermis.

    • Systemic Mycoses: Infect the bloodstream.

Specific Mycoses (Tineas)

  1. Tinea corporis: Ringworm of the body.

  2. Tinea capitis: Ringworm of the scalp.

  3. Tinea pedis: Athlete’s foot.

  4. Tinea barbae: Ringworm of the beard.

  5. Tinea cruris: Jock itch.

  6. Tinea unguium: Nail infection.

Scabies

  • Causative Agent: Sarcoptes scabiei, a mite that burrows into the skin to lay eggs.

  • Incidence: Approximately 500,000 cases per year in the US.

  • Treatment: Topical insecticides.

Pediculosis (Lice)

  • Types:

    • Pediculus humanus capitis: Head louse.

    • P. h. corporis: Body louse.

  • Characteristics:

    • Feed on blood and lay eggs (nits) in hair.

  • Treatment: Topical insecticides, though some strains are rapidly developing resistance.

  • Transmission: Primarily spread through close personal contact and sharing of clothing or personal items.

Multiple choice practice questions 

1._____________ glands produce a lipid-rich substance that contains proteins and minerals and protects the skin.

  1. Sweat

  2. Mammary

  3. Sebaceous

  4. Endocrine

2.Which layer of skin contains living cells, is vascularized, and lies directly above the hypodermis?

  1. the stratum corneum

  2. the dermis

  3. the epidermis

  4. the conjunctiva

3.Staphylococcus aureus is most often associated with being

  1. coagulase-positive.

  2. coagulase-negative.

  3. catalase-negative.

  4. gram-negative

4.M protein is produced by

  1. Pseudomonas aeruginosa

  2. Staphylococcus aureus

  3. Cutibacterium acnes

  4. Streptococcus pyogenes

5.___________ is a major cause of preventable blindness that can be reduced through improved sanitation.

  1. Ophthalmia neonatorum

  2. Keratitis

  3. Trachoma

  4. Cutaneous anthrax

6.Which species is frequently associated with nosocomial infections transmitted via medical devices inserted into the body?

  1. Staphylococcus epidermidis

  2. Streptococcus pyogenes

  3. Proproniobacterium acnes

  4. Bacillus anthracis

7.Warts are caused by

  1. human papillomavirus.

  2. herpes simplex virus.

  3. adenoviruses.

  4. parvovirus B19.

8.Which of these viruses can spread to the eye to cause a form of keratitis?

  1. human papillomavirus

  2. herpes simplex virus 1

  3. parvovirus 19

  4. circoviruses

9.Cold sores are associated with:

  1. human papillomavirus

  2. roseola

  3. herpes simplex viruses

  4. human herpesvirus 6

10.Which disease is usually self-limiting but is most commonly treated with ganciclovir if medical treatment is needed?

  1. roseola

  2. oral herpes

  3. papillomas

  4. viral conjunctivitis

11.Adenoviruses can cause:

  1. viral conjunctivitis

  2. herpetic conjunctivitis

  3. papillomas

  4. oral herpes

12.___________ is a superficial fungal infection found on the head.

  1. Tinea cruris

  2. Tinea capitis

  3. Tinea pedis

  4. Tinea corporis

13.For what purpose would a health-care professional use a Wood’s lamp for a suspected case of ringworm?

  1. to prevent the rash from spreading

  2. to kill the fungus

  3. to visualize the fungus

  4. to examine the fungus microscopically

14.Sabouraud dextrose agar CC is selective for:

  1. all fungi

  2. non-saprophytic fungi

  3. bacteria

  4. viruses

15.The first-line recommended treatment for sporotrichosis is:

  1. itraconazole

  2. clindamycin

  3. amphotericin

  4. nystatin

16.Which of the following is most likely to cause an Acanthamoeba infection?

  1. swimming in a lake while wearing contact lenses

  2. being bitten by deerflies in Central Africa

  3. living environments in a college dormitory with communal showers

  4. participating in a contact sport such as wrestling

17.The parasitic Loa loa worm can cause great pain when it:

  1. moves through the bloodstream

  2. exits through the skin of the foot

  3. travels through the conjunctiva

  4. enters the digestive tract

18.A patient tests positive for Loa loa antibodies. What does this test indicate?

  1. The individual was exposed to Loa loa at some point.

  2. The individual is currently suffering from loiasis.

  3. The individual has never been exposed to Loa loa.

  4. The individual is immunosuppressed.

19.________ is commonly treated with a combination of chlorhexidine and polyhexamethylene biguanide.

  1. Acanthamoeba keratitis

  2. Sporotrichosis

  3. Candidiasis

  4. Loiasis