Microbiology Flashcards
Name five virulence factors
- adhesin
- invasin
- impedin
- aggressin
- modulin
What is adhesin?
Enables binding of the organism to host tissue
What is invasin?
Enables the organism to invade a host cell / tissue
What is impedin?
Enables the organism to avoid host defence mechanisms
What is aggressin?
Causes damage to the host directly
What is modulin?
Induces damage to the host directly
What are virulence factors?
The factors responsible for the variation in virulence within and between species
Describe staph aureus skin infections
- anterior nares and perineum
- nosocomial and community
- coagulase positive
- nasal strain can protect
Describe staph epidermidis skin infections
- 100% colonisation
- skin and mucous membranes
- coagulase negative
- nosocomial infection / immunocomprimised
- associated with foreign devices eg. catheters
Describe MRSA
- defined by flucloxacillin resistance
- mainly nosocomial
- elderly and immunocomprimised
- intensive care units
- burns patients
- surgical patients
- intravenous lines
- dialysis patients
Name presentations of staph aureus skin infections
- rash
- folliculitis
- abscess
- carbuncle (multiocular abscess)
- impetigo
- scalded skin syndrome
Describe the pathogenicity of staph aureus
- superficial lesions; boil to abscesses
- systemic; life threatening
- toxinoses; toxic shock, scalded skin syndrome
What is TSST-1?
- toxinose
- rapid progression (48hrs) high fever, vomiting, diarrhoea, sore throat, muscle pain
What is staphylococcal food poisoning?
- toxinose
- enterotoxin SeA, SeB and SeC
- intoxication, 1-5hrs, vomiting, diarrhoea
What is scalded skin syndrome?
- toxinose
- exfoliatin toxins, often neonatal, face, axilla and groin
- ETA and ETB toxin target desminogen (DG-1)
Describe superantigens
- activate 1 in 5 T cells
- TSST-1 in particular associated with toxic shock
- antigen is not processed by PMN bonds directly to MHC11 complex ie. outside conventional binding groove
- massive release of cytokines and inappropriate immune response
Describe the diagnostic criteria of toxic shock syndrome
- fever; 39 degrees
- diffuse macular rash and desquamation; diffuse macular erythroderma
- hypotension
- 3 or more organ systems involved; liver, blood, renal, mucous membrane, GI , muscular, CNS
Describe adhesins
- attachment and colonisation
- extra cellular matrix are present on epithelial, endothelial surfaces as well as a component of blood clots
Describe panton-valentine leukocidin
- PVL bicomponent toxin
- specificity toxicity for leukocytes
- present in 1-2% clinical strains
- PVL associated with sever skin infections eg recurrent furunculosis, sepsis or necrotising fasciitis
- PVL and alpha toxin linked with CA-MRSA responsible for necrotising pneumonia and contagious severe skin infections
Describe necrotising pneumonia
- preceding influenza like syndrome
- necrotising haemorrhagic pneumonia
- rapid progression
- acute respiratory distress
- deterioration in pulmonary function
- refractory hypoxaemia
- multi-organ failure despite antibiotic therapy
Name the skin infections caused by streptococcus pyogenes
- impetigo
- cellulitis
- necrotising fasciitis
Describe the lancefield system
- serotyping of cell wall carbohydrate
- major serotypes A-H and K-V (20)
- C polysaccharides extracted from cell wall
- group A further subdivided according to M protein antigens
- M1 and M3 major serotype
- M3 and M18 severe invasive disease
Describe impetigo
- usually face
- highly contagious through contact with discharge on the face
- infection immediately below the surface
- GAS skin disease
Describe cellulitis
- GAS skin disease
- deeper skin infection in the dermis that is not associated with necrosis
- erysipelas (localised) fever, rigours and nausea
- range of spreading subcutaneous skin infections
- trauma or primary infection associated with organism spreading through blood and seeding cellulitis
Describe necrotising fasciitis
- invasive group A streptococci
- type 1, clostridia spp
- invasive step a strains penetrate mucous membrane and develop in lesion
- rapidly destroys connective tissue
What is virulence and virulence factors?
- the molecular / genetic basis of pathogenesis that characterises species, sub-species and strains
Where is staph pyogenes usually found?
In the pharynx but also adheres to the skin
Name some competitive bacterial flora (commensals)
- staphylococcus epidermidis
- corynebacterium sp (diphtheroids)
- propionibacterium sp
What is a carbuncle?
A series of abscesses connected by sinuses
What colour of sample bottle is used for aerobic blood cultures?
Green
What colour of sample bottle is used for anaerobic blood cultures?
