BIOL 2051 Handout 4 - Dr. Nita

What is the main cause of bacterial sensitivity to antibiotics?

Antibiotics target essential bacterial processes such as cell wall synthesis, protein synthesis, or nucleic acid synthesis, leading to bacterial death or growth inhibition.

What region on an agar plate indicates bacterial sensitivity?

The zone of inhibition (clear area with no bacterial growth around the antibiotic).

What are the three major antibiotic target categories?

Cell wall synthesis, protein synthesis, nucleic acid synthesis.

What class does penicillin belong to?

β-lactams

What is the origin of penicillin?

Natural product from fermentation of Penicillium molds

What does penicillin target?

Cell wall synthesis

What is penicillin's mechanism of action?

Binds and inactivates PBPs (transpeptidases), blocking peptidoglycan cross-linking

What is the result of penicillin action?

Weak cell wall → osmotic lysis due to high internal pressure

What class does ampicillin belong to?

β-lactams

What type of antibiotic is ampicillin?

Semi-synthetic penicillin

What does ampicillin target?

Cell wall synthesis

What is ampicillin's mechanism?

Binds PBPs to inhibit cell wall synthesis (same as penicillin)

What class does cefoxitin belong to?

β-lactams (cephamycin, related to cephalosporins)

What does cefoxitin target?

Cell wall synthesis

What is cefoxitin's mechanism?

Inhibits PBP-mediated cell wall synthesis

What makes cefoxitin unique?

Contains a methoxy group that protects it from β-lactamase degradation

Why are bacteria sensitive to cefoxitin?

They cannot easily degrade it with β-lactamases

What class does streptomycin belong to?

Aminoglycosides

What is the origin of streptomycin?

Derived from Streptomyces griseus

What does streptomycin target?

Protein synthesis

What is streptomycin's mechanism?

Binds 16S rRNA of 30S subunit → misreading of mRNA

What is the result of streptomycin action?

Incorrect amino acids + premature protein termination → bactericidal

What class does gentamicin belong to?

Aminoglycosides

What does gentamicin target?

Protein synthesis

What is gentamicin's mechanism?

Irreversibly binds 30S subunit disrupting protein synthesis

What special condition is required for gentamicin entry?

Oxygen-dependent active transport

Why is gentamicin ineffective against anaerobes?

It requires oxygen for uptake

What class does erythromycin belong to?

Macrolides

What does erythromycin target?

Protein synthesis

What is erythromycin's mechanism?

Binds 50S subunit and blocks polypeptide elongation

What is the effect of erythromycin?

Bacteriostatic (stops growth)

What class does chloramphenicol belong to?

Amphenicols

What is the origin of chloramphenicol?

Produced by Streptomyces venezuelae

What does chloramphenicol target?

Protein synthesis

What is chloramphenicol's mechanism?

Inhibits peptidyl transferase on 50S subunit

What is the result of chloramphenicol action?

Blocks peptide bond formation → stops protein synthesis

What class does tetracycline belong to?

Tetracyclines

What is tetracycline's origin?

Polyketide antibiotic from Streptomyces species

What does tetracycline target?

Protein synthesis

What is tetracycline's mechanism?

Binds 30S subunit preventing aminoacyl-tRNA binding to A-site

What is the result of tetracycline action?

Inhibits protein synthesis

What class does doxycycline belong to?

Tetracyclines

What type of tetracycline is doxycycline?

Long-acting, lipophilic

What does doxycycline target?

Protein synthesis

What is doxycycline's mechanism?

Binds 30S subunit blocking tRNA binding

What is the result of doxycycline action?

Inhibits protein synthesis and bacterial growth

What class does rifampicin belong to?

Rifamycin

What does rifampicin target?

Nucleic acid synthesis

What is rifampicin's mechanism?

Binds DNA-dependent RNA polymerase → inhibits transcription

What is the result of rifampicin action?

