micro lecture 11

Question 1

list 5 G+ cocci (spherical)

which ones are anaerobes

Answer 1

Staphylococci
Streptococci
Enterococci
Peptococci
Peptostreptococci

anaerobes are Peptococci and Peptostreptococci

Question 2

list 5 G+ rods

and which ones are anaerobes

Answer 2

Clostridia
Corynebacteria (diphtheroids)
Listeria
Bacillus

the anaerobe is clostridia

Question 3

list the Two most common groups of G+ organisms of medical importance:

Answer 3

Staphylococci

Streptococci

Question 4

what looks like a bunch of grapes

Answer 4

Staphylococci

Question 5

list 7 facts about Staphylococci

Answer 5

staphyle - bunches of grapes

coccus – spherical bacterium

catalase-positive (good at dealing with oxidative defensive processes)

hardy (survive quite well outside animal host)

ubiquitous in nature, many are normally harmless commensals

infections can be relatively trivial or rapidly fatal

often resistant to natural penicillins (Pen G/V)

Question 6

list 6 Characteristics of Staphylococcus aureus

Answer 6

golden colonies (hence “aureus”)

most virulent Staphylococcus

coagulase producer: fibrinogen (blood) to fibrin (surface coating and protection from the immune system)

frequently carried by healthy people

even MRSA often found on/in patients and healthcare staff
thus, simple colonisation is NOT infection
… but it can lead to infection

Question 7

what is the function of Protein A (Cell wall factor) in Staphylococcus aureus

Answer 7

Binds to IgG (prevents opsonisation)

Exerts anti-phagocytic effect

Question 8

what is the function of Fibronectin-binding protein/FnBP (Cell wall factor) in Staphylococcus aureus

Answer 8

Promotes binding to mucosal cells and tissue matrices

Question 9

what is the function of Cytolytic exotoxins in Staphylococcus aureus

Answer 9

Haemotoxins: PVL is most significant
(Panton-Valentine Leucocidin)
Attack mammalian cell membranes

Question 10

what is the function of Superantigen exotoxins in Staphylococcus aureus

Answer 10

Stimulate enhanced T cell response

🡪 toxic shock syndrome

Question 11

what happens when Staphylococcus aureus causes Infections of mild / moderate significance?

Answer 11

Localised skin infections most common

Hair follicles
Sweat / sebaceous glands
Subcutaneous abscesses
Impetigo (especially children)  
Superficial, rapidly spreading, contagious

Question 12

what happens when Staphylococcus aureus causes moderate / severe significance?

Answer 12

Localised skin infections (cellulitis)

Dermis / subcutaneous fat (lymph system possibly involved)
May respond to local therapy (if mild)
but can lead to bacteraemia
More rigorous treatment for moderate/severe infection

Question 13

S. aureus can be highly invasive and can cause significant localised damage

true or false

Answer 13

true

Question 14

what are the major Clinical significance of S. aureus

Answer 14

Septicaemia / bacteraemia

• Often from starter lesion on skin wound,
• cannula, drain etc (ie often nosocomial)

Endocarditis (heart valves)

• IV drug users, or after bacteraemia.
• Often quite rapidly damaging infection

Pneumonia (often after ‘flu)
-Can be severe

Question 15

what are the major Clinical significance of S. aureus toxinoses?

Answer 15

toxic shock syndrome
-Fever, rash, vomiting, diarrhoea, hypotension, multi-
organ involvement
-Rapid, severe, caused by “instant” activation of T cell
response
-Related to unremoved tampons, surgical dressings,
nasal packs
(much less common now that this has been
recognised)

Scalded skin syndrome (moderate)

• Result of specific toxins
• Epithelial desquamation (skin peels)

Gastroenteritis (moderate)

• Ingestion of contaminated food
• Nausea and vomiting

Question 16

what are facts of treating S. aureus

Answer 16

Treatment

Destructive infections need aggressive treatments

Source control where possible: Incision / drainage, removal of infected materials
Antibiotics (sometimes to suppress toxin productions as well as to kill)
Choice limited to resistance patterns
β−lactamase resistant lactams (flucloxacillin) would often be first line choice

Question 17

list 2Coagulase-negative staphylococci and their characteristics

Answer 17

Staphylococcus epidermidis

• Part of the normal flora, especially on skin
• Lower virulence than S. aureus
• Implants and catheters (i.e. surface colonisation)
• Acquired drug resistance (β-lactams) is more
• common than in S. aureus
• vancomycin-sensitive

Staphylococcus saprophyticus (saprophyte: digests waste/decaying matter)
-Part of the normal flora of female genital tract
perineum on both male/female
-Cystitis (infection of the bladder)
-especially in younger women
(“honeymoon cystitis”) … within 24 h after intercourse
-Sensitive to most antibiotics