Skin Microbiology; Bacterial & Fungal; Skin infections Flashcards
What does the skin DO that provides a good barrier?
Constant shedding of the skin
Can bacteria use shed skin?
Yes, as a mode of spreading themselves more widely.
Seen in healthcare settings where people are in very close contact with each other.
Pathogenesis of bacteria and humans
- contact (entry) host
- Adhere/colonise and invade
- Multiply and complete life cycle
- EXIT host
Host is damaged in this process.
Bacterial virulence
The capacity of a microbe to cause damage to the host
Virulence factors
> Adhesins > Invasin > Impedin > Aggressin > Modulin
Adhesins
Enable binding of the organism to host tissue.
Extra cellular matrix molecules are present on epithelial, endothelial surfaces as well as a component of blood clots.
Fibrinogen-binding (ClfA ClfB)
Collagen binding (CNA)
Invasin
Enables the organism to invade host cell/tissue
Impedin
Enables the organism to avoid host defence mechanisms
Aggressin
Causes damage to host directly
The “car crash” bacteria
Modulin
Induces damage to the host indirectly
Chronic infections
Leukocidin
killing leukocytes
Staph aureus
where found on body
(types of infections)
Anterior nares and perineum
Nosocomial and community infection
Nasal strain can protect
S epidermidis
100% colonisation
skin & mucous membranes
nosocomial infection/immunocompromised
- catheters
MRSA
Nosocomial
Elderly and immunocompromised Intensive care units Burns patients Surgical patients IV lines Dialysis patients
Staph aureus has a range of infections. It is very flexible.
Due to…
VERY STRONG VIRULENCE FACTORS
Superficial lesions
- boils to abscesses
Systemic
- life threatening
Toxinoses
- toxic shock
- scalded skin syndrome
Evasion of host defences
CAPSULE Protein A Superantigens Coagulase Gamma toxin Alpha toxin PVL cytotoxin
2 types of capsule
Mucoid
Microcapsule
Toxinoses
> TSST-1
- rapid progression high fever, d&v, sore throat, muscle pain
> SSS (scalded skin syndrome)
- exfoliation toxins, often neonatal, face, axillae and groin
- ETA & ETB toxins target desmoglein 1 (DG-1)
What toxins (from S.aureus) bring about SSS?
ETA & ETB toxins
They target desmoglein 1. (integrity of desmosomes)
Superantigens
Activate 1 in 5 t cells. (normally 1 in 10000)
TSST-1 in particular associated with toxic shock
MASSIVE release of cytokines and inappropriate immune response.
What is TSST>
Toxic Shock Syndrome Toxin
Causes toxic shock.
Staph aureus infection.
Toxins absorbed through vaginal walls.
Toxic Shock syndrome - diagnostic criteria
> Fever - 39°C
Diffuse macular erythrodema
> Hypotension (≤90mmHg)
> ≥3 organs systems involved
PVL (Panton Valentine Leukocidin)
Specific toxicity for leukocytes.
Severe skin infections.
e. g. recurrent furunculosis
- sepsis/ necrotising fasciitis
PVL & alpha toxin - linked with CA-MRSA responsible for necrotising pneumonia and contagious severe skin infections.
Necrotising pneumonia
> Preceding influenza link syndrome
> Necrotising haemorrhagic pneumonia
> RAPID PROGRESSION
> Acute respiratory distress
> Deterioration in pulmonary function
> Refractory hyperaemia
> Multi-organ failure despite Abx therapy
Strep pyogenes
Gram +ve cocci Chains Catalase NEGATIVE Haemolysis - Beta haemolytic
Surface antigen
Skin infections
- impetigo
- cellulitis
- necrotising fasciitis
Some infections caused by Strep pyogenes
Impetigo
Cellulitis
Necrotising fasciitis
Lancefield system
> Serotyping of cell wall carbohydrate
Groups A-H & K-V
Group A further subdivided according to M protein antigens.
- M1 and m3 major serotype
- M3 & M18 severe invasive
Adhesion
Oropharynx and nasopharynx
Non ciliated cells covered in mucous
Hyaluronic Acid (capsule) - CD44 +ve keratinocytes
We have hyaluronic acid n our bodies - so the body has difficulty recognising the foreign pathogen.
