Antibiotics — The Arsenal

The Five Mechanisms

Every antibiotic does one of five things. Know the mechanism, and coverage and side effects follow logically.

Quick — penicillin kills bacteria by doing what?

Penicillin binds transpeptidase (PBP) — the enzyme that cross-links peptidoglycan in the cell wall. No cross-links = wall falls apart = bacteria lyse. All beta-lactams work this way: penicillins, cephalosporins, carbapenems. The ring structure mimics D-ala-D-ala. Bacteria that change this target (MRSAMRSA has the mecA gene encoding PBP2a — a modified transpeptidase that beta-lactams can't bind. That's why you need vancomycin or daptomycin.) become resistant to the entire class.

Cell Wall Synthesis

Target: transpeptidase (PBPs) or D-ala-D-ala

Penicillins Cephalosporins Carbapenems Vancomycin

Beta-lactams bind PBPs. Vancomycin binds D-ala-D-ala directly (different target, same wall).

Protein Synthesis — 30S

Target: 30S ribosomal subunit

Aminoglycosides Tetracyclines

Aminoglycosides and Tetracyclines = "AT 30." Aminoglycosides cause misreading (bactericidal). Tetracyclines block tRNA attachment (bacteriostatic).

Protein Synthesis — 50S

Target: 50S ribosomal subunit

Macrolides Clindamycin Chloramphenicol Linezolid

"Buy AT 30, CELL at 50" — Chloramphenicol, Erythromycin (macrolides), Linezolid, cLindamycin. 🔑 Buy AT 30, CELL at 50 — Aminoglycosides & Tetracyclines at 30S. Chloramphenicol, Erythromycin, Linezolid, cLindamycin at 50S.

DNA/RNA Synthesis

Target: DNA gyrase / topoisomerase IV, or DNA directly

Fluoroquinolones Metronidazole Rifampin

FQs block gyrase (gram-neg) and topo IV (gram-pos). Metronidazole forms toxic free radicals that shred DNA. Rifampin blocks RNA polymerase.

Folate Synthesis

Target: dihydropteroate synthase + dihydrofolate reductase

TMP-SMX Dapsone

Double blockade: sulfamethoxazoleSulfa drugs are structural analogs of PABA — they competitively inhibit dihydropteroate synthase, the first step in bacterial folate synthesis. Humans don't have this enzyme (we get folate from food), so sulfa drugs are selectively toxic to bacteria. blocks step 1, trimethoprimTrimethoprim inhibits dihydrofolate reductase (DHFR) — the second step in folate synthesis. Bacterial DHFR and human DHFR are structurally different, giving selectivity. But high doses or long courses can still cause folate deficiency in humans → megaloblastic anemia. blocks step 2. Synergistic — two hits on the same pathway.

Sort the Arsenal

Drag each antibiotic to its mechanism. This is how boards test it — they give you the drug name and expect you to know the target.

Cell Wall

30S Ribosome

50S Ribosome

DNA / RNA

Folate Synthesis

The Coverage Grid

This is the table that lives in your head on test day. Boards give you an infection and expect you to pick the right drug. Know who covers what.

A patient has a UTI caused by Pseudomonas aeruginosa. Which cephalosporin generation covers Pseudomonas?

Most cephalosporins do NOT cover Pseudomonas. The exceptions are ceftazidime (3rd gen but anti-pseudomonal) and cefepime (4th gen). Standard 3rd gen like ceftriaxone misses it. Board trap: "pseudomonal UTI" + ceftriaxone = wrong answer that feels right.

