microbiology sst

• Skin is the largest and thinnest human organ, providing a physical barrier between our internal organs and the external environment.
• Skin is constantly renewing itself, self-repairing through the process of epithelial cell turnover which removes surface substances and microorganisms.
• Skin plays a crucial role in control of body temperature, excretion of water and salts, and the synthesis of hormones.
• Skin is also a sensory organ.
• Skin has resident microflora, organisms able to adapt to high salt and low moisture conditions.
• Skin is subject to frequent trauma, leading to an increased risk of infection.
• Skin can reflect both internal disease and systemic infections.
• Skin is composed of three distinct layers: Epidermis, Dermis, and Hypodermis (subcutaneous tissue).
• The Epidermis is the outermost layer, consisting of many layers of squamous epithelial cells.
• The outermost layer (stratum corneum) of the Epidermis is dead cells, without nuclei, large plate-like envelopes filled with keratin, a water-repellant protein.
• The surface of the Epidermis is constantly shed (epithelial cell turnover) which removes superficial organisms.
• Ulcers are lesions with inflammation and necrosis, can be chronic, and can occur in various stages of healing, such as diabetic foot ulcers and stasis ulcers.
• Bartonellosis is a condition caused by Bartonella bacteria, which can present with a variety of symptoms including fever, fatigue, and skin lesions.
• Petechiae are small hemorrhagic spots that do not 'blanch' (glass test).
• The majority of petechiae are not caused by bacterial infections, but can be caused by low platelets, leukemias, and viral infections.
• Syphilis is caused by Treponema pallidum, a spirochete, with primary syphilis presenting as a genital lesion, called a “hard” chancre, and secondary syphilis presenting as an extensive maculopapular rash.
• Vesicles are fluid-filled blisters that are itchy and may be sterile, such as from poison ivy.
• Papules are small bumps or pimples, vary in size, are raised, can be rough, and have no visible fluid.
• Lyme Disease is caused by Borrelia burgdoferi, a spirochete, with the primary presentation being “erythema migrans”, a red, expanding rash with sharply defined borders and clear center commonly known as a “bull’s eye” lesion.
• Mycobacteria require decontamination and concentration of cutaneous specimens and specialized media, such as Lowenstein-Jensen (LJ), which contains whole eggs.
• Aeromonas infections are more localized than Vibrio infections, and can occur in septic patients, resulting in lesions similar to Vibrio, with bullae with necrosis.
• Vibrio infections are caused by the ingestion of contaminated shellfish or contamination of an existing wound with seawater, resulting in widespread lesions and hemorrhagic bullae which proceed to necrotic ulcers.
• Dermatophyte infections are cultured on potato dextrose agar (PDA), mycosel, inhibitory mould agar (IMA), Sabhi, or Sabourad’s (SAB).
• Bacterial infections are diagnosed by collecting swabs from purulent areas, cleaning the area well prior to collection, and reading the Gram stain STAT.
• Bacterial cultures are kept up to 4 – 8 weeks for better chance of recovery.
• Nocardia grows slowly on routine media but can be identified by fungi and AFB cultures.
• The Dermis is the layer under the Epidermis, a thick layer of connective tissue that contains the blood and nerve supply, hair follicles, sweat gland ducts and oil gland ducts.
• Cultures are kept up to 4 weeks.
• Specialized media for dermatophyte identification may be added if the KOH prep is positive.
• Dermatophyte infections are diagnosed by collecting dry skin scrapings, hair or nail clippings into folded black paper and examining them under a 10% KOH preparation, which clears keratinized tissue.
• Actinomyces requires an anaerobic set up and media containing 'tissue juice' added, for example: brain heart infusion with blood.
• Viral infections are diagnosed by collecting lesion scrapings for immunofluorescent stains, collecting blood for serologic testing, and using PCR methods.
• Calcofluor white stain is added to the KOH preparation to bind to chitin in the cell wall and fluorescent.
• Deeper bacterial infections are diagnosed by surgically collecting tissue samples or aspirating fluid and processing them ASAP.
• Hair follicles provide an entry portal for organisms and are a potential infection site due to friction or blockage by sebum.
• Sweat glands can also be portals of entry, but salt and lysozyme are protective.
• Sebaceous glands secrete sebum (oil) and fatty acids, which are protective.
• Approximately 15% of patients who seek medical care have either a skin disease or lesion.
• Normal skin flora are organisms that live on the skin and are able to withstand high salt concentrations, a relatively low pH and the dry conditions present.
• Normal resident skin microbiota act as competitors to pathogenic organisms, competing for nutrients.