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2.2 Microbiology > Staphylococcus Aureus > Flashcards

Staphylococcus Aureus Flashcards

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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
Q

What is the cause of S. aureus infections

A

Largely the result of an invasive process

boil -> cellulitis -> ulcer -> wound infection -> BSI

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26
Q

What is the cause of S. aureus intoxication

A

Largely the result of toxin production in the absence of invasion e.g. foodborne illness

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27
Q

What is the cause of S. aureus infection and intoxication?

A

Combination of invasion and toxin production at a distant site

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28
Q

What are the three uncomplicated SSTIs?

A

Folliculitis

Furuncles (boils)

Carbuncles

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28
Q

What are the three uncomplicated SSTIs?

A

Folliculitis

Furuncles (boils)

Carbuncles

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29
Q

What are the three uncomplicated SSTIs of hair follicles?

A

Folliculitis

Furuncles (boils)

Carbuncles

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30
Q

What is folliculitis

A

Pus forming infection of hair follicle(s)

Known as a stye if it occur sin the eyelash

Leads to a furuncle

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31
Q

What is a furuncle?
(3)

A

A boil

Extension of an infected hair follicle

Often form around foreign bodies e.g. splinters

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32
Q

What is a carbuncle?
(3)

A

Deep-seated infections of several hair follicles

Can spread to subcutaneous tissues

Can lead to bloodstream infection

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33
Q

Where do carbuncles often form?

A

Nape of the neck

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34
Q

What are the five uncomplicated SSTIs of the superficial skin

A

Impetigo

Pustular impetigo

Bullous impetigo

Cellulitis

Mastitis

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35
Q

What is impetigo?

A

Superficial skin infection involving exposed areas such as the face and legs

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36
Q

What is pustular impetigo?
(3)

A

Small blisters that rupture

Covered by a honey-coloured crust

May also be caused by streptococci

37
Q

What is bullous impetigo?
(5)

A

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
Q

What is cellulitis
(2)

A

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
Q

What is puerperal mastitis?
(2)

A

Blockage of the milk ducts in lactating mothers

Babies carry S. aureus in their nose and give it to their mothers

40
Q

List the two complicated SSTIs

A

Surgical wound infections

Deep seated abscesses

41
Q

What are surgical wound infections
(5)

A

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
Q

What are deep seated abscesses
(5)

A

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
Q

What may cause systemic infections
(2)

A

Spreading of skin infection to bones, joints or deep organs

Result of trauma

44
Q

List the five systemic infections caused by S. aureus

A

Bloodstream infections

Endocarditis

Pneumonia

Osteomyletitis/septic arthritis

Pyelonephritis

45
Q

What may cause bloodstream infections?

A

Septic focus/foci

46
Q

What may cause endocarditis

A

IV drug use

47
Q

What may cause pyelonephritis

A

Kidney infection

48
Q

What are the two main toxin-mediated infections caused by S. aureus

A

Staphylococcal Scalded Skin Syndrome

Toxic Shock Syndrome

49
Q

Write a note on staphylococcal scalded skin syndrome
(7)

A

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
Q

What is ET

A

Exfoliatin Toxin

51
Q

Write a note on toxic shock syndrome

A

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
Q

What percentage of S. aureus isolates produce TSST

A

20% of all S. aureus isolates

53
Q

What does TSST production cause?
(6)

A

Fever
Hypotension
Multi-system shut down
Organ failure
Toxic shock
8% mortality

54
Q

What percentage of people who get TSS die

A

8%

55
Q

When can S. aureus cause food poisoning?

A

Food handler contaminated food or food put in contact with contaminated equipment or preparation surfaces

After prep-contamination food not properly refrigerated

56
Q

What cause S. aureus to cause food poisoning/gastroenteritis

A

30-50% of S. aureus produce enterotoxins

57
Q

What foods can be contaminated with S. aureus ?

