Topic IV: Antibiotics & the Bacterial Cell

Who found the first antibiotic?

Sir Alexander Fleming (1881-1955)

• Studying Staphylococcus

• Some of his bacterial blares become contaminated w/ fungus

• Noticed there was a zone around fungus were Staph couldn’t grow (Penicillium notatum- releasing an antibiotic)

What is an antimicrobial?

Drug to treat microbial infection

What is an antibiotic?

Antimicrobial drug naturally produced by microorganism

What are broad spectrum antibiotics?

Effective against a wide range of microorganisms, including most Gm- & Gm+ bacteria

• Can disrupt normal flora

• Ex. Chloramphenicol

What are narrow spectrum antibiotics?

Effective against a limited number of microorganisms

• Identification of microbe is helpful

• Not as disruptive to normal flora

• Ex. Polymyxin-B (Gm- only)

What is bacteriostatic?

Stops the growth of bacteria

What is bacteriocidal?

Kills bacteria

How do antibiotics target in bacterial cells?

Generally, antibiotics target structures that differ b/w prokaryotic (bacteria) & eukaryotic cells

What are some antibiotic targets?

• Cell wall (peptidoglycan) synthesis

• Nucleic acid synthesis (production of DNA/RNA)

• Protein synthesis

• Metabolic pathways (folate synthesis)

• Cell membrane integrity (for Gm-)

What is transpeptidase?

A bacterial enzyme that crosslinks peptidoglycan strands

What does penicillin inhibit?

Transpeptidase

What does vancomycin do?

Binds to amino acid side chain & blocks transpeptidase binding

What is the antibiotic’s cause to cell wall synthesis?

Bacteriocidal

What is the antibiotic target w/ prokaryotic ribosomes?

• Bind to 30S or 50S subunit

• Can inhibit:

  • Docking of tRNAs

  • Movement of tRNAs through ribosome

  • Formation of peptide bonds b/w amino acids

• Bacteriostatic or bactericidal

What is the antibiotic target w/ metabolic pathways?

Inhibit folic acid metabolism

• Important for purine synthesis

• Eukaryotic cells do not synthesize folic acid

• Bacteriostatic

What is the antibiotic target w/ nucleic acid synthesis?

Inhibit nucleic acid synthesis

• Block DNA polymerase

• Block RNA polymerase

• Block enzymes involved in supercoiling DNA

  • DNA gyrase & topoisomerase IV

• Bacteriostatic

What is the antibiotic target w/ bacterial cell membrane?

• Bind to lipid A (part of LPS) (polymyxins) in outer membrane

  • Effective only against Gm- bacteria

• Often associated w/ toxic side effects

  • Can bind other phospholipids

• Bacteriocidal

How can you choose the best antibiotic for an infection, & an effective concentration of that antibiotic?

Antibiotic susceptibility tests

• A potent antibiotic

• Lowest dose that is effective so you can minimize side effects

• Narrowest spectrum

What is MIC?

Minimum inhibitory concentration

• **Lowest concentration of an antimicrobial needed to prevent growth of a particular bacterial in vitro (in lab)

How is MIC measured?

• Kirby-Bauer test

• E-test

• Serial dilution test (most common)

What is MBC?

Minimum bactericidal concentration

• Lowest concentration that will kill bacteria

What is the Kirby-Bauer method?

• Paper disks are soaked in different antibiotics

• A pure culture of bacteria is grown on the plate

• A “zone of inhibition” forms around disks that contain an effective antibiotic

  • Larger ring= potent antibiotic

• Zone is measured to determine if bacteria is susceptible, intermediate, or resistant to each antibiotic

  • MIC can be calculated from this, but more often used qualitatively

What is the E-test?

• Similar to Kirby’s-Bauer test but uses paper strips rather than disks

• Each strip has a labeled gradient of an antibiotic

• Intersection of elliptical “zone of inhibition” w/ strip indicates the MIC for that antibiotic

What is the serial dilution test?

Different dilutions of antibiotic.. see which one is least amount but effective

• Often automated today

What are the 3 general approaches that bacteria have?

Lead to antibiotic resistance

1. Exclude antibiotic from the cell

• Expel antibiotic or prevent its entry

2. Alter bacterial target

• Aquire a mutation that changes the amino acid sequence (& therefore structure) of target protein

3. Inactivate antibiotic through an enzyme

What is an example of an altered target?

Mutation in gene for transpeptidase

What is an example of enzymatic inactivation?

Some bacteria have a beta-lactase’s enzyme, which cleaves penicillin

What are alternative antimicrobials?

“Phage therapy”

• Bacteriophage- viruses that kill just bacteria

  • Each phage can only attack 1 species of bacteria