Microbiology for Surgery

Question 1

What are the most common causative organisms for surgical site infections following clean procedures (i.e. where the GI tract has not been exposed) ?

What organisms are unlikely to be found following the above procedure?

Answer 1

Common:

1. Staphylococcus Auerus - COMMONEST
2. Streptococcus Pyogenes (Group A beta-heamolytic strep)
3. Group C or G beta-heamolytic streptococci

Rare:

1. Anaerobic organisms E.g. bacterioids or clostridium spp
2. Group B beta-heamolytic strep

Question 2

What organisms will microbiologists routinely ignore on wound swab cultures and why?

Answer 2

• Enterococci
• Pseudomonas spp.
• Other aerobic gram negative bacillis

Why: Likely due to contamination or colonisation rather than true infection by these organisms. Only relevant if cultured heavily from the same wound on multiple occasions or the patient is immunocompromised.

Question 3

What empirical antibiotic therapy is most appropriate for a surgical wound infection?

Answer 3

IV Flucloxacillin with switch to oral flucloxacillin when well enough. Administer for 7-10 days.

Question 4

What are two possible side effects of Co-Amoxiclav treatment?

Answer 4

1. Cholestatic jaundice

2. Predisposition to C. Difficile

Question 5

What antibiotic therapy is most appropriate for treatment of meticillin sensitive staph aureus ?

What is an alternative in penicillin allergy?

Answer 5

IV Flucloxacillin with oral switch after 48-72 hours.

or…

Clindamycin (Under 65 years only), clarithromycin or vancomycin.

Question 6

What antibiotic therapy is most appropriate for treatment of Streptococcus Pyogenes?

What would you add in necrotising fasciitis?

What is an alternative in penicillin allergy?

Answer 6

IV Benzylpenicillin with oral switch to phenoxymethylpenicillin or amoxicillin after 48-72hours.

Add in clindamycin in necrotising fasciitis.

or…

Clindamycin (under 65 years only), clarithromycin or vancomycin.

Question 7

What antibiotic therapy is most appropriate for treatment of Meticillin resistant staph aureus?

Answer 7

IV Vancomycin with oral switch to clarithromycin, tetracycline or linezolid.

Question 8

What are the most likely causative organisms in early onset prosthetic valve endocarditis?

Answer 8

• Coagulase negative staphylococci

- Staph aureus

Question 9

What is the definition of early onset prosthetic valve endocarditis?

Answer 9

Endocarditis of a prosthetic heart valve occurring less than 60 days after valve replacement.

Question 10

What is the definition of late onset prosthetic valve endocarditis?

Answer 10

Endocarditis of a prosthetic heart valve occurring more than 60 days after valve replacement.

Question 11

What are the most likely causative organisms in late onset prosthetic valve endocarditis?

Answer 11

• Coagulase negative staphylococci

- Staph aureus possible but less common than in early onset.

Question 12

What is the most appropriate antibiotic therapy for prosthetic valve endocarditis?

Answer 12

IV vancomycin + rifampicin for 6-8 weeks.
+
IV gentamycin for the first 2 weeks.

Question 13

Why are coagulase negative staphylococci commonly found in prosthetic infections?

Answer 13

Because they produce a slime (exopolysacharide) which allows adhesion to plastic surfaces.

Question 14

What is an appropriate prophylactic antibiotic regimine for a patient undergoing bowel surgery?

Answer 14

LTHT: Single dose of co-amoxiclav

Other options:

• Cefuroxime + metronidazole (single dose)
• Gentamycin + metronidazole (single dose)

Question 15

What organisms are typically found in the normal gut flora?

Answer 15

• Anaerobes

- Gram negative bacilli