Hyaluronic acid capsule
Similar to human hyaluronic acid
Mucoid colonies produced by high levels of capsule production
Produced in early exponential growth
Reduces phagocytosis
Highly encapsulated & M rich GAS are virulent
More encapsulated =
more virulent
Impetigo
Group A strep skin disease
Highly contagious through contact with discharge on the face.
Infection immediately below surface (stratum corneum)
What does GAS stand for?
Group A Streptococcal
Cellulitis
Group A strep infection
Deeper skin infection in the dermis
Not associated with necrosis
Cellulitis is not associated with…
necrosis
Erysipelas
Fever
Rigours
Nauseas
GAS
Necrotising fasciitis (type ii)
Invasive Strep A strains penetrate mucous membrane and develop in lesion.
Rapidly destroys connective tissue.
AMPUTATION TERRITORY
Streptolysins
Pore forming cytolysin
Toxic to PMN (neutrophils), organelles, platelets
Toxic Shock LIKE syndrome (TSLS)
S. pyogenes
Pyrogenic exotoxin (produces fever)
Complication of invasive infection
30% mortality
Hours to days
Hypotension to organ failure
SpeB and SpeC (superantigens)
Toxic Shock - S. aureus and S pyogenes
S. aureus localised infection no bacteremia Menstrual TSST-1 Non-menstural SEB or SEC pyrogenic toxin
Virulence Factor
Superantigen
S. pyogenes
invasive disease (pharyngitis)
SpeA & SpeC most common toxin
pyrogenic toxin
Virulence factor
Superantigen
Where is S. pyogenes normally found?
Pharynx
Variation of virulence factors caused by…
Variation in genes
Defence against infection
Less likely to become infected if skin is intact
dry - desiccation of microorganisms
Sebum - fatty acids (inhibit bacterial growth)
Competitive bacterial flora
Concept of resident and transient flora
Competitive bacterial flora
Staph. epidermis
Corynebacterium sp. (diphtheroids)
Propionibacterium sp.
Medical name for a boil
Furuncle
When would you swab an ulcer/boi; etc
If lesion surface is broken/looks infected
Pus or tissue if deeper lesion
Is Staph aureus coagulase negative or positive?
Positive.
All other staph are coagulase negative.
Which species of Staphylococcus is Novobiocin resistant?
S. saprophyticus
All other staph are SENSITIVE
Staph species
Gram positive cocci in clusters
Aerobic and facultatively anaerobic
S. aureus
Clots plasma
Causes wound, skin, bone and joint infections
Enterotoxin - food poisoning
SSSST
PVL
Treatment for a staph aureus infection?
FLUCLOX
FLUCLOX
FLUCLOX
SSSST
Staphylococcal scalded skin syndrome toxin
If resistant to methicillin…it will be resistant to
Fluclox
Carbuncle
red, swollen, and painful cluster of boils that are connected to each other under the skin.
Staph aureus - skin infections
> Boils and carbuncles
> Minor skin sepsis
> Cellulitis
> Infected eczema
> Impetigo
> Wound infection
> Staphylococcal SSS
Classic sign of Impetigo
Golden crusting of lesions
MRSA treatmetn
Doxycycline
Co-trimoxazole
Clindamycin
Vancomycin
Coagulase negative Staphs (Staph epidermidis)
> Skin commensals (not usually pathogenic)
> Infection in association with implanted artificial material (heart valves, joints, IV catheters)
What can Staph. saprophyticus cause?
Urinary tract infection in women of child-bearing age.
Strep species
Gram positive and in chains (strips)
Aerobic and facultatively anaerobic
Classified initially by haemolytic on blood agar
- Beta (complete)
- alpha (partial)
- gamma (none)
Further classified by antigenic structure on surface
- Group A
- Group B
If bacteria appears as a “chain” and is beta haemolytic… what is it?
