micrb lec 5
What are chemotherapeutic agents
Chemicals used to treat disease: antibacterials, antifungals, antivirals.
Where do chemotherapeutic agents come from
Plants, bacteria, fungi (and many are ancient remedies).
Who discovered penicillin and what was it first called? Who helped make penicillin usable clinically (names to recognize)?
Alexander Fleming; he called it “mould juice” before “penicillin.” Howard Florey, Ernst Chain, Norman Heatley, Mary Hunt (Mary Hunt found a strong-producing strain).
Define selective toxicity.
Drug kills/inhibits pathogen while harming host as little as possible.
Define therapeutic dose.
Drug level needed for clinical treatment.
Define toxic dose.
Drug level where it becomes too toxic (side effects).
Define therapeutic index and what you want it to look like.
Toxic dose / therapeutic dose; want it as large as possible.
What are “side effects” in this context?
Undesirable effects of a drug on host cells.
Why can MIC and MLC differ? If you remove drug at MIC, what might happen?
Inhibition doesn’t always mean killing; you must measure—you can’t predict. Pathogen can often grow again (because MIC is inhibitory, not necessarily lethal).
how do you find MIC?
Dilution susceptibility test . MIC = lowest concentration with no visible growth in broth/agar.
how do you determine MLC?
broth dilution test. Subculture “no growth” tubes into drug-free medium; the lowest concentration where microbe can’t be recovered = MLC.
Kirby-Bauer disk diffusion—what does a “zone of inhibition” mean? What does “no zone of inhibition” imply? how can you infer “broad spectrum” from the pictures?
Antibiotic diffuses and inhibits growth; bigger zone = more susceptible (for that setup). Antibiotic is not effective against that bacterium.
If the same antibiotic inhibits both a Gram+ (e.g., S. aureus) and Gram− (e.g., E. coli) it suggests broad spectrum.
The 4 major antibiotic target categories
Cell wall synthesis, protein synthesis, metabolic antagonists, nucleic acid synthesis.
What practical factors determine if a drug will work in a patient? (8)
Reach infection site, pathogen susceptibility, body levels exceed MIC, dose amount, route (topical/oral/IV), uptake speed, clearance rate, toxicity.
Penicillins (β-lactams): spectrum + key MOA + when they work best.
Mainly Gram+ (narrow); block transpeptidation (cross-linking) → incomplete wall → lysis; act best on actively growing cells making new PG.
whats wrong with beta lactams
This class is “not great” because resistance can develop readily.
cephalosporins: relationship to penicillins + spectrum trend across generations.
Structurally/functionally similar; broad-spectrum; early generations mainly Gram+, later modified to include more Gram−.
Why were Gram+ easier targets than Gram− for cell wall drugs?
Gram+ have lots of PG; Gram− have an outer membrane barrier that reduces drug entry.
Vancomycin/Teicoplanin: what class + what do they bind + why Gram− resistant?
Glycopeptides; bind D-Ala–D-Ala motif in PG; too large to pass Gram− outer membrane.
Why is vancomycin clinically famous
Used for antibiotic-resistant MRSA and enterococcal infections.
Aminoglycosides (ex: streptomycin): structure vibe + who they hit + mechanism.
Cyclohexane ring + amino sugars; active vs Gram− aerobic & facultative anaerobes; bind 30S → inhibit protein synthesis + cause mRNA misreading.
tetracyclines: key properties + what exactly they block.
Broad-spectrum, bacteriostatic; bind 30S; block aminoacyl-tRNA binding to the A site.
What common use is mentioned for tetracyclines?
Sometimes used to treat acne.
Macrolides (ex: erythromycin): target + effect on translation + clinical note.
: Bind 23S rRNA of 50S; inhibit peptide chain elongation; often used for patients allergic to penicillin.
Why is folic acid important
Needed for DNA synthesis and as a cofactor in many reactions (Vitamin B9).
Sulfa drugs: what do they mimic and what do they inhibit?
