Microbial Diseases of the Skin and Eyes

Microbiology: An Introduction - Chapter 21: Microbial Diseases of the Skin and Eyes

Structure and Function of the Skin

  • Epidermis:

    • Thin outer portion of skin composed of layers of epithelial cells.

  • Keratin:

    • A waterproofing protein coating the outer layer of epidermis.

  • Dermis:

    • Inner, thick portion of skin, composed mainly of connective tissue.

Key Functions of Skin

  • Perspiration:

    • Provides moisture and nutrients for microbial growth.

    • Contains salt that inhibits the growth of microorganisms.

  • Lysozyme:

    • An enzyme that breaks down bacterial cell walls.

  • Antimicrobial Peptides:

    • Contribute to skin’s defense against infection.

  • Sebum:

    • Secreted by oil glands; contains fatty acids that inhibit pathogens.

Mucous Membranes

  • Location:

    • Line body cavities open to the exterior.

  • Structure:

    • Tightly packed epithelial cells attached to the basement membrane (extracellular material).

    • Some cells secrete mucus; some possess cilia.

    • Often acidic to inhibit pathogen growth.

  • Eye Membrane:

    • Washed by tears containing lysozyme.

  • Viral Attachments:

    • ACE2 protein for SARS-CoV-2, and sialic acid for influenza viruses.

Normal Microbiota of the Skin

  • Resilience:

    • Microbiota are resistant to drying and high salt concentrations.

  • Composition:

    • Large numbers of gram-positive cocci, primarily Staphylococci and Micrococci.

    • Vigorous washing can reduce, but not eliminate these microbes.

    • Higher populations in moist areas that metabolize sweat, contributing to body odor.

  • Variety:

    • Includes gram-positive pleomorphic rods (diphtheroids) like Cutibacterium acnes and Corynebacterium xerosis.

    • Yeast: Malassezia furfur, associated with dandruff.

Microbial Diseases of the Skin

  • Types of Skin Lesions:

    • Vesicles: Small, fluid-filled lesions.

    • Bullae: Vesicles larger than 1 cm in diameter.

    • Macules: Flat, reddened lesions.

    • Papules: Raised lesions.

    • Pustules: Raised lesions containing pus.

    • Exanthem: Skin rash due to disease.

    • Enanthem: Rash on mucous membranes due to disease.

Staphylococcal Skin Infections

  • General Description:

    • Staphylococci: Spherical, gram-positive bacteria arranged in irregular clusters.

    • Two main groups based on coagulase production:

    • Coagulase-positive: Clots fibrin in blood, includes pathogenic strains.

    • Coagulase-negative: Generally non-pathogenic.

  • Staphylococcus epidermidis:

    • Accounts for 90% of normal skin microbiota and is a healthcare-associated pathogen.

    • Produces biofilm on catheters.

  • Staphylococcus aureus:

    • Carried by 20% of the population in the nasal passages.

    • Recognized by golden-yellow colonies; it is coagulase-positive.

    • May produce toxins causing various diseases including sepsis.

    • Avoids host defenses by secreting proteins and toxins that kill phagocytes.

    • MRSA strains are resistant to many antibiotics.

Types of Staphylococcal Skin Infections

  • Folliculitis: Infections of hair follicles.

  • Sty: Folliculitis of an eyelash.

  • Furuncle (Boil): Abscess containing pus, surrounded by inflamed tissue.

  • Carbuncle: Severe inflammation and damage in deep tissue from a spreading furuncle.

  • Impetigo: Crusting sores that can spread via autoinoculation; may also be caused by Streptococcus pyogenes.

  • Scalded Skin Syndrome:

    • Bullous impetigo due to exfoliative toxins A and B.

    • Pemphigus neonatorum: impetigo in newborns.

  • Toxic Shock Syndrome (TSS):

    • Caused by toxic shock syndrome toxin 1 (TSST-1).

    • Symptoms include fever, vomiting, shock, and organ failure; associated with tampon use and nasal surgery.

Streptococcal Skin Infections

  • General Description:

    • Gram-positive cocci organized in chains, associated with numerous infections.

    • Produce hemolysins that lyse red blood cells.

    • Divided into groups A through T based on antigenic cell wall carbohydrates.

  • Group A Streptococci (GAS):

    • Identified as Streptococcus pyogenes, produces various virulence factors including streptolysins and M proteins that enhance evasion of the immune system.

  • Virulence Factors:

    • Streptolysins: Lyse red blood cells.

    • M Proteins: Aid in adhesion to mucous membranes and evasiveness against phagocytes.

    • Hyaluronidase: Dissolves connective tissue aiding infection spread.

    • Streptokinases: Dissolve blood clots, facilitating further spread.

Major Streptococcal Diseases

  • Erysipelas:

    • Infection of the dermal layer, causing tissue destruction and potential sepsis.

  • Necrotizing Fasciitis:

    • A severe disease characterized as “flesh-eating,” propelled by pyrogenic toxins acting as superantigens.

