Staphylococcus Aureus Flashcards
S. aureus is a commensal found in what percentage of people?
30-40%
Where is S. aureus found as a commensal
(4)
Nasal passages
Skin
Skin glands
Mucous membranes
Why is S. aureus so significant
(2)
One of the most common and virulent pathogens
Causes more frequent and varied types of infections in community and healthcare environments than any other human pathogen
S. aureus can cause what three types of infections
Suppurative infections
Systemic infections
Toxin-associated infection
What is a suppurative infection
Pus producing infection
What acts as the primary reservoir of S. aureus?
Humans -> especially the nose
Why is S. aureus good at colonising the nose and skin?
Grow at high salt and lipid concentrations
How does S. aureus use the environment as a reservoir?
(2)
S. aureus can survive for long periods on dry inanimate objects
S. aureus can remain alive for several months when dried in pus, bed clothes or dust
In general where can S. aureus survive?
In almost any environment in which humans coexist
S. aureus can survive on inanimate objects for how long?
Months
What are the two types of transmission
Direct transmission
Indirect transmission
In what two ways does direct transmission of S. aureus occur?
(2)
Contact with an infected person
Inhalation with an infected droplet
Give an example how you can get infected with S. aureus through contact with an infected person
(2)
Contact with contaminated hands
In healthcare -> cross contamination from an infected or colonised patient/staff member
By what two means does indirect transmission of S. aureus occur?
Contact with contaminated objects/surfaces
Consumption of contaminated food
How does contact with contaminated objects/surfaces cause HCAI?
Contact with the critical devices, bedding or clothing of infected patients
How does S. aureus live as a commensal on humans?
(4)
It exists in low numbers on a healthy host
It co-exists as a commensal
It colonises the skin and mucous memebranes
It does not penetrate the tissue
When can S. aureus switch from being a commensal to a pathogen
(3)
If skin or mucous membranes are damaged
If ingested from contaminated food
Infection will only occur if there is sufficient numbers
What type of suppurative infections does S. aureus cause
Skin and soft tissue infections
What four systemic infections does S. aureus cause
Bloodstream infections
Endocarditis
Pneumonia
Osteomyelitis
What three toxin-associated infections does S. aureus cause
Toxic shock syndrome
Scaled skin Syndrome
Gastroenteritis
What does SSTI stand for
Skin and Soft Tissue Infections
Classify SSTIs
Uncomplicated SSTI
Complicated SSTI
What are uncomplicated SSTIs
Superficial infections of the hair follicles and skin
What are complicated SSTIs
(4)
Deep seated infections
Infections that require surgical intervention
Wound infections
Deep abscesses of the breast, kidney, brain etc
What is the cause of S. aureus infections
Largely the result of an invasive process
boil -> cellulitis -> ulcer -> wound infection -> BSI
What is the cause of S. aureus intoxication
Largely the result of toxin production in the absence of invasion e.g. foodborne illness
What is the cause of S. aureus infection and intoxication?
Combination of invasion and toxin production at a distant site
What are the three uncomplicated SSTIs?
Folliculitis
Furuncles (boils)
Carbuncles
What are the three uncomplicated SSTIs?
Folliculitis
Furuncles (boils)
Carbuncles
What are the three uncomplicated SSTIs of hair follicles?
Folliculitis
Furuncles (boils)
Carbuncles
What is folliculitis
Pus forming infection of hair follicle(s)
Known as a stye if it occur sin the eyelash
Leads to a furuncle
What is a furuncle?
(3)
A boil
Extension of an infected hair follicle
Often form around foreign bodies e.g. splinters
What is a carbuncle?
(3)
Deep-seated infections of several hair follicles
Can spread to subcutaneous tissues
Can lead to bloodstream infection
Where do carbuncles often form?
Nape of the neck
What are the five uncomplicated SSTIs of the superficial skin
Impetigo
Pustular impetigo
Bullous impetigo
Cellulitis
Mastitis
What is impetigo?
Superficial skin infection involving exposed areas such as the face and legs
What is pustular impetigo?
(3)
Small blisters that rupture
Covered by a honey-coloured crust
May also be caused by streptococci
What is bullous impetigo?