Group A Strep (GAS)
Beta-haemolytic Strep
> Pathogenic
Haemolysin is one of many toxins
> Further classified by antigenic structure on surface
- Group A
- Group B
Group A strep
Throat
Severe skin infections
Group B strep
Meningitis in neonates
Important categories of alpha haemolytic strep
> Strep pneumoniae - pathogen, commonest, cause of pneumonia
> Strep viridian’s group - commensals of mouth, throat, vagina - cause infection, endocarditis
Non-haemolytic Streptococci
Enteroccus species (E. faecalis, E calcium)
Commensals of bowel
Common causes of UTI
Scalded Skin syndrome
Caused by staphylococcal infection
Shearing/sloughing/peeling of skin
Seen in children/new borns
Sometimes seen in drug users
Strep pyogenes
GAS
Infected eczema Impetigo Cellulitis Erysipelas (superficial form of cellulitis) Necrotising fasciitis
Crepitus of muscles can be caused by?
Necrotising fasciitis
Strep pyogenes treatment
Penicillin
flucloxacillin too
Necrotising fasciitis treatment
Immediate surgical debridement as well as Abx
Necrotising fasciitis
2 types.
Bacterial infection spreading along fascial planes below skin surface –> rapid tissue destruction.
SEVERE PAIN. Little skin signs.
2 types
- I: mixed anaerobes and coliform, usually post abdo surgery
- II: Group A strep Infection
Urgent surgical opinion and debridement required
Abx treatment depends on organisms
Fournier gangrene
Gangrene affecting the perineum
More likely in diabetics
Tinea means…
Ringworm. Not a worm, but raised outer borders of lesions in an annular pattern looks like a ringworm.
Tinea capitals - scalp " barbae - beard " corporis - body " manuum - hand " unguium - nails " cruris - groin " pedis - foot
DERMATOPHYTE
Dermatophyte pathogenesis
Fungus enters abraded or soggy skin
Hyphae spread in stratum corneum
Infects keratinised tissues only (skin, hair, nails)
Increased epidermal turnover causes scaling
Inflammatory response provoked (dermis)
Hair follicles and shafts invaded
Lesion grows outward and heals in centre, giving a “ring” appearance
Who is more commonly affected by dermatophyte infections?
Men
mainly foot and groin
Scalp ringworm normally affects…
Children
Sources of dermatophyte infection
- other infected humans*
- antrhopophilic fungi
Animals (cats, dogs, cattle)
- zoophilic fungi
Soil (less common UK)
- geophilic fungi
Dermatophytes - causal organisms
**Trichophyton rubrum (human-human) **
Trichophyton mentagraphytes (next most common, human-human)
Microsporum canis
- cats, dogs, humans
Main cause of dermatophyte infection?
TRICHOPHYTON RUBRUM
Dermatophyte infection - diagnosis
Clinical appearance
Woods light (fluorescence)
Skin scrapings, nail clippings, hair
- send to lab in a dermapak
- culture takes 2 weeks+
When sending samples of suspected dermatophytes, what do you send them in?
Dermapaks.
Dermatophyte infection - treatment
> Small areas of infected skin, nails = Clotrimazole cream or similar
, topical nail paint
> Extensive skin infections > nail infections > Scalp infections - terbinafine orally - itraconazole orally
Candida skin infection
causes infection in skin folds where area is warm and moist (candida intertrigo)
Seen under breasts in females, groin areas, abdo skin folds, nappy area in babies
SWAB for culture
Candida - treatment
Clotrimazole cream
Oral fluconazole
Scabies
Sarcoptes scabiei
6 weeks’ incubation
Intensely itchy rash affecting finger webs, wrists, genital area
Chronic crusted form of scabies =
Norwegian scabies
Scabies - treatment
Malathion lotion (applied to whole body, washed off next day)
Benzyl benzoate (avoid in children)
What treatment should be avoided in children with scabies?
Benzyl benzoate
Lice infestation (pediculosis)
INTENSE ITCH
Pediculis wapitis (head louse)
Pediculus corporis (body louse)
Phthirus pubis (pubic louse)
Malathion for treatment
Lice - treatment
Malathion
Infection control - dermatology
Exfoliative skin conditions are an issue.
Gram pos bacteria can survive in the environment because of their cell wall
Source of infection for other patients
SO
Gloves and plastic aprons (contact precautions)
Single room isolation for some patients
- GAS infection
- MRSA infection
- Scabies (long sleeved gowns required)
