Microbiology Flashcards
What are virulence factors
Proteins that contribute to an organisms virulence.
Which bacteria tend to survive on the skin
Usually gram positive as they can cope with the dryness
Gram negatives tend to be found in moister areas such as the armpit or perineum
What defences does the skin have against infection
The structure - should be impenetrable
Shedding layer prevents a biofilm forming
Sebaceous glands and sweat pores
What is MRSA
Methicillin resistant staph aureus
Defined by its resistance to flucloxacillin
Often seen in hospital patients, particularly elderly or immunosuppressed
What clinical presentations can be caused by a staph aureus infection
superficial lesions - boils to abscesses
Systemic effects - can be fatal
Toxinoses such as toxic shock, scalded skin syndrome
What virulence factor will all staph aureus strains carry
Coagulase
All are coagulase positive organisms
Do all strains of staph aureus carry the same virulence factors
NO
will have different combinations and lead to different presentations
Variety makes it an effective pathogen
Name some toxinoses that can be caused by staph aureus
TSST-1 can lead to fever, vomiting, diarrhoea, pain etc - toxic shock
Staph food poisoning caused by enterotoxin
Scaled skin syndrome
Describe scalded skin syndrome
Often occurs in neonates
exfoliatin toxins attack cross-bridges that hold the skin together
As a result the dermis and epidermis slide apart
What are the clinical signs of toxic shock syndrome
Fever
Defuse macular rash
Hypotension - <90mmHg
more than 3 organ system involved - life threatening
what is the cause of toxic shock syndrome
Particularly associated with TSST-1 (staph aureus toxin)
Tampon use
There is an overreaction of the immune system due to a massive release of cytokines
What is PVL
Panton-Valentine Leukocidin
Toxic to leukocytes
Associated with severe and recurrent skin infections
Describe necrotising pneumonia
Preceded by a flu like syndrome
rapidly progresses
Leads to acute respiratory distress, deterioration of lung function and organ failure
Organism destroys the respiratory tissue
What are the features of strep pyrogenes
Gram + cocci in chains
B haemolysis
Describe impetigo
Red/orange crusty rash, usually on face
Infection is just below skin surface
Common in nursery age children
Highly contagious - spread through direct contact with discharge
Describe necrotizing fasciitis
Caused by invasive Strep A strains
They penetrate the mucous membrane and develop
Rapidly destroys connective tissue
Irreversible
What type of virulence factor is responsible for toxic shock
Super antigens
Either in S. aureus or S. pyogenes
How can gene transfer occur in bacteria
Bacterial transformation - taking up DNA from another cell and incorporating it
Transduction -release of bacteriophage which transfer DNA
Conjugation - sex pili exchange plasmids
Where does staph aureus colonise
Multiple strains colonise the skin and mucous membranes
How does the skin act as an immune defence
It works if the skin is intact
Dry surface
Sebum - inhibits bacterial growth with fatty acid
Competitive bacterial flora
How do you diagnose a skin infection
Swab the lesion if the surface is broken
Bacterial and viral swabs used as appropriate
Take a pus or tissue sample if deeper
Blood cultures if necessary
How do you determine what type of staph is present
Best way is a coagulate test
Staph aureus is coagulase positive - gold appearance on plate
Other staphs are negative
Which strep strains are alpha haemolytic
Pneumoniae - cause of pneumonia
Viridans - commensal of mouth etc, can cause endocarditis
Which strep strains are beta haemolytic
Group A , B and C
Which strep strains are non haemolytic
Enterococcus - commensal of bowel
Can cause UTI
What is the choice of treatment for staph aureus
Flucloxacillin
What infections can staph aureus cause
Wound, skin and joint infections Cellulitis Infected eczema Impetigo SSS
What toxins can staph aureus produce
Enterotoxin - food poisoning
SSSST - causes scalded skin syndrome
PVL - caused multiple necrosing skin infections
What are the treatment options for MRSA
Doxycycline
Co-trimoxazole