(5)
Large, fragile, water filler blisters
Leave a bright ref, inflamed area
Nearly always caused by S. aureus
Not deep -> usually resolved with ointment
Occur mostly in kids
What is cellulitis
(2)
Infection of connective tissue underlying the skin especially the arms and legs
Can spread to lymph nodes and bloodstream
Burns patients are particularly susceptible -> burn cellulitis/impetigo
What is puerperal mastitis?
(2)
Blockage of the milk ducts in lactating mothers
Babies carry S. aureus in their nose and give it to their mothers
List the two complicated SSTIs
Surgical wound infections
Deep seated abscesses
What are surgical wound infections
(5)
Infections of a surgical wound within 30 days of operation or 1yr after implant
1-3% of surgeries end in infection
Risk increases with underlying conditions, age and immune compromised
Significant risk of bloodstream infection
Burns patient very high risk of BSI -> if infected graft will lift off
What are deep seated abscesses
(5)
Abscess = pus-filled area under the skin
S. aureus can cause abscesses at almost any site in the body
Can cause single or multiple
Breast abscess occurs in 1-3% of mothers -> result of blocked milk ducts
Often seen in kidney and brain from septic foci
What may cause systemic infections
(2)
Spreading of skin infection to bones, joints or deep organs
Result of trauma
List the five systemic infections caused by S. aureus
Bloodstream infections
Endocarditis
Pneumonia
Osteomyletitis/septic arthritis
Pyelonephritis
What may cause bloodstream infections?
Septic focus/foci
What may cause endocarditis
IV drug use
What may cause pyelonephritis
Kidney infection
What are the two main toxin-mediated infections caused by S. aureus
Staphylococcal Scalded Skin Syndrome
Toxic Shock Syndrome
Write a note on staphylococcal scalded skin syndrome
(7)
Caused by acute exfoliation of skin following cellulitis
S. aureus produces exfoliatin toxin (ET) in lesion
ET is absorbed into bloodstream and causes desquamation of skin at remote site
ET acts on desins in the skin -> skin separates and falls off
ET produced by 5% of S. aureus isolates
Outbreaks in nurseries -> most cases seen in children under 5
Child usually recovers after 10 days when new skin replaces the old skin
What is ET
Exfoliatin Toxin
Write a note on toxic shock syndrome
Used to only be associated with vaginal colonisation of S. aureus that produce TSS toxin (TSST)
- used to be only caused by use of super absorbent tampons
Recently incidence in women reduced to 1/100,000
Now its recognised more so as a complication of S. aureus surgical wound infections
20% of S. aureus produce TSST
TSST results in fever, hypotension, multi-system shut down, organ failure, toxic shock
8% mortality rate
What percentage of S. aureus isolates produce TSST
20% of all S. aureus isolates
What does TSST production cause?
(6)
Fever
Hypotension
Multi-system shut down
Organ failure
Toxic shock
8% mortality
What percentage of people who get TSS die
8%
When can S. aureus cause food poisoning?
Food handler contaminated food or food put in contact with contaminated equipment or preparation surfaces
After prep-contamination food not properly refrigerated
What cause S. aureus to cause food poisoning/gastroenteritis
30-50% of S. aureus produce enterotoxins
What foods can be contaminated with S. aureus ?
Foods that are prepared with hand contact and require no additional cooking - ready to eat
- salads, ham, egg, tuna, chicken, potato
- bakery products such as cream-filled pastries
- sandwiches
- milk and dairy products
How does S. aureus cause a foodborn illness
(3)
Toxigenic strain of S, aureus multiples in contaminated food and releases an enterotoxin
Food poisoning results from intoxication
Need very high concentration of S. aureua (10^5 organisms/gram) to produce 1ug enterotoxin and cause symptoms
What is intoxification?
Ingesting of pre-formed enterotoxins in food
What are the tell tale signs of foodborne illness caused by S. aureus
(3)
Acute onset 2-6 hours after ingestion
Severe nausea, vomiting, abdominal pain and diarrhoea triggered by toxin rather than infection
Self-limiting infection and resolved in 8 to 24 hours
What are the five mechanisms of adherence in S, aureus?
Many surface proteins that promote attachment
Most strains express fibronectin binding proteins
Possess Teichoic acid in cell wall-binds fibronectin
Biofilm
Bound coagulase and free extracellular staphylocoagulase
What does teichoic acid do?
Binds fibronectin
What is biofilm?