Drug Class	Gram+	Gram−	Anaerobes	Pseudomonas	MRSA	Atypicals
Penicillin G/V	+++	−	+	−	−	−
Ampicillin/Amoxicillin	+++	+	+	−	−	−
Pip/Tazo (Zosyn)	+++	+++	+++	✓	−	−
Cefazolin (1st gen)	+++	+	−	−	−	−
Ceftriaxone (3rd gen)	+	+++	−	−	−	−
Cefepime (4th gen)	++	+++	−	✓	−	−
Carbapenems	+++	+++	+++	✓	−	−
Azithromycin	+	+	−	−	−	✓
Fluoroquinolones	++	+++	−	✓	−	✓
Vancomycin	+++	−	−	−	✓	−
TMP-SMX	++	++	−	−	✓	−
Metronidazole	−	−	+++	−	−	−
Doxycycline	+	+	−	−	+	✓
Linezolid	+++	−	−	−	✓	−

The Big Three MRSA Drugs: Vancomycin, TMP-SMX, Linezolid. Doxycycline has some activity (skin/soft tissue). Everything else — all beta-lactams, all cephalosporins, all carbapenems — fails against MRSA.

The Pseudomonas Shortlist: Pip/tazo, ceftazidime/cefepime, carbapenems, fluoroquinolones. Most other antibiotics miss it. If the question says "Pseudomonas" and the answer choices include ampicillin — trap.

Atypical Coverage: AtypicalsMycoplasma, Chlamydia, Legionella — they lack standard cell walls or live intracellularly. Beta-lactams target the cell wall, so they're useless against organisms without one. You need drugs that get inside cells: macrolides, FQs, tetracyclines. need macrolides, fluoroquinolones, or doxycycline. Cell wall agents don't work on organisms without cell walls.

Cephalosporin Generations

The pattern is simple: as generations go up, gram-negative coverage increases and gram-positive coverage decreases. Except 4th gen, which gets both back.

You need surgical prophylaxis (skin flora = gram-positives). Which generation?

Cefazolin (1st gen) = surgical prophylaxis. Best gram-positive coverage in the cephalosporin family. 3rd gen is overkill (broader than needed, worse gram-pos). 5th gen covers MRSA but is reserved for serious infections, not prophylaxis.

Gen	Example	Gram+	Gram−	Special	Board Use
1st	Cefazolin, Cephalexin	+++	+	—	Surgical prophylaxis, skin infections
2nd	Cefoxitin, Cefuroxime	++	++	Cefoxitin: anaerobes	Mixed infections, aspiration PNA
3rd	Ceftriaxone, Ceftazidime	+	+++	Crosses BBB. Ceftazidime: Pseudomonas	Meningitis, gonorrhea, serious GNR
4th	Cefepime	++	+++	Pseudomonas + expanded	Febrile neutropenia, nosocomial
5th	Ceftaroline	+++	++	MRSA coverage	The ONLY beta-lactam that covers MRSA

Ceftaroline is the exception that boards love. It's the only cephalosporin (only beta-lactam) that covers MRSA. If a question asks about MRSA and lists ceftaroline as an option, it might be the answer — they're testing whether you know the 5th gen exception. 🔑 CeftAROLINE — the A-RONE that stands alone. Only beta-lactam vs MRSA.

The Board Traps

Side effects are half of antibiotic questions. They describe a clinical scenario and expect you to name the drug. These are the ones that always show up.

⚠ Red Man Syndrome

Vancomycin (too-fast infusion)

Patient turns flushed, red, itchy — upper body, face, neck. NOT an allergy. It's direct histamine release from mast cells. The fix: slow the infusion rate + pretreat with diphenhydramine. Boards love to offer "switch to a different antibiotic" as a wrong answer. Don't switch. Slow down. Why not allergic?True IgE-mediated allergy causes anaphylaxis (hypotension, airway compromise). Red Man is a rate-dependent histamine dump — no IgE involved. Slow the drip, problem solved. Allergic reactions don't resolve by slowing the drug.

⚠ C. difficile Colitis

Clindamycin (classic), Fluoroquinolones, Ampicillin

Any antibiotic can cause it, but clindamycin is the board answer. Kills anaerobic gut flora → C. diff overgrows → toxin A (enterotoxin) and toxin B (cytotoxin) → watery diarrhea, pseudomembranes. Treat with oral vancomycin or fidaxomicin. NOT IV vancomycin — IV vanco doesn't reach the gut lumen.

