Lecture 21-22: Antimicrobial Chemotherapy and Clinical Microbiology

What are antibiotics?

Compounds that are produced by one species of microbe that can inhibit the growth or kill other microbes.

Who discovered penicillin?

Alexander Fleming.

Who discovered sulfa drugs?

Gerhard Domagk.

Should antibiotics affect patients?

No, they should only affect infectious organisms.

What are broad-spectrum antimicrobials?

Antibiotics that are effective against many species.

What are narrow-spectrum antimicrobials?

Antibiotics that are effective against few or a single species.

What is a bactericidal?

Antibiotic that kills the target organism.

What does a bacteriostatic do?

Prevents the growth of the target organism.

Do bacteriostatics kill the target organism?

No, the immune system takes care of the infection.

What is the Minimal Inhibitory Concentration?

The lowest concentration of an antibiotic that prevents growth.

What is the Minimal Lethal Concentration?

Where no colonies grow.

Between the MIC and the MLC, which may still have living (but not growing) organisms?

The MIC (minimal inhibitory concentration).

What does the Kirby-Bauer Disk Diffusion Test test for?

Bacterial sensitivity to multiple antibiotics.

How is the Kirby-Bauer Test done?

Multiple disks are soaked with different antibiotics.

In the Kirby-Bauer Test, what represents sensitivity?

The size of cleared zones.

How is MIC determined in Disk Diffusion Tests?

The size of cleared zones is measured.

What does a bigger clear zone in Disk Diffusion Tests mean?

More sensitivity to antibiotics.

What are the clear zones in the Kirby-Bauer Test called?

Zone of inhibition.

What is the zone of inhibition?

The edge of no-growth.

How do you ensure an antibiotics effectiveness in terms of the MIC?

The drug concentration should be higher than the MIC.

What do antibiotics generally target?

Microbe-specific physiology.

Gram negative peptidoglycan structure is characterized by:

Direct cross-linking, thin layer.

Gram positive peptidoglycan structure is characterized by:

Peptide interbridge, thick layer.

Penicillin’s:

Competitive inhibitors of crosslink transpeptidation.

Vancomycin:

Binds ends of peptides.

Cycloserine:

Blocks formation of peptide for crosslink.

Bacitracin:

Blocks movement across membrane.

Do disaccharides ever reach the periplasm?

No.

Peptidoglycan (cell wall) antibiotics:

Penicillin’s, Vancomycin, Cycloserine, and Bacitracin.

What do beta-lactams do?

They inhibit cell wall synthesis by binding to penicillin binding proteins (PBPs).

What are beta-lactamases?

Enzymes that cleave beta-lactam rings of beta-lactams.

What do Quinolones do?

Block the bacterial DNA gyrase, prevents DNA replication.

Which antibiotics interfere with DNA (synthesis or integrity)?

Quinolones (i.e. nalidixic acid).

What do sulfa drugs do?

Interfere with the synthesis of metabolic intermediates.

What is PABA needed for in sulfa drugs?

DNA synthesis.

Which antibiotics interfere with bacterial metabolism?

Sulfa drugs.

What does Rifampicin do?

Blocks bacterial RNA Polymerase.

What does Actinomycin D do?

Binds DNA and inhibits polymerase movement.

What do RNA synthesis inhibitors do?

Inhibit transcription, they are bactericidal and the most active against growing bacteria.

Which antibiotics are RNA synthesis inhibitors?

Rifampicin and Actinomycin D.

What do Macrolides, chloramphenicols, and lincosamides do?

They bind the large subunit and block the transfer of peptides.

What do aminoglycosides do?

They prevent the 30S and 50S subunits from binding each other.

What do tetracyclines do?

They bind the small subunit and block the binding of aminoacyl-tRNA.

Which antibiotics are ribosomal inhibitor antibiotics?

Erythromycin, Azithromycin, and Streptomycin.

How does bacteria destroy antibiotics?

It destroys penicillin and its analogues, lowers penicillin concentration, and protects nearby cells, via beta-lactamase.

How do bacteria modify the antibiotic?

They use enzymes to modify aminoglycoside antibiotics (using acetylation, phosphorylation, and adenylation) and change the antibiotic to no longer interfere with ribosomes.

How do bacteria alter the target?

They modify the target enzyme of penicillin, or ribosome.

What is drug efflux?

It pumps the drug out of the cell and often confers resistance to several different classes of antibiotics simultaneously.

What is an MDR?

Multidrug resistance transporter.

First thing that happens in antibiotic resistance?

Lots of germs, a few are drug-resistance.

Second thing that happens in antibiotic resistance?

Antibiotics kill bacteria causing illness, as well as good bacteria protecting the body from infection.

Third thing that happens in antibiotic resistance?

The drug-resistant bacteria are now allowed to grow and take over.

Fourth thing that happens in antibiotic resistance?

Some bacteria give their drug-resistance to other bacteria, causing more problems.

What are the two main factors contributing to the rise in antibiotic resistance?

Indiscriminate use of antibiotics and not finishing your prescription.

What are strategies to prevent or treat drug-resistant pathogens?

Chemically altering the antibiotic and using more than one antibiotic (like clavulanic acid in combination with penicillin).

How do bacteria naturally have antibiotic resistance??

They have resistance genes to protect themselves from their own antibiotics and to protect themselves against others in their community.

What leads to increased selective pressure on bacteria?

Incomplete use and/or overuse of antibiotics.

What are antimicrobial resistance mutations?

Very rare spontaneous mutations which confer resistance (usually changes the drug target).

What is horizontal gene transfer in terms of antibiotic resistance?

Getting resistance genes from another organism.

What does the R plasmid do?

Carry multiple resistance genes.

Why are antiviral agents hard to find?

They’re often toxic to us and use host enzymes.

What does Amantadine do?

Prevent entry of virus into host cell.

What does Zanamivir do?

Prevents release of mature viruses.

Selective media:

Removes non-pathogenic strains and allows growth of the pathogen.

Differential media?

Stains grow, but differ in indicator color.

Analytical profile index (API):

Strips include several biochemical tests in one strip with a separate well for each test, each well contains chemicals needed for a different biochemical test.

PCR:

Amplify small fragment of DNA and allow detection of tiny numbers of bacteria.

ELISA (enzyme-linked immunosorbent assay):

Can detect antigens or antibodies present in nanogram and picogram quantities, an immunotest.

Point-of-care (POC) laboratory tests:

Designed to be used directly at the site of patient care, such as physician’s offices and outpatient clinics.