Red
Describe staphylococcus species
- gram positive cocci in clusters
- aerobic and facultatively anaerobic (grows best aerobically, but also grows anaerobically)
- 2 important types; staph aureus and coagulase negative staph
Describe the features of staph aureus
- common human pathogen
- produces enzymes, including coagulase, an enzyme that clots plasma
- causes wound, skin, bone and joint infections
- resistant strains (MRSA)
- some strains produce toxins
Describe staph aureus skin infections
- 30% of hospital staff are carriers
- boils and carbuncles
- other minor skin sepsis (infected cuts etc)
- cellulitis
- infected eczema
- impetigo
- wound infection
- staphylococcal scalded skin syndrome
Name the strains of staph aureus
- enterotoxin (food poisoning)
- SSSST (staphylococcal scaled skin syndrome toxin)
- PVL (panton valentine leucocidin, more often in systemic disease)
Name the antibiotics of choice for staph aureus
- flucloxacillin
Name MRSA treatment options
- doxycycline (bacteriostatic)
- co-trimoxazole
- clindamycin
- vancomycin
NOT FLUCLOXACILLIN
Describe the features of coagulase negative staphs (eg staph epidermidis)
- skin commensals - not usually pathogenic
- may cause infection in association with implanted artificial joints, artificial heart valves, IV catheters
- staph saprophyticus causes urinary tract infection in women of child bearing age
Describe the features of streptococcus species
- gram positive cocci in chains
- aerobic (and facultatively anaerobic)
- classified initially by haemolysis on blood agar
Describe beta haemolytic streptococci
- pathogenic organisms
- haemolysin is one of many toxins (enzymes) produced that damage tissues
- further classified by antigenic structure on surface (serological grouping)
- group A (throat, severe skin infections)
- group B (meningitis in neonates)
Describe alpha haemolytic streptococci
- 2 important categories - strep pneumoniae; commonest cause of pneumonia
- strep viridans; commensals of mouth, throat, vagina - cause infection endocarditis
Describe non-haemolytic streptococci
- enterococcus (e faecalis, e faecium)
- commensals of bowel
- common cause of UTI
Describe the diseases strep pyogenes is involved with / causes (group a strep)
- infected eczema
- impetigo
- cellulitis
- erysipelas
- necrotising fasciitis
Describe the treatment of staphylococcal and streptococcal infections
- minor skin sepsis may not require antibiotics
- staph aureus - flucloxacillin
- group a step - penicillin (not flucloxacillin)
- necrotising fasciitis - life threatening, requires surgical debridement as well as antibiotics
Describe necrotising fasciitis
- bacterial infection spreading along fascial planes below skin surface > rapid tissue destruction
- little to see on skin but severe pain
- 2 types
- urgent surgical debridement and opinion required
- antibiotic treatment depends on organisms isolated form tissue taken at operation
Describe the two types of necrotising fasciitis
- type 1; mixed anaerobes and coliforms, usually post abdominal surgery
- type 2; group a strep infection
When would you take a swab of a leg ulcer?
Only is signs of cellulitis or infection are present
Name the different dermatophyte (fungal) infections (ringworm)
- tinea capitis; scalp
- tinea barbae; beard
- tinea corporis; body
- tinea manuum; hand
- tinea unguium; nails
- tinea cruris; groin
- tinea pedis; foot (athletes foot)
Describe the pathogenesis of dermatophyte
- 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?
Males
Scalp ringworm usually affects who?
Children
Name some sources of infection of dermatophyte infections
- other infected humans; the most likely source
- animals - zoophilic fungi
- soil (less common in UK)
Name some casual organisms of dermatophyte infection
- trichophyton rubrum (>70% of lab isolates)
- trichophyton mentagraphytes
- microsporum canis (occasional isolate, cats / dogs to humans)
Describe the diagnosis of dermatophyte infection
- clinical appearance
- woods light (fluorescence)
- skin scrapings, nail clippings, hair
- send to laboratory in a dermapak for microscopy and culture
- culture takes 2 weeks +
Describe the treatment of dermatophyte infections
- small areas of infected skin, nails
- clotrimazole (canestan) cream or similar
- topical nail paint (amorolfine)
- extensive skin infections
- nail infections
- scalp infections
- terbinafine orally (topically for athletes foot and on occasion with cellulitis)
- itraconazole orally
Describe candida skin infection
- candida causes infection in skin folds where area is warm and moist
- seen under the breasts in females, groin areas, abdominal skin folds etc, nappy area in babies
- diagnosis; swab for culture
- treatment; clotrimazole cream, oral fluconazole
Describe scabies
- caused by sacroptes scabiei
- chronic crusted form is termed norweigan scabies (highly infectious)
- incubation period up to 6 weeks
- intensely itchy rash affecting finger webs, wrists and genital area
- treatment; malathion lotion, applied overnight to whole body and washed off the next day
- benzyl benzoate (avoid in children)
Describe lice (pediculosis)
- pediculus captitis (head louse)
- pediculus corporis (body louse)
- phthirus pubis (pubic louse)
- associated with intense itch
- treatment; malathion
Describe infection control in dermatology
- gloves and plastic aprons required for dressing changes
- patients who need single room isolation and contact precautions;
- patients with group a strep infection
- MRSA patients
- patients with scabies (long sleeved gowns also required for norweigan scabies)
