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2.2 Microbiology > Staphylococcus Aureus > Flashcards

Staphylococcus Aureus Flashcards

1
Q

S. aureus is a commensal found in what percentage of people?

A

30-40%

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2
Q

Where is S. aureus found as a commensal
(4)

A

Nasal passages

Skin

Skin glands

Mucous membranes

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3
Q

Why is S. aureus so significant
(2)

A

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

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4
Q

S. aureus can cause what three types of infections

A

Suppurative infections

Systemic infections

Toxin-associated infection

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5
Q

What is a suppurative infection

A

Pus producing infection

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6
Q

What acts as the primary reservoir of S. aureus?

A

Humans -> especially the nose

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7
Q

Why is S. aureus good at colonising the nose and skin?

A

Grow at high salt and lipid concentrations

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8
Q

How does S. aureus use the environment as a reservoir?
(2)

A

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

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9
Q

In general where can S. aureus survive?

A

In almost any environment in which humans coexist

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10
Q

S. aureus can survive on inanimate objects for how long?

A

Months

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11
Q

What are the two types of transmission

A

Direct transmission

Indirect transmission

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12
Q

In what two ways does direct transmission of S. aureus occur?
(2)

A

Contact with an infected person

Inhalation with an infected droplet

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13
Q

Give an example how you can get infected with S. aureus through contact with an infected person
(2)

A

Contact with contaminated hands

In healthcare -> cross contamination from an infected or colonised patient/staff member

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14
Q

By what two means does indirect transmission of S. aureus occur?

A

Contact with contaminated objects/surfaces

Consumption of contaminated food

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15
Q

How does contact with contaminated objects/surfaces cause HCAI?

A

Contact with the critical devices, bedding or clothing of infected patients

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16
Q

How does S. aureus live as a commensal on humans?
(4)

A

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

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17
Q

When can S. aureus switch from being a commensal to a pathogen
(3)

A

If skin or mucous membranes are damaged

If ingested from contaminated food

Infection will only occur if there is sufficient numbers

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18
Q

What type of suppurative infections does S. aureus cause

A

Skin and soft tissue infections

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19
Q

What four systemic infections does S. aureus cause

A

Bloodstream infections

Endocarditis

Pneumonia

Osteomyelitis

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20
Q

What three toxin-associated infections does S. aureus cause

A

Toxic shock syndrome

Scaled skin Syndrome

Gastroenteritis

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21
Q

What does SSTI stand for

A

Skin and Soft Tissue Infections

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22
Q

Classify SSTIs

A

Uncomplicated SSTI

Complicated SSTI

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23
Q

What are uncomplicated SSTIs

A

Superficial infections of the hair follicles and skin

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24
Q

What are complicated SSTIs
(4)