PABA analogs; competitively inhibit enzymes in folic acid synthesis → selective toxicity.
Sulfa drugs: bacteriostatic or bactericidal?
Bacteriostatic.
trimethoprim: what pathway does it hit and what’s special about combining it?
Also blocks folic acid production; combining with sulfa blocks two steps → higher efficacy + harder resistance.
trimethoprim: main side effects + common use mentioned.q
Side effects include abdominal pain and photosensitivity; used for UTIs.
Quinolones (ex: ciprofloxacin): target enzymes + kill/static + other key perk.
Inhibit DNA gyrase and topoisomerase II; bactericidal; excellent tissue penetration (good for internal infections).
Daptomycin (lipopeptide): who it hits + why it’s “new class” material.
Very effective vs Gram+ (e.g., S. aureus) but can be toxic; inserts near phosphatidylglycerol, aggregates → forms holes/depolarizes membrane; “new class” because new mode of action.
Why is daptomycin hard to make?
Difficult to synthesize in lab—easier to isolate from environment.
Resistance mechanism: efflux pumps (what’s the point)?
Pumps (often ABC transporters) export antibiotics → lowers intracellular concentration; often non-specific.
Resistance mechanism: decreased uptake (how does that happen)?
Alter entry routes—mutate transporters or reduce transporter presence.
Resistance mechanism: inactivating enzymes (what do they do)?
Chemically modify or degrade the antibiotic (can act inside or outside cell).
Resistance mechanism: target alteration.
Change the structure of the molecule the drug binds to → drug binds poorly.
Resistance mechanism: alternative enzyme.
Use a different enzyme that performs the same function but isn’t inhibited.
Resistance mechanism: shut down growth / sporulation (why it helps).
If growth stops, some antibiotics (like wall synthesis inhibitors) have little effect; spores/growth shutdown are less susceptible.
Three big “drivers” of antimicrobial resistance listed.
Microbial warfare, horizontal gene transfer, evolutionary pressure from widespread antibiotic use.
Where are resistance genes often located (why that matters)?
On plasmids → easy transfer between bacteria.
Why are sub-MIC antibiotic levels especially dangerous?
They apply selection pressure without fully eliminating bacteria → enrich resistant cells.
What mutation rate range is given for E. coli DNA polymerase?
~10⁻⁶ to 10⁻⁷
E. coli genome size given? What’s the “so what” statement about mutations per cell?
~4,600,000 bp. On average ~1 mutation per cell.
Why do infections make resistance likely
Infections can contain billions to trillions of cells → rare mutants are practically guaranteed.
Once resistance originates, what’s the major spread risk?
It can be transmitted to other bacteria (even different species).
Why isn’t resistance limited to one drug class?
One mechanism (e.g., efflux) can impact multiple classes; mechanisms aren’t “confined.”
Why are abscesses/biofilms specifically mentioned?
Cells may be slow-growing, so many antibiotics are less effective
What’s the “don’t stop early” antibiotic adherence point (selection story)?
If you stop early, symptoms may improve but survivors remain; resistant/persister cells get selected and the infection can rebound.
What’s the fastest way to answer “possible resistance mechanisms” for a drug?
ask: (1) Does the drug need to enter the cell? (2) What’s the target?
If it must enter, then permeability changes + efflux + target change can all matter.
If it acts outside, permeability/efflux may matter less.
Penicillin-style drug (targets peptidoglycan machinery in periplasm/outside): which resistance mechanism is most “on-target” here?
Change the structure of a protein (alter the target enzyme / binding). Per the notes: it doesn’t need to enter the cytoplasm.
For a drug that does need to enter the cell (like the structure in Q6), what resistance set is plausible?
decrease permeability, alter target protein, efflux pumps.
If a bacteriostatic drug is added mid-exponential phase (OD measurement), what should the curve do?
Flatten/plateau (cells stop increasing, but OD doesn’t “crash”).