M4: Staphylococci and staphylococal infections

Question 1

Outline characteristics of the staphylococcus genus

Answer 1

1. Gram-positive arranged in grape-like clusters
2. Non-motile, non-spore forming
3. Catalase-positive
4. Aerobic and anaerobic metabolism
5. Resistant to dry and high salt conditions
6. Found mainly on skin and mucous membranes

Question 2

Give examples of coagulase positive staphylococcus species

Answer 2

S. aureus

S. intermedius

Question 3

Give examples of coagulase negative staphylococcus species

Answer 3

S. epidermidis
S. haemolyticus
S. lugdunensis

Question 4

What are the main diagnostic features of staphylococcus aureus coagulase positive pathogens

Answer 4

• Colonies are pigmented (carotenoid pigments)
• Extra-cellular coagulase production
• Nuclease production that breaks down DNA
• Production of cell surface-associated enzyme clumping factors or bound coagulase

Question 5

What are the pyogenic infections caused by staphylococci aureus

Answer 5

1. Boils, carbuncles
2. Wound (surgical( infection
3. Abscesses
4. Impetigo
5. Mastitis
6. Septicaemia
7. Osteomyelitis
8. Pneumonia
9. Endocarditis

Question 6

What are the toxin-mediated infections caused by staphylococci aureus

Answer 6

1. Scalded skin syndrome
2. Pemphigus neonatorum
3. Toxic shock syndrome
4. Food poisoning

Question 7

What is the pathogenesis of staph. aureus

Answer 7

It is an opportunistic pathogen which causes infection at sites of lowered host resistance e.g. damaged skin or mucous membranes/ staphylococcal pneumonia which is a complication of influenza

Question 8

What do enterotoxins/pyrogenic exotoxins produced by staph. aureus result in

Answer 8

These heat stable proteins induce nausea, vomiting and diarrhoea within a few hours and is most commonly due to contaminated foods e.g. processed meats, custard filled pastries, potato salads and ice cream

Question 9

What do epidermolytic toxins/exfoliatins produced by staph. aureus result in

Answer 9

There are two types of toxins, A and B which cause

• Pemphigus neonatorum (distended blisters)
• Impetigo (flattened blisters)
• Scalded skin syndrome (extensive area of skin lost) - this is mainly seen in children which lack the neutralising antitoxin

Question 10

Describe pustular impetigo

Answer 10

Vesicles are at different stages of development so some are pus-filled vesicles on an erythematous base, dry or crusted lesions

Question 11

How does staph. aureus cause toxic shock syndrome

Answer 11

This is associated with highly absorbent tampons and vaginal colonisation with TSST-1 producing S. aureus in presence of tampon creates the conditions for bacterial multiplication and toxin production

Question 12

What do haemolysins produced by staph. aureus result in

Answer 12

They are cytotoxic and result in lysis of RBCs and other phagocytic tissue cells

Question 13

What do Pantones-valentine leucocidin produced by staph. aureus result in

Answer 13

Destruction of WBCs increasing resistance to phagocytosis and this is associated with community acquired MRSA

Question 14

What do hyaluronidase produced by staph. aureus result in

Answer 14

Break down of intracellular ground substance (HA) of tissues

Question 15

How do cell wall polymers act as virulence factors for staph. aureus

Answer 15

Peptidoglycans - inhibit inflammatory response, endotoxin-like activity

Lipoteichoic acid - interacts with toll-like receptors

Question 16

How do cell surface proteins act as virulence factors for staph. aureus

Answer 16

Protein A - reacts with Fc region go IgG
Clumping factor - binds to fibrinogen
Fibronectin-binding protein - binds to fibronectin

Question 17

What are the sources of infection for staph. aureus

Answer 17

1. Infected lesions
2. Healthy carriers
3. Animals

Transferred by direct contact, air-borne dust and air-borne droplet nuclei

Question 18

What samples can be taken for the laboratory diagnosis of staph. aureus

Answer 18

• Pus from abscesses, wounds and burns
• Sputum from cases of pneumonia
• Faeces or vomit from those with food poisoning
• Blood from patients with bacteraemia
• Mid-stream urine for those with cystitis or pyelonephritis
• Anterior nasal or perineal swabs from suspected carriers

Question 19

How can patients with staph. aureus infections be treated

Answer 19

Sensitive to many antimicrobial agents but are resistant to benzyl penicillin

Question 20

How does the production of penicillinase enzyme induce antibiotic resistance

Answer 20

Because it inactivates penicillin drug by opening the beta-lactam ring of penicillin molecule as well as ampicillin and amoxicillin

Question 21

Which drugs are resistant to penicillinases

Answer 21

1. Methicillin
2. Cloxacillin
3. Cephalosporin drugs
4. Enzyme inhibitors clavulanic acid

Question 22

How can a mutation induce antibiotic resistance

Answer 22

If the essential penicillin-binding protein found in bacterial cell envelope mutates

Question 23

How can glycopeptides induce antibiotic resistance

Answer 23

GISA causes thickened cell walls

Question 24

What drug is used in place of vancomycin

Answer 24

Synercid because people have vancomycin resistance

Question 25

Describe coagulase-negative staphylococci

Answer 25

• Morphologically similar to S. aureus
• Non-pigmented
• Carried as part of the normal skin flora
• Don’t normally cause infection

Question 26

What infections are caused by coagulase-negative staphylococci

Answer 26

Infected prostheses/implants
Ventriculitis (shunt associated)
Peritonitis 
Septicaemia 
Endocarditis (with prosthetic valves)

Question 27

What treatment is given for coagulase-negative staphlococci

Answer 27

Resistant to; penicillin, gentamicin, erythromycin and chloramphenicol

Treated with; vancomycin