Clindamycin - risk of c. Diff
Vancomycin
Where might staph epidermidis be found in the body
Common skin commensal
May cause infection in association with artificial material such as heart valves or joints
How do you treat necrotising fasciitis
Needs urgent surgical debridement
Back up with antibiotics
What is the key clinical sign for necrotising fasciitis
Pain that exceeds the visual presentation
Patient will be in excruciating pain
What is the underlying cause of leg ulcers
Vascular problems - venous or arterial
When do you need to swab a leg ulcer
ONLY when there is clinical signs of infection
There will always be commensals growing due to moist, warm nature of wound but not always requiring treatment
Name the different types of tinea infection (by location)
Tinea pedis - foot Tinea cruris - groin Tinea capris - scalp Tinea manum - hand Tinea corporis - body
What is the medical name for ringworm
Tinea
Fungal infection
Who is most commonly affected by ringworm
Men - particularly foot and groin
Children are the main group that get scalp ringworm
What are some of the sources of dermatophyte infection
Other infected humans
Animals - e.g. Cats and dogs
Soil - less common in the UK
How do you diagnose a dermatophyte
Clinical appearance
Woods light
Skin scraping, nail clippings - for microscopy and culture
Describe a candida infection
Fungal infection that infects skin folds
Looks for warm moist areas - under breasts, groin, ab skin folds
Diagnoses by swab
How do you treat a candida infection
Clotrimazole cream
Oral fluconazole
What is Norwegian scabies
A chronic crusted form of scabies - thick white crust
HIGHLY INFECTIOUS
Thousands of mites across the body
Common in elderly, debilitated or immunosuppressed patients
What is the main symptom of a louse infection
Intense itching
Can affect head, lashes body (rare now) and pubic area (sexual contact)
How do you treat lice
Malathion lotions
Physical removal with a fine tooth comb, repeated regularly to catch new lice as they emerge
Which skin infections need isolation
Group A strep
MRSA
Scabies - with extra PPE precaution if Norwegian
Describe the chickenpox virus
Varicella - clinical presentation of first exposure
Primary infection usually occurs in childhood
Presents with a generalised rash and fever
Usually only lasts a week or so - self-limiting
Virus can then become latent
Describe shingles
Caused by herpes zoster
Reactivation of the same virus as chickenpox
Often in old age
Affects the dermatome supplied by the nerve root it resided in
Describe the appearance of the chickenpox rash
Starts as macules, then to papules Moves on to vesicles which then scab over and fade Small chance of scarring Skin looks inflames comes with itch and fever
which groups are more likely to suffer severe consequences of chickenpox
The very young and very old
Immunosuppressed - e.g. leukaemia patients
What is neonatal varicella zoster virus
When a new-born develops the virus
Due to maternal infection in late pregnancy - if she has it within 5 days of delivery
Comes with higher mortality
How do you prevent neonatal VZV
If mother has never had chickenpox and has been exposed to someone with it you can give the VZ immunoglobulin to lessen the severity or prevent the case
How does the shingles rash present
Tingling and pain is the first sign
Then erythema to vesicles then crust
what is post-herpetic neuralgia
Zoster pain that continues for 4 weeks
Common in the elderly and in trigeminal shingles
what type of pain does shingles cause
sharp
neuralgic
What are the symptoms of ramsay hunt syndrome
Pain and vesicles in the ear canal and throat
Facial palsy
If CNVIII is irritated then deafness, vertigo and tinnitus
Is there a vaccine for chickenpox
Yes
A live attenuated vaccine is available
Not routine in the UK
Is there a vaccine for shingles
Same vaccine for chickenpox can be used in high titre
Can reduce chance and impact of shingles in the elderly
Routinely given to 70 year olds in the UK
What can HSV type 1 cause
Main cause of oral lesions - cold sores
Causes 1/2 of genital herpes
Encephalitis - very rare
What can HSV type 2 cause
Causes 1/2 of genital herpes
rare cause of oral lesions
