Biomedical Sciences: Antibiotics and Resistance (Lecture 4; Exam 1)

Selective Toxicity

Greater harm to microbes than to humans and other animal hosts.

• Interfere with essential structures or biochemical processes that are common or accessible in microbes, but not host cells.

Therapeutic Index

Is the ratio of the LOWEST dose toxic to host (patient), to dose required to kill microorganism.

• Ex. 1000ug/ml begins to harm patient, and 1ug/ml kills bacteria; therapeutic index = 1000

What is a good site of action for bacteria?

• Bacterial peptidoglycan

• Ribosomes (bacterial ribosomes are different than animal cells)

Broad Spectrum Antibiotics

Affect different types of bacteria.

(i.e. both gram negative and positive)

Narrow Spectrum Antibiotics

Affects only a few related types of bacteria.

Bacteriostatic

Inhibits the growth of bacteria.

Bactericidal

Kills bacteria.

Antibiotic Side Effects

• Allergic reactions (i.e. common with penicillin; initially amoxicillin)

• Toxic effects

  • Affects teeth, hearing, blood forming systems in some cases.

  • Inhibition of mitochondrial processes

• Reduces normal microbiota

• Antibiotic associated colitis: Clostridioides difficile (C. diff infections)

Classifications of Antibiotics

• Antibacterial

• Antifungal

• Antiprotozoan / antiparasitic

• Antiviral

Generations of Antibiotics

Antibiotics are usually made by some natural organism.

• Newer generation uses chemical modifications to improve antibiotics.

• Semi-synthetic: Second & third generation antibiotics

• New approach is to design antibiotics using computers to get best structure.

Assay of Antibiotics: Zone of Inhibition

A clear, circular area where bacteria cannot grow around an antimicrobial disk or substance on an agar plate.

Assay of Antibiotics: Dilution Assay

A lab test used to find the lowest concentration of an antibiotic that stops bacteria from growing.

Minimum Inhibitory Concentration (MIC)

The lowest concentration of an antimicrobial drug that completely stops the visible growth of bacteria.

Minimum Lethal Concentration (MLC)

The lowest concentration of an antimicrobial agent that kills 99.9% of a bacterial population; identifies the exact threshold required to caused bacterial death.

Penicillins

*** Affects cell wall (cell synthesis) ***

• Penicillin G = first antibiotic

• Was extremely effective

• Prevents synthesis of cell wall by blocking cross-linking of peptidoglycan

What causes resistance to penicillins?

Often due to beta-lactamases that inactivate (cleave) the antibiotic (pencillinases).

What does penicillin activity depend on?

beta-lactam ring; which is cleaved by beta-lactamases.

• Sometimes overcome with augmentin: amoxicillin + clavulanic acid (a B-lactamase inhibitor).

Methicillin

Is a beta-lactam antibiotic that is resistant to penicillinases, but bacteria developed methicillinases, etc.

Inhibitor of cell wall synthesis.

Cephalosporins

Inhibitor of cell wall synthesis.

Are also B-lactam antibiotics that are resistant to penicillinases

• Can have cephalosporinases.

Carbapenems

Inhibitor of cell wall synthesis

• Imipenem: First carbapenem in use to treat complex bacterial infections (1985).

• Has activity against both gram negative and positive bacteria.

• Has both peptidoglycan synthesis activity and B-lactamase inhibitor activity

*Resistance has emerged

How has resistance emerged for carbapenems?

• Efflux pumps

• New B-lactamases

• Modified porins in Gram-negatives

• Modified penicillin-binding proteins

Inhibitors of Protein Synthesis

• Tetracyclines (like doxycycline)

• Chloramphenicol (has bad side effects, used as last resport)

  • Bacteriostatic; stops protein synthesis)

• Aminoglycosides

• Erythromycin

• Clindamycin

Polypeptide Antibiotics

• Polymyxin B and bacitracin

Polymyxin B

Inserts & disrupts bacterial membranes.

Bacitracin

Affects membranes, blocking secretion of peptidoglycan precursors.

What part of the cell does polymyxin B and bacitracin affect?

Cell membranes.

Rifampin

Binds to RNA polymerase and inhibits

mRNA synthesis.

Inhibitors of DNA Replication

(DNA gyrase is target)

• nalidixic acid

• Fluoroquinolones (i.e. ciprofloxacin)

• novobiocin

What is the target of DNA replication inhibitors?

DNA Gyrase

Metronidazole

Specific for anaerobes (flagyl)

• Disrupts electron transport and leads to DNA damage.

• Must be activated by anaerobic organisms.

Trimethoprim

Inhibits folic acid synthesis

• (Folic acid synthesis is essential for bacteria and other microbes)

• Inhibits dihydrofolate reductase

• Often delivered as part of the formulation: bactrim

Modes of Action of Antibacterial Medications

• Inhibition of cell wall synthesis

• Inhibition of protein synthesis

• Disruption of cytoplasmic membrane

• Inhibition of nucleic acid synthesis

• Competitive inhibition

Limitations of Antimicrobial Agents

• Selection of antibiotic-resistant bacteria:

  • Can have spontaneous reaction due to mutation

    • R plasmids

• Antimicrobial hand soaps:

  • Can lead to disinfectant-resistant cells (triclosan)

• Antibiotics given to farm animals

Antibiotic Resistance Mechanisms

• Impermeability

• Binding site altered

• Alternative pathway

• Inactivation of antibiotic

• Increase in amount of sensitive site in cell

• Production of a system to excrete the antibiotic

Development of Drug Resistance

Often develop antibiotic resistance within 5-10 years of deployment of a new antibiotic.

How do beta-lactams cause antibiotic resistance?

Interfere with peptidoglycan synthesis

ex. Penicillins

S. aureus Drug Resistance Patterns

• Sensitivity to penicillin

• B-lactamase

• Methicillin resistance - MecA

What does penicillinase do?

Is an enzyme produced by certain bacteria to destroy penicillin and cause drug resistance.

What does tetracycline inhibit?

Protein synthesis

What does Chloramphenicol inhibit?

Protein synthesis

What do aminoglycosides inhibit?

Proteins synthesis

What are a few examples of aminoglycosides?

• Streptomycin

• Gentamycin

What does erythromycin inhibit?

Protein synthesis

What does Clindamycin inhibit?

Protein synthesis

What is an example of Tetracycline?

Doxycycline

What class of antibiotic is erythromycin?

Macrolide antibiotic

What class of antibiotic is clindamycin?

Lincosamide antibiotic

What kind of organisms have to activate Metronidazole?

Anaerobic organisms.

What happens when cell wall synthesis is inhibited?

The cell will grow and lyse.

What happens when protein synthesis is inhibited?

Stops bacterial growth.

What happens when cytoplasmic membranes are disrupted?

Materials leak out of the cell.

What happens during competitive inhibition of antibacterial medications?

Prevents action of an enzyme by competing for active site.

What is affected when nucleic acid synthesis is inhibited?

Both RNA and DNA.

Why are penicillin-binding proteins important?

They cross-link peptidoglycan polymers, which gives the bacterial wall its strength and rigid structure.

Vital for bacterial growth.

What class of antibiotics does penicillin fall under?

Beta-Lactam antibiotics.

What does penicillinase do?

Destroys beta-lactam ring of penicillin.

What are PBP’s? (Penicillin-binding protein)

Enzymes that cross-link peptidoglycan.

What is MecA?

methicillin resistance

Is a variant of bacterial PBP2A.

Bacteria that become resistant to first-line B-lactam drugs, do so by what method of resistance?

Inactivation of the antibiotic.