19. Anti-Microbial Therapy

Question 1

Non-selective killing of microbes

Answer 1

• Sterilization: kill ALL microbes present. use pressurized steam, high temperature; ionizing radiation, ethylene oxide gas, and plasma gases, if you can’t heat the material)
• Disinfection: kills most of the microbes (Phenolics, iodophores and quaternary ammonium compounds)
• Antisepsis: subset of disinfecting agents that can be applied to the skin. (Alcohol, iodophores and chlorhexidine, Purell)

Question 2

Some drugs can be ____ at low concentrations and ___ at higher concentrations

Answer 2

bacteriostatic

bactericidal

Question 3

Bactericidal drugs

Answer 3

will actually kill the bacteria (penicillin)

Question 4

Bacteriostatic drugs:

Answer 4

• stop the growth of the bacteria but do not kill the organisms. (tetracycline)
• Once the drug is removed the bacteria can continue to grow.

Question 5

sources of antibiotics:

Answer 5

• 50% of antibiotics are made from Streptomyces spp. Bacterium (grows in soil)
• fungi cephalosporium spp – penicllium notatum

Question 6

MIC

Answer 6

Minimum inhibitory concentration – minimum amount of antibiotics needed to prevent growth of the bacteria

Question 7

Susceptibility to antibiotics can be tested in 2 ways:

Answer 7

• tube dilution assay– determine MIC, min concentration of drug needed to prevent growth
• disk diffusion assay– zone of inhibition around antibiotic paper discs are measured

Question 8

Tube dilution essay

Answer 8

• fixed amount of bacteria is inoculated in liquid culture with drug. Serial dilutions are performed.
• Turbidity in a vial signals bacterial growth, concent before this dilution point is MIC.

Question 9

MBC

Answer 9

Minimum bactericidal concentration – lowest concentration of drug that will kill 99.9% of bacteria

Question 10

How do we find MBC?

Answer 10

• culture the bacteria from the tube dilution assay

- If the bacteria grows drug is bacteriostatic, if doesn’t grow it is bactericidal.

Question 11

Downside of using broad spectrum antibiotics:

Answer 11

wipe out commensal bacteria/resident microbiota of the intestines, and can lead to resistance

Question 12

3 ways bacteria develop resistance:

Answer 12

1) Prevent uptake of drug into the cell by altering cell permeability. Evolving pumps that remove the drug from their cytoplasm
2) Modifying structure of the target of the drug or amount necessary to activate the target (overexpression of target). ex: penicillin binding protein can be modified so antibiotics can no longer bind OR develop mutation that makes excessive PBP, so a higher dose of drugs is required.
3) can develop enzymes that hydrolyze or alter the structure of antibiotics Ex: NDM-1: new beta-lactamase that cleaves cabapenmes; inactivate drugs by adding a side chain to it.

Question 13

Major mechanistic classes of antibiotics

Answer 13

1) Inhibitors of cell wall biosynthesis: beta lactam, and non beta lactam
2) Inhibitors of nucleic acid synthesis
3) Metabolic inhibitors
4) Membrane disrupting agents

Question 14

bacterial cell wall is a _____ layer

Answer 14

murein layer- peptidoglycan compound of sugar chains, N-acetylglucosamine and N-acetylmuramic acid, and tetrapeptides attached via interpeptide bonds.

Question 15

Two types of inhibitors of cell wall synthesis

Answer 15

• Beta-lactams: penicillins, cephalosporins, monbctams and carbpenems
• Non-beta lactams: vancomycin, cycloserine, bacitracin, fosfomycin

Question 16

antibiotics that inhibit steps of murein layer synthesis

Answer 16

• Fosfomycin: inhibits conversion of NAG to NAM
• Cycloserine: inhibits addition of the peptide amino acids to make the peptide side chain
• Bacitracin: inhibits translocation of the precursor from inside of the cell to the outside
• Vancomycin: inhibits linking of the precursors to the glycan chain
• Penicillin: inhibits cross-linking of the peptide chains, the last step of the process.

Question 17

Penicillin

Answer 17

• inhibits cross-linking of the peptide chains, the last step of the process.
• penicillin derivatives have altered side chains in order to change, solubility, uptake ect. Anything with “illin” at the end is a penicillin derivative

Question 18

All beta-lactam antibiotics have a _____

Answer 18

beta lactam ring

Question 19

Bacterial resistance to beta lactams

Answer 19

• Bacteria have developed the enzyme B-lactamase, which hydrolyzes the lactam ring and inactivates the drug.
• beta-lactamase inhibitors used in conjunction with antibiotics. ex: Augmentin = amoxicillin and clavulanic acid, a beta lactamase inhibitor.