Microbiology of skin infections Flashcards
What do staphlococcus look like under a microscope?
Gram positive (purple) cocci in clusters
Are staphlococcus aerobic or anaerobic bacteria?
Aerobic and facultatively anaerobic
What are the two important groups of staph. and how can they be differentiated?
Staph. aureus and coagulase negative staph (e.g staph. epidermidis). They are differentiated with the coagulase test
In culture what colour does staph aureus appear?
Gold
What kind of infections do staph. aureus commonly cause?
Wound, skin, bone and joint
Name some toxins which certain strains of s.aureus produce
Enterotoxin
PVL
SSSST
What is the drug of choice used to treat staph. aureus skin infections?
Flucloxacillin
In which group of people is staphlococcal scalded skin syndrome usually found?
Infants
What might a PVL containing strain of s.aureus usually cause?
Chronic abscesses (axilla and buttocks)
Coagulase negative staph are usually skin commensals. T/F
True
When might coagulase negative staph cause infections?
In association with implanted artificial material (e.g heart valves)
What type of infections does staph. saprophyticus usually cause?
Urinary tract infection (woman mostly)
What do streptococci look like under a microscope?
Gram positive (purple) cocci in chains
strep=strips
Are streptococci aerobic or anaerobic?
Aerobic
How are streptococci classified?
By type of haemolysis on a blood agar plate (alpha, beta and gamma). Where beta is complete haemolysis, alpha is partial haemolysis and gamma is no haemolysis
Which class of steps are the most pathogenic?
Beta haemolytic (group A strep)
When does group B strep cause infection?
Usually in neonates
What is the major toxin produced by group B strep which causes destruction of tissues?
Haemolysin
How is strep classified within the groupings?
Surface antigen structure
What types of infection do group B streps usually cause?
Throat and skin infections
What types of infection do group A streps usually cause?
Meningitis (neonates)
What are the two most important alpha haemolytic streps? Which infections do they cause respectively?
Strep. pneumonia and strep. viridans. Pneumonia and usually commensals of the mouth, throat and vagina
What are the non-haemolytic strep? Which infections do they usually cause?
Enterococcus sp. Gut commensals but common cause of urinary tract infection
Which features of the skin protect against infection?
Dry surface (doesn’t affect gram positive organisms much), sebum inhibits bacterial growth & competitive resident flora
Name some competitive bacterial flora?
Staph. epidermidis, diptheroids, proprionobacterium
What types of skin infections do staph.aureus cause?
- boils and carbuncles (hair follicles)
- minor skin sepsis (infected cuts)
- cellulitis
- infected eczema
- impetigo
- wound infection
- SSSS
Boils and carbuncles must be treated with oral antibiotics (flucloxacillin). T/F
False - only carbuncles need treated
Where do carbuncles most commonly occur?
At the base of the neck
Which pathogens usually cause impetigo?
S.aureus or group A strep
Where does impetigo present?
Mucocutaneous junction around the mouth and nose (crusty yellow scabs)
In which age group is impetigo most commonly found?
Children
How is SSSS treated?
Aggressive antibiotics and rehydration therapy
What is another name for group A strep?
Strep pyrogenes
Which types of skin infections do strep pyrogenes cause?
- infected eczema
- impetigo
- cellulitis
- erysipelas
- necrotising fasciitis (often along with other bacteria)
How does cellulitis present?
Erythematous skin with no obvious breakage
How does erysipelas present?
Butterfly pattern of facial swelling and erythema
How does necrotising fasciitis present?
Purple/erythematous areas with no obvious skin breakage. Patient appears to be suffering pain disproportionate to what can be seen. Rapid spread
How are bacterial skin infections diagnoses?
Bacterial swab if the skin is broken. If a deeper tissue infection pus/tissue can be used. Blood cultures if pyrexial
How are bacterial skin infections treated?
+ minor skin sepsis - no treatment
+ s.aureus - flucloxacillin
+ strep. pyrogenes - penicillin (but also flucloxacillin)
+ necrotising fasciitis - immediate surgical debridement & antibiotics
What is the pathogenesis of necrotising fasciitis?
Bacterial infection which spreads along the fascial planes below the skin surface causing rapid tissue destruction
What are the two types of necrotising fasciitis?
Type 1 - mixed anaerobes and coliforms
Type 2 - group A strep
When does Type 1 necrotising fasciitis usually occur?
Post abdominal surgery
Which antibiotics is Type 2 necrotising fasciitis treated with?
Clindomycin and penicillin
When should swabs be taken from leg ulcers?
If there are signs of infection/cellulitis
Which organisms found in leg ulcers are worth treating?
Strep. pyrogenes, s.aureus & anaerobes (diabetic patients)
Tinea is another word for what?
Ringworm
How does tinea capitis present?
An area of hair loss (hair itself is infected and breaks off) with scale
Why doe tinea corporis present as round lesions?
Fungus grows outwards from central infected point
Is tinea cruris more common in men or woman?
Men (may grow on scrotum)
How does tinea unguum present?
Yellow nails which flake off easily
How does tinea pedis present? What is another name for tinea pedis?
Itch & soggy area of broken skin between toes (usually between fifth and fourth toes). Athlete’s foot
What is the pathogenesis of dermatophyte infection?
Fungus enters broken/soggy skin > hyphae spread in keratin layer > increased epidermal turnover causing scaling > inflammatory response > hair follicles invaded > lesion grows outward and heals in centre (target lesion)
Dermatophytes only infect keratinised tissues (skin, nails, hair). T/F
True
What is the epidemiology of dermatophyte infection?
Men more commonly affected (pedis and cruris most commonly). Children more commonly affected with tinea capitis.
How do dermatophyte infections spread?
Human-human (most common), animals & soil
What are the three most common dermatophytes to cause infection? How are the spread?
Trichophyton rubrum, trichophyton mentagraphytes & microsporum canis. Human, human and animal
How are dermatophyte infections diagnosed?
Clinically, woods light, microscopy (most accurate) and culture of skin scrapings, nail clippings & hair
How must skin scrapings, nail clippings and hair be sent to the laboratory?
Within a dermapak (black so samples are more easily seen)
What is the disadvantage of culturing dermatophytes?
It takes forever
How does a culture of trichophyton rubrum appear?
White and fluffy on top and brown and orange below
How are dermatophyte infections treated?
+ small areas of infection - clotrimazole cream, topical nail paint (amorolfine)
+ extensive infection - oral terbinafine and oral itraconazole
Where does candida most commonly infect?
Within skin folds (under breasts, groin area, abdominal folds)
How are candida infections diagnosed?
Swab for culture
How are candida infections treated?
Clotrimazole cream, oral fluconazole
What organisms is scabies caused by?
Sarcoptes scabiei
What is the chronic, crusted form of scabies called? How does this occur?
Norwegian scabies. In immunocompromised patients
What is the incubation period of scabies? How is it transferred?
Up to six weeks. Human to human
How does scabies present?
Itchy rash often forming burrows and affecting the genital area, wrists and finger webs
How is scabies treated?
Malathion lotion (applied overnight to whole body and washed off next day). Benzyl benzoate (NOT IN CHILDREN)
What are the different types of lice?
Pediculus capitis (head lice), pediculus corporus (body louse), phthirus pubis (pubic lice)
How does lice infection present?
Intense itch
When might someone contract pediculus corporus?
Homeless/sleeping on the streets
What is the treatment of lice?
Malathion
What are the infection control procedures needed in dermatology?
Gloves and aprons required for dressing changes, single room isolation
Which patients need single room isolation?
Group A strep infection, MRSA, scabies (long sleeved gowns required