  • Streptococcal Toxic Shock Syndrome:

    • Similar effects to staphylococcal TSS, but with greater lethality.

Infections by Pseudomonads

  • Pseudomonas aeruginosa:

    • A gram-negative, aerobic rod known for its resistance to antibiotics.

    • Produces blue-green pus due to the pigment pyocyanin.

    • Capable of forming biofilms and producing exo- and endotoxins.

    • Associated with conditions like Pseudomonas dermatitis (self-limiting rash from hot tubs) and "swimmer's ear" (otitis externa).

  • Opportunistics:

    • Particularly dangerous for burn patients and those with cystic fibrosis.

Buruli Ulcer

  • Causative Agent:

    • Mycobacterium ulcerans produces the toxin mycolactone, leading to deep, damaging ulcers.

  • Transmission:

    • Enters through breaks in the skin or insect bites; prevalent in Western and Central Africa.

  • Treatment:

    • Antimycobacterial drugs, serious cases may require amputation.

Acne

  • Overview:

    • The most common skin disease in humans, categorized into three types:

    • Comedonal Acne: Non-inflammatory, mild cases treated with topical formulations.

    • Inflammatory Acne: Caused by Cutibacterium acnes, responds to antibiotics.

    • Nodular Cystic Acne: Severe form with pus-filled lesions and potential scarring; treated with isotretinoin (Accutane®) but may be teratogenic.

Viral Diseases of the Skin

  • Transmission:

    • Many are transmitted through respiratory routes, often affecting children and fetuses.

Warts

  • Papillomas:

    • Small skin growths caused by papillomavirus (over 50 types); some related to cancer.

  • Treatment:

    • Includes cryotherapy, electrodesiccation, salicylic acid, or prescription medications such as podofilox and imiquimod.

Smallpox (Variola)

  • Causative Agent:

    • An orthopoxvirus with two forms: variola major (20-60% mortality) and variola minor (<1% mortality).

  • Transmission:

    • Respiratory route leading to widespread infection of internal organs and skin.

  • Control:

    • Completely eradicated by vaccination; considered a potential bioterrorism threat.

MPOX

  • Monkeypox:

    • Related to smallpox but less severe; endemic to small animals in Africa.

    • Spill-over events to humans can occur; 2022 outbreak in the U.S. with over 30,000 cases.

  • Prevention:

    • Smallpox vaccination provides protection.

Chickenpox (Varicella) and Shingles (Herpes Zoster)

  • Chickenpox (Varicella):

    • Caused by herpesvirus varicella-zoster; transmitted via respiratory route leading to vesicular rash after 10-14 days.

  • Reye's Syndrome:

    • Severe complication associated with chickenpox, characterized by vomiting and brain dysfunction; aspirin use increases the risk.

  • Treatment and Prevention:

    • Prevented by a live attenuated vaccine; breakthrough infections can occur.

  • Shingles (Herpes Zoster):

    • Reactivation of the latent chickenpox virus, causing painful lesions usually on one side of the body.

    • Predominantly affects older adults and can lead to postherpetic neuralgia (long-lasting pain).

    • Treatment:

    • Acyclovir or related antivirals may alleviate symptoms; prevention through Shingrix® vaccine.

Herpes Simplex

  • Types:

    • HSV-1 (primarily oral transmission leading to cold sores) and HSV-2 (primarily sexual transmission causing genital herpes).

    • Approximately 90% of U.S. population has HSV-1.

  • Latency:

    • HSV-1 remains latent in trigeminal nerve ganglia; HSV-2 in sacral nerves.

    • Triggers for HSV outbreaks include stress and hormonal changes.

Measles (Rubeola)

  • Transmission:

    • Via respiratory route, with infected individuals shedding virus days before symptoms appear.

  • Symptoms:

    • Include cold-like symptoms, macular rash, and Koplik’s spots (red spots in the mouth).

  • Complications:

    • Risk of encephalitis, occurring in 1 in 1000 cases, and subacute sclerosing panencephalitis, a rare late complication.

  • Prevention:

    • Vaccination with the MMR (measles, mumps, rubella) vaccine, with restrictions for children under 1.

Rubella

  • Description:

    • Also known as German measles, caused by rubella virus; milder compared to measles with macular rash and low fever.

  • Transmission and Complications:

    • Can cause severe fetal damage including deafness, heart defects in 35% of congenital rubella syndrome cases; 15% mortality rate within the first year of life.

  • Prevention:

    • Part of the MMR vaccination schedule, not recommended for pregnant women.

Other Viral Rashes

  • Fifth Disease:

    • Caused by human parvovirus B19, featured by mild flu-like symptoms and a “slapped-cheek” facial rash.

  • Roseola:

    • Caused by human herpesvirus 6 and 7, presents a high fever followed by a rash, recovery typically within 1-2 days.

  • Hand-Foot-Mouth Disease:

    • Linked to enteroviruses like Coxsackievirus A16; spreads through saliva and causes fever, sore throat, and rash.