Stops RNA and protein production

What class does nalidixic acid belong to?

Quinolones

What does nalidixic acid target?

Nucleic acid synthesis

What enzymes does nalidixic acid inhibit?

DNA gyrase (topoisomerase II) and topoisomerase IV

What is the result of nalidixic acid action?

Blocks DNA replication, transcription, repair → rapid cell death

Why are bacteria sensitive to cell wall inhibitors?

They target peptidoglycan, which is unique to bacteria → leads to osmotic lysis

Why are bacteria sensitive to protein synthesis inhibitors?

They target 70S ribosomes, which differ from eukaryotic 80S ribosomes

Why can protein synthesis inhibitors be toxic to humans?

Mitochondria have ribosomes similar to bacterial 70S ribosomes

Why are bacteria sensitive to nucleic acid inhibitors?

They target bacterial enzymes distinct from human enzymes

What region on an agar plate indicates bacterial resistance?

Areas where bacteria grow despite antibiotic presence

What is the antibiotic resistance crisis?

Loss of effective antibiotics makes infections harder to treat

Why do bacteria evolve resistance quickly?

Rapid mutations and gene transfer (conjugation, transduction, transformation)

What is the goal of studying resistance mechanisms?

Develop smarter antibiotics that bacteria cannot easily resist

How do Gram-positive bacteria resist antibiotics structurally?

Thick peptidoglycan layer blocks access to PBPs

What is peptidoglycan thickening?

Increased thickness acts as a physical barrier

What is target modification (D-Ala-D-Lac/D-Ser)?

Alters peptidoglycan precursors reducing antibiotic binding

What is increased cross-linking?

Reduces access to PBPs for β-lactams

What is O-acetylation (OatA)?

Addition of acetyl groups to peptidoglycan → resistance to lysozyme and antibiotics

Which bacteria use O-acetylation?

Staphylococcus aureus

How do Gram-negative bacteria resist antibiotics?

Outer membrane with LPS restricts drug entry

What role do porins play?

Selective channels that can be reduced or modified to limit entry

How do efflux pumps contribute?

Actively pump antibiotics out of the cell

How do mycobacteria resist antibiotics?

Mycolic acid-rich cell wall creates hydrophobic barrier

Why are endospores resistant?

Multi-layer structure + dormancy prevents antibiotic entry and action

Examples of endospore-forming bacteria

Clostridioides difficile, Bacillus anthracis

How do capsules contribute to resistance?

Polysaccharide layer blocks antibiotic access

What is a biofilm?

A structured bacterial community in an extracellular matrix

Why are biofilms resistant?

EPS matrix blocks antibiotics and slows penetration

What are persister cells?

Dormant cells resistant to antibiotics targeting active processes

What is VBNC state?

Viable but nonculturable deep dormancy state

How do biofilms increase resistance genetically?

Close proximity enhances gene transfer

What is target modification resistance?

Mutation alters antibiotic binding site

Example of target modification

DNA gyrase mutation → quinolone resistance

Example of RNA polymerase mutation

Leads to rifampicin resistance

Example of ribosomal mutation

16S rRNA mutation → streptomycin resistance

What is efflux pump resistance?

Mutations increase antibiotic removal from the cell

What is reduced permeability resistance?

Porin mutations decrease antibiotic entry

What are beta-lactamases?

Enzymes that degrade β-lactam antibiotics by cleaving the ring

What antibiotics are affected by beta-lactamases?

Penicillins, cephalosporins, carbapenems

What are aminoglycoside-modifying enzymes?

Enzymes that add chemical groups to aminoglycosides

Types of aminoglycoside-modifying enzymes

AAC, APH, ANT

What is the effect of aminoglycoside modification?

Prevents binding to 30S ribosome

What is chloramphenicol acetyltransferase?

Enzyme that acetylates chloramphenicol

What is the result of chloramphenicol acetylation?

Prevents binding to 50S subunit → drug inactive