A

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
Q

How does S. aureus cause a foodborn illness
(3)

A

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
Q

What is intoxification?

A

Ingesting of pre-formed enterotoxins in food

60
Q

What are the tell tale signs of foodborne illness caused by S. aureus
(3)

A

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
Q

What are the five mechanisms of adherence in S, aureus?

A

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
Q

What does teichoic acid do?

A

Binds fibronectin

63
Q

What is biofilm?
(2)

A

Majority of clinical S. aureus isolates express capsular polysaccharide

Polysaccharide intracellular adhesion + teichoic acid facilitates adherence to host cells and medical devices

64
Q

What does biofilm promote
(3)

A

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
Q

What is bound coagulase also called?

A

Clumping factor

66
Q

Where is clumping factor expressed and with what?

A

Expressed on the surface of the cell wall with free extracellular staphylocoagulase

67
Q

What percentage of staph strains express clumping factor and free extracellular staphylocoagulase?

A

97%

68
Q

What does coagulase do?
(3)

A

Binds to fibrinogen on cell surfaces

This converts fibrinogen to fibrin

This produces fibrin clots

69
Q

What does the production of fibrin clots do?
(3)

A

Causes agglutination of bacterial cells

Promotes bacterial attachment

White cells penetrate fibrin clots poorly

70
Q

What does coagulase do?
(4)

A

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
Q

What are the four mechanisms of immune evasion by the host?

A

Coagulase - protects from phagocytosis

Capsular polysaccharide inhibits phagocytosis

Catalase production allows for survival in phagocytes

Protein A prevents opsonisation by antibodies

72
Q

How does catalase production facilitate the survival of S. aureus in phagocytes?

A

Catalase neutralises the production of oxygen free radicals in phagocytes

73
Q

What does protein A do?
(4)

A

A surface protein

Binds IgG molecules by the Fc region

IgG is bound in he wrong orientation

This disrupts opsonisation and phagocytosis

74
Q

What is S. aureus main mechanisms of invasions
(4)

A

Invasins
Membrane-damaging exotoxins
Tissue-damaging toxins
Food poisoning toxins

75
Q

What are invasins?

A

Degradative enzymes promote spread in tissues

76
Q

What are the three invasins

A

Staphylokinase
Hyaluronidase
Thermostable deoxyribonuclease

77
Q

What does staphylokinase do?

A

Dissolves fibrin clots laid down by during inflammation to try to wall of the infection

78
Q

What does hyaluronidase do?

A

Hydrolyses matrix of connective tissue

79
Q

What does thermostable deoxyribonuclease do

A

Releases nutrients for organism

80
Q

What do membrane-damaging exotoxins do?

A

Lyse host cells

81
Q

What are the two membrane-damaging exotoxins?

A

Panton-Valentine Leukocidan (PVL)

Haemolysins

82
Q

What does Panton-Valentine Leukocidan (PVL)?

A

Produces pores in leukocytes - common in community-associated infection

83
Q

What does haemolysins do?

A

Produces pores in red blood cells

84
Q

What are the two tissue-damaging toxins

A

Exfoliatin toxin (ET)

Toxic shock syndrome toxin-1 (TSST-1)

85
Q

What does Exfoliatin toxin do?
(3)

A

Causes staphylococcal scalded skin syndrome (SSSS)

Act on desmosomes causing epidermal splitting

Produced by 5% of S. aureus isolates

86
Q

What does TSST-1 do?

A

Causes S. aureus Toxic Shock Syndrome
Produced by -20% of S. aureus isolates

87
Q

Write about the food poisoning toxins of S. aureus
(5)

A

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
Q

Describe the relationship between exfoliatin, TSST-1 and enterotoxin and the immune syste,

A

These all act as superantigens to stimulate the enhanced immune response

89
Q

Write about antibiotic resistance in S. aureus
(5)

A

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
Q

Comment on the trends in S. aureus BSI infection in Ireland, as well as MRSA
(2)

A

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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