ID, Microbiology and Antimicrobial Cases (Small Group Session 18)

Question 1

What are the common organisms that are associated with liver abscesses? Most common?

Answer 1

• Bacterial
  
  • Aerobic organisms (most common cause): E. coli, Klebsiella, Strep. anginosus
  • Anaerobic organisms: Bacteroides fragilis and other gut anaerobes, Fusobacterium (rare), Staph. aureus
• Parasitic (amoebic): Entamoeba histolytica, echinococcal cyst (hytadid/tape worm)
• Fungal: Candida

Question 2

Describe the mechanisms by which liver abscesses may develop?

Answer 2

Bacteria can access the liver and cause a liver abscess via a number of routes:

1. Biliary tree: blockage of the biliary tree that causes cholangitis (gall stone, tumour); ERCP
2. Hematogenous
   
   • Hepatic artery: systemic bacteremia can spread to the liver via this route
   • Portal vein: portal venous system drains the abdominal viscera. Infection from diverticulitis, pancreatitis, inflammatory bowel disease can result in pyogenic liver abscess.
3. Direct extension from contiguous focus of infection ie. cholecystitis, subphrenic abscess or perinephric abscess
4. Trauma: penetrating trauma to the liver can result in an infection

Question 3

What is the management for liver abscesses?

Answer 3

Liver abscesses require antibiotic treatment and drainage. Abscesses larger than 4 cm in diameter require drainage as antibiotics alone cannot penetrate and clear larger abscesses. The abscess fluid should be sent to the microbiology laboratory for areobic and anaerobic bacterial culuture to source of infection. Do not drain a hytadid cyst.

Question 4

What possible antimicrobial regimens might you use in the treatment of polymicrobiial bacterial liver abscesses?

Answer 4

1. Ceftriaxone (covers gram negatives and streptococci) and metronidazole (anaerobe and parasite coverage)
2. Piperacillin-tazobactam (covers gram -, streptococci,enterococci, and anaerobes)
3. Ampicillin/Cell bactam (gram +, anaerobes, enterococcus)

Question 5

Name two gram negative bacilli resembling coliforms that are associated with liver abscesses.

Answer 5

1. E. coli
2. Klebsiella

Question 6

Name two gram positive cocci in chain bacteria types associated with liver abscesses.

Answer 6

1. Enterococcus species
2. Streptococcus anginosus

Question 7

What bacteria and viruses are the common causes of traveller’s diarrhea?

Answer 7

Bacterial

• Enterotoxigenic E. coli (most common cause)
• Salmonella
• Campylobacter
• Shigella
• Vibrio

Viral

• Norovirus
• Rotavirus

Protozoal is rare but cyclospora is endemic to Peru (as per travel hx of small group case).

Question 8

Name two antibiotics that may be used in the treatment of acute traveler’s diarrhea.

Answer 8

1. Ciprofloxacin (fluoroquinolone)
2. Azithromycin (macrolide)

Question 9

What is the mechanism of action of ciprofloxacin?

Answer 9

Ciprofloxacin is a fluoroquinolone. It inhibits bacterial DNA replication by inhibiting the DNA gyrase enzyme that coils/uncoils DNA. This results in cell death (bacteriocidal).

Question 10

What is the mechanism of action of azithromycin?

Answer 10

Azithromycin is a macrolide. It inhibits protein synthesis by binding the 50S ribosome. It is bacteriostatic.

Question 11

A 55 yr old woman presents with a one month history of epigastric abdominal pain and nausea. You are suspicious for H. pylori infection and order a urea breath test, which is positive. An upper endoscopy is performed which shows 2 shallow ulcers in the duodenum. Biopsy of these lesions demonstrate active inflammatory cells and curved gram negative bacilli confirmed to be H. pylori.

What is the first line treatment?

Second line?

Answer 11

First Line Therapy = Triple Therapy

1. PPI + amoxicillin 5 days + clarithromycin following 5 days
2. PPI + metronidazole 5 days + amoxicillin following 5 days

• Sequential therapy is best

Second Line Therapy = Quadruple Therapy

1. PPI + bismuth qid + 250 mg metronidazole 5 days + 500 mg tetracycline following 5 days

Question 12

What is the mechanism of action of Amoxicillin (penicillin)?

Answer 12

Amoxicillin inhibits peptidoglycan cross-linkage and is therefore a cell wall inhibitor.

Question 13

What is the mechanism of action of clarithromycin?

Answer 13

Clarithromycin inhibits protein synthesis via bacterial 50S subunit.

Question 14

What is the mechanism of action of metronidazole?

Answer 14

Metronidazole inhibits nucleic acid synthesis.

Question 15

What is the mechanism of action of tetracycline?

Answer 15

Tetracycline inhibits protein synthesis via 30S ribosomal subunit.

Question 16

Name 2 microbial agents from 2 different classes that may be used to treat a candida infection.

Answer 16

Fluconazole: inhibits fungal cytochrome P450, which prevents production of ergosterol (essential to cell membrane)

Nystatin: binds to ergosterol and forms pores in cell membrane

Echinocandin: inhibits synthesis of glucan, which is part of the fungal cell wall

Amphotericin = amphoterrible; don’t use unless you have to.