Encephalitis
What is erythema multiforme
Triggered by drugs or infections
Target lesions with erythema appear
At worst can be life threatening
Describe molluscum contagiosum
Viral infection - common in kids
Fleshy, firm, umbilicated, pearlescent nodules
Usually self-limiting
how can you treat molluscum contagiosum
Usually self-limiting but can take months to fade
Can use local application of liquid nitrogen
What diseases can HPV cause
Warts/verrucas
Genital warts
Cervical cancer
head and neck cancer
what is herpangina
Blistering rash of back of mouth
Caused by enterovirus
Self-limiting
Describe hand, foot and mouth disease
caused by enteroviruses - particularly coxsackie
Can cause lesions on hand, foot and buttocks
Typically occurs in kids
Not common in UK
What is erythema infectiosum
Caused by erythrovirus
Red rash appears on cheek - known as slapped cheek disease
In adults it may present as arthritis in the small joints
What are some complications of erythema infectiosum
Spontaneous abortion
Aplastic crisis - drop in haemoglobin
Chronic anaemia
What is orf
Virus comes from sheep
Firm, fleshy nodule appears on hand
Common in farmers
Self-limiting
How does syphilis present
Priamry - painless ulcer at infection site
Secondary - red rash all over body
- prominent on soles and palms
Tertiary - CNS and cardio presentations
What causes lyme disease
Spirochete borrelia burgdorferi
Passed to humans by tick saliva when they bite - they get from infected host
Must be attached for around 24 hours to transmit
How does lyme disease present
First stage - erythema migrans
2nd stage - lymphocytoma numbess, arthralgia and myalgia, , facial paralysis, meningitis, arrythmia
3rd stage - arthritis, chronic pain and neuro problems
How do you treat lyme disease
Best is prevention! - tick repellent and early removal
Doxycycline or amoxicillin
2-3 weeks course
If more severe – IV penicillin / Ceftriaxone 14 – 21
How do you treat shingles
Treat with Acyclovir 800mg 5 times daily for 7-10 days,
Tramadol 50mg 4 times daily
Advise local cooling agents
How do insect bites typically present
Variable - small papules to bullae
Typically itchy
Often a linear pattern with grouped lesions
Asymmetrical
How do you treat insect bites
Prevention with repellents
Symptomatic - antihistamine or topical steroid
Treat pets and environment if fleas
Describe erythema migrans seen in Lyme disease
Occurs within a month of the bite and resolves in a month (days if treated)
Usually this is seen as a solitary macule, or annular (ring-shaped) lesion which can vary considerably in size - typical target appearance
How does lymphocytoma present in lyme disease
Approximately 6 months after the initial bite
Firm, bluish red swelling on earlobes of children / nipple in adults
Associated with tender, local lymphadenopathy
Describe Acrodermatitis Chronica Atrophicans seen in lyme disease
Late stage - 6/12 – 8yrs after initial infection
Characteristic blue/red discolouration progressing to atrophy
Treatable when in initial inflammatory stage
What is the causative oragnism is scabies
The arthropod sarcoptes scabiei
How is scabies spread
Direct skin-to-skin contact
It is a contagious disease and may spread extensively in residential establishments
How does scabies present
Significant itch - particularly at night
Burrows are best seen on the sides of the fingers or flexor aspect of wrist, with the mite appearing as a small dark dot at the end of the burrow
Excoriations, vesicles, eczematous or urticated papules and rubbery nodules may be seen
Describe the pathogenesis of scabies
Female mites burrow through the keratin layer of skin and lay eggs as they go
Affected individuals are asymptomatic for up to 6 weeks then delayed hypersensitivity reaction develops
Which sites are commonly affected by scabies
Fingers and web spaces
Flexor wrists
Nipples + genitals
Feet - particularly in infants
How do you treat scabies
Treat any secondary infection
Permethrin cream - top to toe , done twice, 1 week apart
Or malathion
Treat everyone in household and close contacts
Treat itch symptomatically with steroid or crotamiton cream - may take 1-4 weeks to settle
