Infectious diseases of the skin and musculoskeletal system

===Lecture Details===
Anton Pelig; Week 6 MED1022; Microbiology
===Lecture Content===
Skin's microbial load is controlled by the amount of moisture present, pH of normal skin, surface temperature, salty sweat, excreted chemicals such as sebum, fatty acids and urea; competition between normal flora. Approach infection- is primary infection skin? Distant to skin (deposition of organisms, immune complex, toxin mediated)? Other than infection? Infection is generally from breach of skin, spread to skin of systemic infections, toxin-mediated damage. Predisposing conditions are eczema, burns, surgery, pressure sores, IV drip sites, carriage of pathogens.
Papula is raised patch of inflammation, vesicle is when epithelium is invaded, ulcer is rupture of epithelium, papilloma is when epithelium grows with microbe within, shedding includes microbe; macule is flat immune response of infiltrating leukocytes in the skin. Bacterial infections involve different layers of the skin with different patterns; staph aureus and staph pyogenes are the most common. Toxin-producing bacteria produces systemic infection from local skin infection (TSS).
Group A streptococcal infection causes pharyngitis and skin infection, nasal carriage and auto-inoculation occurs, skin contact, virulence factors (M proteins) resist immune system and enable rapid spread, immune sequelae (rheumatic fever, glomerulonephritis), toxin mediated disease can also occur (scarlet fever). Erysipelas occurs on the face and is due to group A strep (Strep pyogenes).
Acne is caused by increased sebum production at puberty and desquamation leadng to blockage of sebaceous ducts. Propionibacterium acnes infects the wound. Breaks down sebum to give fatty acids and peptides which induce inflammation. Leprosy is caused by mycobacterium leprae which is mildly infectious. Lepromatous has poor immunity and spreads widely. Tuberculoid is strong response.
Fungal infections can be superficial or deep, superficial eg ringworm are very common, very prevalent and spread by contact. Sources are animals, humans or soil. Systemic fungal infections have skin manifestations. Tinea can be corporis, cruris (genitals) or capitus. Candida is another common infection.
Viral infections can be local (papillomavirus, herpes simplex) or systemic which causes rash (virus in vesicles, immune reaction is maculopapular rash, can be measles, rubella, chickenpox). DNA viruses include papillomavirus and herpesvirus (HSV, EBV, CMV, VSV). Measles have Koplic spots and maculo-papular rash.
Parasitic infections can enter through direct penetration (hookworm, schistosomes) or blood feeding vectors. Insect infestations can be fleas, lice, ticks, mites. 
Infections of joint and bone are typically of growing bone so disease of children and teenagers. Can be damaged by immunological processes (RA, ankylosing spondylitis, rubella, Hep B, shigella, campylobacter). Osteomyelitis is due to staph. Septic arthritis commonly due to S aureus, streptococci, gram negative rods or gram negative cocci (Neisseria gonorrhoeae).
Ringworm involves keratnised epithelium (central pallor with spreading redness), impetigo the epidermis, cellulitis (Strep pyogenes) subcutaneous fat. Pyogenes can also cause scarlet fever which has an erythematous rash. Impetigo is mixed strep/staph infection, commonly known as school sores.
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