Antibiotics

Question 1

Describe 2 major problems with antibiotics

Answer 1

1. Bacterial resistance always happens
2. Companies not interested in developing new antibiotics
   
   • New ABx = resistance = poor money makers

Question 2

How is antibiotic activity measured?

Describe the method

Answer 2

Minimum inhibitory concentration (MIC): how much ABx is required to kill a bacteria

1. Inject test tubes with microbes
2. Add ABx to each tube in increasing []s
3. MIC: the concentration where the bacteria does not grow anymore

Question 3

How can ABx strips be used to measure MIC?

Answer 3

• 6 different paper strips with antibiotics
• More ABx at the top of the strip than at the bottom
• Bacteria are set to grow - the point at where they stop growing is the MIC (ex., 0.38 mcg)

Question 4

Describe the β Lactam antibiotics (ex., penicillin)

Answer 4

• Contain a β lactam ring
• Inhibit cell wall synthesis
  
  • Bind to proteins called PBPs (penicillin binding proteins)
  • Breaks cross links = cell wall is weak, cell dies

Question 5

Describe resistance to β lactam antibiotics

How do we combat it?

Answer 5

• Some bacteria produce β lactamase: destroys β ring = destroys ABx
• Methicillin contains a β ring that is modified to prevent β lactamase cleavage
  
  • Some bacteria produce PBP2a encoded by mec which does not bind methicillin

Question 6

Describe the vancomycin antibiotic

Answer 6

• Glycopeptide ABx, inhibits cell wall synthesis in Gram positives only (cannot get across Gram negative membranes)
• Last resort
• Binds peptide linkages at terminal D-Ala residues, inhibiting cross linkages

Question 7

Describe resistance to vancomycin antibiotic

Answer 7

Resistance van genes change D-Ala-D-Ala terminal residues to D-Ala-D-Lac to prevent vancomycin from binding

Question 8

How do bacteria develop resistance, genetically?

Answer 8

Bacteria require new genes to develop resistance as opposed to a single mutation; most resistance mechanisms are intrinsic and genetically encoded

Question 9

Where are resistance genes located?

Describe the 3 types of horizontal gene transfer

Answer 9

Encoded on mobile genetic elements (ex., plasmids) that allows for horizontal gene transfer

1. Bacterial transformation
   
   • A donor cell that is resistant to an ABx dies
   • Bacteria take up the DNA and become resistant themselves
2. Bacterial transduction
   
   • If a bacteriophage infects a bacteria, it will kill it and package its own viral genome (usually) but may pick up the ABx resistant gene
     
     • Whoops, now it infects a new bacteria and gives it ABx resistance
3. Bacterial conjugation
   
   • Sometimes plasmids encode machinery that specifically moves DNA from one bacterium to another
   • This is very efficient at giving resistance

Question 10

Describe the Klebsiella pneumoniae pathogen

Answer 10

• Gram negative, causes nosocomial (hospital) pneumonia
• Gooey capsules, MDR
• NDM-1 resistance makes it resistance to carbapenem ABx, which are resistant to β-lactamase

Question 11

Describe the Clostridia pathogen

Answer 11

• Gram positive, rod shaped
• Form endospores
• Anaerobes
• Soil, intestinal tracts

Question 12

List the 4 important human Clostridia pathogens and describe their diseases

• Note they produce endotoxins and exotoxins

Answer 12

1. Clostridium difficile (C. diff) - pseudomembranous colitis
2. Clostridium tetani - tetanus
3. Clostridium botulinum - botulism
4. Clostridium perfringens – food-borne illness and gas gangrene

Question 13

How is pseudomembranous colitis linked to ABx?

Answer 13

ABx can kill normal microbiota

• Provides space for C. diff
• A-B toxins (aka large clostridial cytotoxins, enter host cell) damage large intenstine

Question 14

What are A-B toxins?

Answer 14

Called A-B because they have 2 domains

• A domain: active portion, carries enzymatic activity
  
  • Inactivates regulatory proteins in host cell leading to dysregulation of cell processes –> inflammation, cell death
• B domain: binding portion, binding and uptake of spore into host cell
  
  • Translocates A component to cytoplasm

Question 15

Describe pseudomembranous colitis

Answer 15

• Life-threatening pseudomembranous colitis (antibiotic-associated diarrhea)
• Nosocomial
• Difficult to eradicate endospores from the environment
• The transmission route is via the spore: fecal-oral
• Risk factor: receiving antibiotic (symptoms occur days to weeks after discontinuing)
• Endoscopy reveals yellow lesions which peel off

Question 16

How is pseudomembrane colitis diagnosed?

What is the treatment

Answer 16

Diagnosis:

• History (ABx usage), asses symptoms, lab tests confirm C. diff
• Endoscopy and toxin detection assays

Treatment

• Discontinue ABx, start ABx for C. diff
• Oral vancomycin or I.V. metronidazole
• Avoid antidiarrheal agents
• Fecal microbiota transplantation, for recurrent infection
  
  1. Donor fecal sample from a healthy individual is infused into the patient (we replace the patient’s microbiome)
  2. The symptoms resolve in most cases