Fungal Diseases of the Skin and Nails

  • Mycosis:

    • A general term for fungal infections, including cutaneous mycoses which colonize hair, nails, and outer epidermis, metabolizing keratin.

  • Dermatomycoses:

    • Often referred to as tinea or ringworm, with specific types:

    • Tinea Capitis: Scalp ringworm.

    • Tinea Cruris: Jock itch.

    • Tinea Pedis: Athlete's foot.

    • Tinea Unguium: Ringworm of the nails.

  • Treatment:

    • Typically involves topical antifungal medications such as miconazole and clotrimazole.

Subcutaneous Mycoses

  • Characteristics:

    • More serious than cutaneous mycoses and penetrate the stratum corneum.

  • Example:

    • Sporotrichosis caused by Sporothrix schenckii, often presenting as a small ulcer after entering through a wound; treated with itraconazole or potassium iodide.

Candidiasis

  • Cause:

    • Overgrowth of Candida albicans, producing pseudohyphae, making it resistant to phagocytosis.

  • Locations:

    • Affects skin and mucous membranes; oral infections are known as thrush.

  • Trigger:

    • Antibiotic use may suppress competing bacteria, or shifts in mucosal pH may enhance infection.

  • Severity:

    • Can be a significant concern in immunosuppressed individuals.

Scabies

  • Causative Agent:

    • Sarcoptes scabiei mites burrow under the skin to lay eggs, causing inflammatory lesions and itching.

  • Transmission:

    • Generally via intimate contact; treated with permethrin.

Pediculosis (Lice)

  • Types:

    • Pediculus humanus capitis (head) and P. h. corporis (body).

  • Life Cycle:

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

  • Treatment:

    • Involves topical insecticides such as permethrin or pyrethrin; may require stronger medications for resistant cases.

Inflammation of the Eye Membranes: Conjunctivitis

  • Definition:

    • Inflammation of the conjunctiva, often referred to as pink eye.

  • Common Causes:

    • Haemophilus influenzae and adenoviruses; linked to unsanitary contact lens use.

Specific Eye Infections

  • Ophthalmia Neonatorum:

    • Caused by Neisseria gonorrhoeae, leading to significant pus and potential blindness if untreated.

    • Prevented by antibiotic treatment for newborns.

  • Inclusion Conjunctivitis:

    • Resulting from Chlamydia trachomatis; treated with tetracycline; can spread through swimming pool water.

  • Trachoma:

    • Leading cause of blindness worldwide, caused by certain serotypes of Chlamydia trachomatis. Infection can lead to scarring and secondary infections; oral azithromycin for treatment.

Other Infectious Diseases of the Eye

  • Keratitis:

    • Inflammation of the cornea; bacterial in the U.S. and caused by Fusarium and Aspergillus in Africa and Asia.

  • Herpetic Keratitis:

    • Caused by HSV-1, can lead to blindness; treated with trifluridine.

  • Acanthamoeba Keratitis:

    • Caused by ameba from water/soil; severe pain and inflammation, treatment may involve chlorhexidine drops or corneal transplant.

Diseases in Focus: Bacterial Diseases

  • List of Bacterial Diseases with Pathogen, Portal of Entry, Signs and Symptoms, Transmission, and Treatment:

    • Conjunctivitis: Haemophilus influenzae, redness, itchiness, through direct contact.

    • Ophthalmia Neonatorum: Neisseria gonorrhoeae, pus-filled infection, via birth canal, treated prophylactically.

    • Inclusion Conjunctivitis: Chlamydia trachomatis, swelling and discharge, through birth canal and swimming pools, treated with tetracycline.

    • Trachoma: C. trachomatis, conjunctivitis symptoms, direct contact and fly vector, treated with azithromycin.

Diseases in Focus: Viral and Protozoan Eyes Diseases

  • List of Viral Diseases:

    • Conjunctivitis: Adenoviruses, direct contact causes redness, with no specific treatment.

    • Herpetic Keratitis: Herpes simplex virus, keratitis, direct contact; treated with trifluridine.

  • Protozoan Disease:

    • Acanthamoeba Keratitis: Acanthamoeba spp., keratitis from contact with water, treated with chlorhexidine and may require surgical intervention.

Neglected Tropical Diseases (NTDs)

  • Overview:

    • Affect 1 billion people yearly with approximately half a million deaths, disproportionately impacting impoverished populations; lead to various health issues including blindness and neurological damage.

Management Strategies for NTDs

  • Protozoan Diseases:

    • Control of vectors (e.g., tsetse fly for African trypanosomiasis).

  • Helminthic Infections:

    • Strategies like improved sanitation and veterinary public health.

  • Bacterial and Viral Infections:

    • Control of vectors and preventive chemotherapy.

Concluding Notes on NTDs and Strategies for 2030

  • WHO targets set for significant NTD reduction, advocating improved sanitation, vector control, and intensified disease management.

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