Microbiology of the Skin and Eye

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• The skin acts as a physical barrier, a cutaneous membrane of closely packed cells, making it hard for substances to move in and out.

• The skin has normal flora, producing acidic secretions and shedding its top layer, which is constantly replaced.

• Sebum from oil glands and perspiration cleanse the skin and produce lysozyme, an enzyme that breaks down bacterial cell walls.

• Mucous membranes, including those covering the eyes, act as physical and chemical barriers.

• The normal microbiota, or normal flora, primarily consists of gram-positive cocci and bacilli.

• Examples of normal microbiota include Staphylococcus aureus in the nose, Staphylococcus epidermidis on the skin, and Micrococcus species.

• Staphylococcus aureus can cause folliculitis, boils, abscesses, and carbuncles, as well as infantigo, scalding skin syndrome, and toxic shock syndrome.

• Staphylococcus aureus infections can be mild to severe, and the bacteria can produce invasive factors, such as coagulase.

• Streptococcus pyogenes is a group A strep that causes infantigo, erysipelas, and necrotizing fasciitis.

• Streptococcus pyogenes is sensitive to bacitracin and produces hemolysin, which breaks down red blood cells.

• Pseudomonas aeruginosa is an opportunistic infection that can cause otitis externa (swimmer's ear) and is resistant to many antibiotics.

• Pseudomonas aeruginosa produces a greenish pigment and has a distinct smell, often compared to corn tortillas.

• Propionibacterium acnes is a gram-positive bacillus that causes acne and is sensitive to topical treatments.

• Viral diseases of the skin include warts, smallpox, chickenpox, shingles, measles, German measles, and roseola.

• Warts are caused by the papilloma virus and are typically benign growths that can be treated with cryotherapy or topical creams.

• Smallpox is a pox virus that has been eradicated, but the vaccine is still kept in high-security areas due to bioterrorism concerns.

• Chickenpox is a herpes virus that can cause shingles, a latent infection that can be triggered by stress.

• Shingles can be treated with antiviral medications like acyclovir, and a vaccine is available for prevention.

• Cold sores, also known as fever blisters, are caused by herpes simplex type 1 and are highly contagious.

• Herpes simplex type 1 can remain latent in nerve ganglia and reactivate as cold sores.• Herpes Simplex 1 (HSV-1) infects a significant portion of the population, and it should not be confused with canker sores. • HSV-1 remains dormant in the trigeminal nerve, which is a cranial nerve found in the head with three branches: to the forehead, maxilla (top lip), and mandible (lower jaw). • The trigeminal nerve allows the virus to access the mouth region, and triggers can include sunlight, UV light, stress, and certain hormones. • Herpes simplex encephalitis is a rare and severe complication of HSV-1 infection, with a high fatality rate and potential for serious neurological damage in survivors. • Fever blisters caused by HSV-1 are found on the lips and in the mouth, whereas canker sores are found on the tongue, cheek, or inside the mouth. • Macular rashes include measles, rubella, and Rosalia, which are transmitted through the respiratory system and can be spread through aerosols. • Measles is extremely contagious and can be a dangerous infection, with the virus shed prior to the outbreak, making herd immunity difficult to attain. • The MMR vaccine is available for measles, mumps, and rubella, and is typically given at 1 year of age, with limited protection provided by maternal antibodies. • Rubella, also known as German measles, is a milder case than measles, but is associated with birth defects if the mother is exposed during pregnancy. • Rosalia, or herpes virus 6, causes a mild rash and high fever that lasts for a couple of days, but provides good immunity after recovery. • Fungal diseases of the skin include cutaneous mycoses or dermatophytoses, which can cause ringworm, jock itch, athlete's foot, and nail infections. • Candida albicans causes thrush or oral thrush, especially in children and immunocompromised individuals. • Fungal infections can be diagnosed through skin scrapings treated with potassium hydroxide, and may require topical or oral treatment. • Skin parasite infestations include scabies (skin mites) and pediculosis (hair lice), which can cause red lines and itchiness. • Skin mites burrow under the skin, lay eggs, and cause red lines, while hair lice adapt to different body areas and lay eggs on hair follicles. • Both skin mites and hair lice are highly contagious and can be treated with medications and topical treatments. • Bacterial infections of the eye include conjunctivitis, which can be caused by Haemophilus influenzae, Neisseria gonorrhoeae, and Chlamydia trachomatis. • These infections can cause pink eye, weeping, and potential blindness if left untreated.