Prosthetic hip wound

Question 1

What are the main sources or wound infections?

Answer 1

Environment - operating room and staff
Surrounding skin - endogenoud flora
If appropriate, flora from closest endogenous mucous membrane

Question 2

What are the top six pathogens isolated from surgical wounds?

Answer 2

S. aureus
CoNS
Enterococcus spp
E Coli
Pseudomonas Aeruginosa
Enterobacter spp

Question 3

What is CoNS?

Answer 3

• Normal flora on skin and mucous
• Key opportunistic pathogens – forms biofilm
• Common cause of infection of indwelling devices, postsurgical endophtalmitis, endocarditis, UTIs, bacteremia
• S. Epididermis causes over 80% CoNS infections

Question 4

What clinical cues should be considered for wound infections?

Answer 4

• Is there a blister over the incision? Suggests staph or strep
• Is the site warm / tender? Is there discharge? Suggests staph
• Is there an odor? Sweet and fruity suggests pseudomonas; putrid implies anaerobes
• On palpitation, is there crepitance / crackling sound? This is from gas production; suggests anaerobes or gram negative rods

Question 5

How should samples of wound infections be taken?

Answer 5

a) Request both routine and anaerobic cultures of purulent material and joint fluid. Take blood cultures if fever, because wound cultures frequently cause bacteremia
b) Specimen must be delivered within two hours to preserve anaerobic sample

Question 6

How can you store a sample anaerobically?

Answer 6

In a gaspak jar

Question 7

Is gram staining used for diagnosis of wound cultures?

Answer 7

Yes

Question 8

Which tests are typically performed on wound cultures?

Answer 8

Gram staining
Hemolysis pattern to differentiate staph and stre
Catalase test
Coagulase test

Question 9

What is a MSA / mannitol-sugar agar?

What does it distinguish?

Answer 9

7.5% NaCl, Mannitol, PH indicator, various nutrition

Selects for halotolerant bacteria and for mannitol fermentation.
Those that ferment mannitol produce lactic acid => PH indicator turns pink.

Distinguishes S. epididermis and S. aureus

Question 10

What percentage of hospital-based Staph Aureus strains are MRSA?

Answer 10

50-60%

Question 11

What is the treatment of choice for MRSA?

Answer 11

Vancomycin (although vancomycin-resistant MRSA and Vancomycin-intermediate have emerged)

Fifth-generation cephalosporins now have some resistance to beta-lactamases

Question 12

How can you tell whether an S Aureus infection is MRSA?

Answer 12

Perform an antibiotic susceptibility test.

Question 13

What is oxacillin

Answer 13

A penicilliase-resistant beta-lactam. Similar to methicillin; has replaced methicillin in clinical use

Question 14

What is sublactam?

Answer 14

A beta-lactamase inhibitor. Given in combination with beta-lactam antibiotics to inhibit beta-lactamase

Question 15

What are the virulence factors of S. Aureus?

Answer 15

Special peptidoglycan – more endotoxic than usual

Polysaccharide capsule; Protein A – antiphagocytic

Surface proteins – aid adhesion

Pyrogenic superantigens

Enterotoxin – food poisining

Exfoliative toxins – cause skin erythema and separation

Panton-valentine leukocidin – causes severe cutaneous infection w/ leukocyte destruction and tissue necrosis

Protease, lipase, hyaluronidase

Question 16

What skin lesions can S. Aureus cause?

Answer 16

Lesions:
Folliculitis = pimple
Furnuncle = boil; deeper pus-filled infection
Carbuncle = multi-head infection

Scalded skin syndrome – through exfoliative toxin

Question 17

What factors predispose to toxigenic S. aureus infection?

Answer 17

TSS - Vaginal / nasal tampons

Food poisining – improper food storage

Question 18

What factors predispose to pyogenic S. aureus infection?

Answer 18

Local infections: Poor skin hygeine

Systemic [e.g. sepsis, endocarditis] - IV drug use

Question 19

What factors predispose to pyogenic S. epidermidis infection?

Answer 19

Infection of indwelling / prosthetic devices

Question 20

What factors predispose to pyogenic S. saprophyticus infection?

Answer 20

Sexual activity => UTIs

Question 21

What is the main reservoir for s. aureus?

Answer 21

The human nose

Question 22

What antibiotics can be used for vancomycin resistant MRSA?

Answer 22

Linezolid
Quinipristin-dalfopristin
Daptomycin

Question 23

What is the difference in mechanism between methicillin and vancomycin?

Answer 23

Methicillin – inhibits transpeptidase activity
Class = penicillin

Vancomycin – large molecule that binds to peptide chain of peptidoglycan subunits, blocking transglycolysation / transpeptidation
Class = glycopeptides

Question 24

What is the mechanism of action of daptomycin?

Answer 24

Inserts into the cytoplasmic membrane of gram-positive bacteria. Disrupts membrane and kills cell.

Class = Lipopeptide
Narrow-spectrum, commonly used against MRSA

Question 25

Describe the mechanism of action of quinupristin / dalfopristin

Answer 25

Bind sequentially to different sites of the 50S and 70S subunits => prevent ribosomal protein synthesis

Individually, they are bacteriostatic. But bacteriocidal together

Broad spectrum

Question 26

What is the mechanism of action of linezolid?

Answer 26

Interferes with the formation of the initiation complex between 50S and 30S subunits and other factors
Class = Oxazolidinone
Bacteriostatic
Broad spectrum