(2)
Majority of clinical S. aureus isolates express capsular polysaccharide
Polysaccharide intracellular adhesion + teichoic acid facilitates adherence to host cells and medical devices
What does biofilm promote
(3)
Chronic and persistent infections
Device associated infection - medical devices e.g. catheters implanted artificial heart valves and joint prosthetics
Increased tolerance to disinfectants, antibiotics and phagocytosis
What is bound coagulase also called?
Clumping factor
Where is clumping factor expressed and with what?
Expressed on the surface of the cell wall with free extracellular staphylocoagulase
What percentage of staph strains express clumping factor and free extracellular staphylocoagulase?
97%
What does coagulase do?
(3)
Binds to fibrinogen on cell surfaces
This converts fibrinogen to fibrin
This produces fibrin clots
What does the production of fibrin clots do?
(3)
Causes agglutination of bacterial cells
Promotes bacterial attachment
White cells penetrate fibrin clots poorly
What does coagulase do?
(4)
Converts fibrinogen to fibrin -> fibrin is deposited
The clot sticks to staphylococci and promotes bacterial adhesion
This helps to wall off the staph from the host and protect them from phagocytosis
Coagulase aids in invasion and evasion
What are the four mechanisms of immune evasion by the host?
Coagulase - protects from phagocytosis
Capsular polysaccharide inhibits phagocytosis
Catalase production allows for survival in phagocytes
Protein A prevents opsonisation by antibodies
How does catalase production facilitate the survival of S. aureus in phagocytes?
Catalase neutralises the production of oxygen free radicals in phagocytes
What does protein A do?
(4)
A surface protein
Binds IgG molecules by the Fc region
IgG is bound in he wrong orientation
This disrupts opsonisation and phagocytosis
What is S. aureus main mechanisms of invasions
(4)
Invasins
Membrane-damaging exotoxins
Tissue-damaging toxins
Food poisoning toxins
What are invasins?
Degradative enzymes promote spread in tissues
What are the three invasins
Staphylokinase
Hyaluronidase
Thermostable deoxyribonuclease
What does staphylokinase do?
Dissolves fibrin clots laid down by during inflammation to try to wall of the infection
What does hyaluronidase do?
Hydrolyses matrix of connective tissue
What does thermostable deoxyribonuclease do
Releases nutrients for organism
What do membrane-damaging exotoxins do?
Lyse host cells
What are the two membrane-damaging exotoxins?
Panton-Valentine Leukocidan (PVL)
Haemolysins
What does Panton-Valentine Leukocidan (PVL)?
Produces pores in leukocytes - common in community-associated infection
What does haemolysins do?
Produces pores in red blood cells
What are the two tissue-damaging toxins
Exfoliatin toxin (ET)
Toxic shock syndrome toxin-1 (TSST-1)
What does Exfoliatin toxin do?
(3)
Causes staphylococcal scalded skin syndrome (SSSS)
Act on desmosomes causing epidermal splitting
Produced by 5% of S. aureus isolates
What does TSST-1 do?
Causes S. aureus Toxic Shock Syndrome
Produced by -20% of S. aureus isolates
Write about the food poisoning toxins of S. aureus
(5)
Responsible for staphylococcal food poisoning
Produced by 30-50% of S. aureus strains
Heat stable (100 degrees for 10 minutes)
Resistant to gastric and jejunal enzyme
Ingestion cause vomiting, as it acts on the vagus nerve endings in the stomach) and diarrhoea (stimulate intestinal peristalsis)
Describe the relationship between exfoliatin, TSST-1 and enterotoxin and the immune syste,
These all act as superantigens to stimulate the enhanced immune response
Write about antibiotic resistance in S. aureus
(5)
Virtually all strains of S. aureus are penicillin resistant
Methicillin was the drug of choice until 1970 when MRSA emerged
MRSA also resistant to other B-lactam antibiotics and other class such as fluoroquinolones
Initially MRSA was confined to HCAI where it become endemic but recently HA-MRSA has decreased and community-acquired CA-MRSA has emerged
CA-MRSA is often more virulent (PVL positive) but does not display a broad spectrum of antibiotic resistance
Comment on the trends in S. aureus BSI infection in Ireland, as well as MRSA
(2)
Proportion of S. aureus BSI caused by MRSA has been decreasing for the last 12 years
In 2021 11.6% of S. aureus BSI was caused by MRSA