A

Deep seated infections

Infections that require surgical intervention

Wound infections

Deep abscesses of the breast, kidney, brain etc

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25
What is the cause of S. aureus infections
Largely the result of an invasive process boil -> cellulitis -> ulcer -> wound infection -> BSI
26
What is the cause of S. aureus intoxication
Largely the result of toxin production in the absence of invasion e.g. foodborne illness
27
What is the cause of S. aureus infection and intoxication?
Combination of invasion and toxin production at a distant site
28
What are the three uncomplicated SSTIs?
Folliculitis Furuncles (boils) Carbuncles
28
What are the three uncomplicated SSTIs?
Folliculitis Furuncles (boils) Carbuncles
29
What are the three uncomplicated SSTIs of hair follicles?
Folliculitis Furuncles (boils) Carbuncles
30
What is folliculitis
Pus forming infection of hair follicle(s) Known as a stye if it occur sin the eyelash Leads to a furuncle
31
What is a furuncle? (3)
A boil Extension of an infected hair follicle Often form around foreign bodies e.g. splinters
32
What is a carbuncle? (3)
Deep-seated infections of several hair follicles Can spread to subcutaneous tissues Can lead to bloodstream infection
33
Where do carbuncles often form?
Nape of the neck
34
What are the five uncomplicated SSTIs of the superficial skin
Impetigo Pustular impetigo Bullous impetigo Cellulitis Mastitis
35
What is impetigo?
Superficial skin infection involving exposed areas such as the face and legs
36
What is pustular impetigo? (3)
Small blisters that rupture Covered by a honey-coloured crust May also be caused by streptococci
37
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
38
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
39
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
40
List the two complicated SSTIs
Surgical wound infections Deep seated abscesses
41
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
42
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
43
What may cause systemic infections (2)
Spreading of skin infection to bones, joints or deep organs Result of trauma
44
List the five systemic infections caused by S. aureus
Bloodstream infections Endocarditis Pneumonia Osteomyletitis/septic arthritis Pyelonephritis
45
What may cause bloodstream infections?
Septic focus/foci
46
What may cause endocarditis
IV drug use
47
What may cause pyelonephritis
Kidney infection
48
What are the two main toxin-mediated infections caused by S. aureus
Staphylococcal Scalded Skin Syndrome Toxic Shock Syndrome
49
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
50
What is ET
Exfoliatin Toxin
51
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
52
What percentage of S. aureus isolates produce TSST
20% of all S. aureus isolates
53
What does TSST production cause? (6)
Fever Hypotension Multi-system shut down Organ failure Toxic shock 8% mortality
54
What percentage of people who get TSS die
8%
55
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
56
What cause S. aureus to cause food poisoning/gastroenteritis
30-50% of S. aureus produce enterotoxins
57
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
58
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
59
What is intoxification?
Ingesting of pre-formed enterotoxins in food
60
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
61
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
62
What does teichoic acid do?
Binds fibronectin
63
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
64
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
65
What is bound coagulase also called?
Clumping factor
66
Where is clumping factor expressed and with what?
Expressed on the surface of the cell wall with free extracellular staphylocoagulase
67
What percentage of staph strains express clumping factor and free extracellular staphylocoagulase?
97%
68
What does coagulase do? (3)
Binds to fibrinogen on cell surfaces This converts fibrinogen to fibrin This produces fibrin clots
69
What does the production of fibrin clots do? (3)
Causes agglutination of bacterial cells Promotes bacterial attachment White cells penetrate fibrin clots poorly
70
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
71
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
72
How does catalase production facilitate the survival of S. aureus in phagocytes?
Catalase neutralises the production of oxygen free radicals in phagocytes
73
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
74
What is S. aureus main mechanisms of invasions (4)
Invasins Membrane-damaging exotoxins Tissue-damaging toxins Food poisoning toxins
75
What are invasins?
Degradative enzymes promote spread in tissues
76
What are the three invasins
Staphylokinase Hyaluronidase Thermostable deoxyribonuclease
77
What does staphylokinase do?
Dissolves fibrin clots laid down by during inflammation to try to wall of the infection
78
What does hyaluronidase do?
Hydrolyses matrix of connective tissue
79
What does thermostable deoxyribonuclease do
Releases nutrients for organism
80
What do membrane-damaging exotoxins do?
Lyse host cells
81
What are the two membrane-damaging exotoxins?
Panton-Valentine Leukocidan (PVL) Haemolysins
82
What does Panton-Valentine Leukocidan (PVL)?
Produces pores in leukocytes - common in community-associated infection
83
What does haemolysins do?
Produces pores in red blood cells
84
What are the two tissue-damaging toxins
Exfoliatin toxin (ET) Toxic shock syndrome toxin-1 (TSST-1)
85
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
86
What does TSST-1 do?
Causes S. aureus Toxic Shock Syndrome Produced by -20% of S. aureus isolates
87
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)
88
Describe the relationship between exfoliatin, TSST-1 and enterotoxin and the immune syste,
These all act as superantigens to stimulate the enhanced immune response
89
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
90
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

2.2 Microbiology (60 decks)

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