Microbiology of Skin Infections Flashcards
What properties of the skin allow it to defend against micro-organisms?
Dry (desiccation of micro-organisms), sebum/fatty acids (inhibit bacterial growth), competitive bacterial flora
What are some competitive bacterial flora of the skin?
Staph. epidermidis, Corynebacterium sp. (diphtheroids), Propionibacterium sp.
How can skin infections be diagnosed?
Swab lesion if surface broken, pus or tissue if deeper lesion, +/- blood cultures if appropriate
What is Staphylococcus?
Gram positive cocci in clusters, aerobic and facultatively anaerobic (grows best in air but can grow anaerobically)
What are the two important types of Staphylococcus?
Staph. aureus (coagulase positive), coagulase negative staph. (all other types except staph. aureus, not normally important for skin infections)
How is staph. aureus distinguished from all other staph. species?
Produces, enzymes, including coagulase (clots plasma)
What kind of infections does Staph. aureus cause?
Wound, skin, bone and joint infection
What is the treatment of choice for Staph. aureus infections?
Flucloxacillin
What is the resistant strain of Staph aureus called?
Methicillin Resistant Staphylococcus Aureus (MRSA)
Why can’t MRSA be treated with flucloxacillin?
Methicillin is very similar to flucloxacillin, its basically a laboratory version of the drug
What are some toxins produced by Staph. aureus?
Enterotoxin (food poisoning), Staphylococcal Scalded Skin Syndrome Toxin (SSSST), Panton valentine leucocidin (PVL)
What skin infections does staph. aureus cause?
Boils and carbuncles, cellulitis, other minor skin sepsis, infected eczema, impetigo, wound infection, Staph. scalded skin syndrome
What are the treatments for skin and soft tissue infections caused by MRSA?
Doxycycline (oral, bacteriostatic), co-trimoxazole (excellent oral bioavailability), Clindamycin and Linezolid (not routinely recommended)
What are the bactericidal treatment options for systemic MRSA infections?
Vancomycin, Daptomycin
What do coagulase negative staph. tend to be?
Skin commensals (not usually pathogenic)
When do coagulase negative staph. cause infection?
May cause infection in association with implanted artificial material (e.g heart valves)
What does Staph. saprophticus cause?
UTIs in women of child bearing age
What is Streptococci?
Gram positive cocci in chains, aerobic (and facultatively anaerobic)
How is Streptococcus classified initially?
By haemolysis on blood agar = beta (complete), alpha (partial), gamma (no haemolysis)
What is an example of a toxin produced by beta haemolytic strep?
Haemolysin=damages tissues
How can beta haemolytic strep be further classified?
By the antigenic structure on their surface = Group A (throat, severe skin infections), Group B (meningitis in neonates)
What are the two important examples of alpha haemolytic strep?
Strep. pneumoniae (commonest cause of pneumonia, pathogen), Strep. viridans group (commensals of mouth, throat and vagina, cause endocarditis)
What is an example of a gamma haemolytic strep?
Enterococcus species=commensals of bowel, common cause of UTI
What are some examples of infections caused by Strep. pyogenes (Group A strep)?
Infected eczema, impetigo, cellulitis, erysipelas, necrotising fasciitis (also caused by mixed bacterial infection)
What is the treatment for Strep. pyogenes infections?
Penicillin (also flucloxacillin)
What is necrotising fasciitis?
Bacterial infection that spreads along fascial planes below skin surface causing rapid tissue destruction (little to see on skin surface but severe pain)
What causes type 1 necrotising fasciitis?
Mixed anaerobes and coliforms, usually post-abdominal surgery
What causes type 2 necrotising fasciitis?
Group A strep infection
What is the treatment for necrotising fasciitis?
Urgent surgical opinion and debridement needed, antibiotic treatment depends on the organisms isolated from the tissue taken at operation
What is the underlying problem of leg ulcers?
Vascular problems
When do you take swabs of leg ulcers?
If there are signs of cellulitis or infection
What are some organisms worth treating if found in a leg ulcer?
Strep. pyogenes, Staph. aureus, other beta haemolytic strep (B, C, G), anaerobes (especially in diabetic patients)
What are some examples of dermatophyte (fungal infections)?
Tinea (ringworm), candida
How is tinea named?
According to where it is found (e.g tinea pedis = foot)
What kind of skin does tinea enter?
Abraded or soggy skin
Where does the hyphae of tinea spread?
Stratum corneum
What is the pathogenesis of tinea once it has entered the stratum corneum?
Increases epidermal turnover causing scaling, inflammatory response is provoked in dermis, hair follicles and shafts are invaded, lesions grow outwards and heal in centre (causing a ring appearance)
What kind of tissue does tinea infect?
Only keratinised tissues (hair, nail, skin)
Who is more likely to get tinea infection?
Males (especially foot and groin ringworm)
Who commonly gets scalp ringworm?
Mainly children
What are some sources of tinea infection?
Other infected humans (anthropophilic), animals (cats, dogs, cattle-zoophilic), soil (less common in UK-geophilic)
What is the most common cause of tinea infection?
Trichophyton rubrum = >70% of isolates, human-human transmission
What is the second most common cause of tinea infection?
Trichophyton mentgraphytes = >20% of isolates, human-human transmission
What is a rare cause of tinea infection?
Microsporium canis = occasional isolate, cat/dog-human transmission
How can tinea infection be diagnosed?
Clinical appearance, Woods light (fluorescence), skin scrapings/nail clippings sent to lab in Dermapak for microscopy and culture
How long does a tinea culture take, and where should skin scrapings be taken from?
2 weeks +; scrapings should be taken from scaly edge of lesion
How are small areas of skin/nails infected with tinea treated?
Clotrimazole (Canestan) cream, topical nail paint (Amorolfine)
How are extensive tinea infections, or nail/scalp infections treated?
Oral terbinafine or Itraconazole
Where does Candida intertrigo infection occur?
In skin folds where area is warm and moist = under breast in females, groin areas, abdominal skin folds, nappy area in babies
How is candida infection diagnosed?
Using a swab for culture
How is candida infection treated?
Clotrimazole cream, oral Fluconazole
What organism causes scabies?
Sarcoptes scabies
What is the chronic crusted form of scabies called?
Norwegian scabies (highly infectious)
What is the incubation period of scabies?
Up to 6 weeks
How does scabies manifest?
Intensely itchy rash affecting finger webs, wrists or genital area
How is scabies treated?
Malathion lotion (applied overnight to whole body and washed off next morning), Benzyl benzoate (avoid in children)
What organism causes head lice?
Pediculus capitus
What organism causes body lice (Vagabond’s disease)?
Pediculus corporis
What is associated with a lice infection and how is the infection treated?
Intense itch; infection treated with malathion
What are some infection control precautions for changing dressings?
Wear gloves and plastic aprons
What patients need single rooms?
Patients with Group A strep, MRSA or scabies
What kind of gown needs to be worn when seeing to a patient with Norwegian scabies?
Long sleeved gowns
Why are patients with exfoliative skin conditions an infection risk?
They shed huge numbers of skin scales and associated bacteria (gram positive bacteria can survive in environment)