⚠ Tendon Rupture

Fluoroquinolones (ciprofloxacin, levofloxacin)

Achilles tendon snaps — especially in patients >60, on corticosteroidsCorticosteroids weaken tendons by inhibiting collagen synthesis. FQs chelate magnesium in tendon tissue, disrupting collagen cross-linking. Both hit the same target. Together, it's catastrophic., or with renal impairment. Also: seizures (GABA antagonism), QT prolongation, and aortic dissection (newer black box warning). Boards describe an elderly diabetic on steroids who suddenly can't walk → answer is always FQ.

⚠ Gray Baby Syndrome

Chloramphenicol

Neonates can't glucuronidateNeonatal liver lacks mature UDP-glucuronosyltransferase. Chloramphenicol accumulates because it can't be conjugated for excretion. The toxic metabolite causes cardiovascular collapse — the baby turns gray and limp. chloramphenicol → toxic levels → cardiovascular collapse → ashen gray, limp, distended. Almost never used in the US anymore but boards still test it. If the question says "neonate" + "gray" + "antibiotic" → chloramphenicol.

⚠ Serotonin Syndrome

Linezolid + SSRIs

Linezolid is a weak MAO inhibitorMAO-A breaks down serotonin in the CNS. Linezolid reversibly inhibits MAO-A. Add an SSRI (which blocks serotonin reuptake) and you get serotonin flooding the synapse. The triad: hyperthermia + clonus + altered mental status.. Combined with SSRIs, SNRIs, or meperidine → serotonin excess → hyperthermia, clonus, agitation. Board setup: patient on sertraline needs MRSA coverage → linezolid is contraindicated → use vancomycin or daptomycin instead.

⚠ Nephrotoxicity + Ototoxicity

Aminoglycosides (gentamicin, tobramycin)

Hearing loss (cochlear damage, irreversible) + kidney damage (reversible with dose adjustment). Monitor trough levels. Risk increases with vancomycin co-administration (synergistic nephrotoxicity). If the question describes progressive hearing loss during treatment → aminoglycoside.

⚠ Photosensitivity + Teeth Staining

Tetracyclines (doxycycline, minocycline)

Severe sunburn-like rash with sun exposure. Deposits in developing teeth → permanent yellow-brown discoloration. Contraindicated in pregnancy and children <8. Boards love to put doxycycline as an option for a pregnant patient with Chlamydia → wrong. Use azithromycin instead.

⚠ Disulfiram-Like Reaction

Metronidazole + alcohol

Patient drinks while on metronidazole → severe nausea, vomiting, flushing, headache. Metronidazole blocks aldehyde dehydrogenase (same as disulfiram/Antabuse). "Don't drink on metro." If the question describes a patient who vomits after a beer while being treated for C. diff or a parasitic infection → metronidazole interaction.

Elimination Game

A patient walks in. Four drugs are options. Clinical clues eliminate one at a time until only the answer remains.

A 22-year-old woman presents with dysuria and urinary frequency. She is 14 weeks pregnant. Urine culture grows E. coli. She has no drug allergies.

Ciprofloxacin

Fluoroquinolone

Doxycycline

Tetracycline

TMP-SMX

Folate inhibitor

Nitrofurantoin

Urinary antiseptic

Clue 1: She is pregnant. FQs cause cartilage damage in the fetus.

Ciprofloxacin is eliminated.

Clue 2: She is pregnant. Tetracyclines cause teeth staining and bone growth inhibition.

Doxycycline is eliminated.

Clue 3: She is in the first trimester. TMP-SMX is a folate antagonist — teratogenic in the 1st trimester (neural tube defects) and causes neonatal kernicterus near term.

TMP-SMX is eliminated. (Usable in 2nd trimester only if necessary.)

The Decision Tree

Patient has an infection. Pick the drug. This is how it shows up on test day.

What type of infection?

Community-acquired pneumonia — outpatient or inpatient?

Outpatient CAP (healthy) → Amoxicillin OR Doxycycline

Monotherapy is fine. Azithromycin used to be first-line but S. pneumoniae resistance rates are too high now. If the question gives azithromycin as the only option for healthy outpatient CAP, it might still be "correct" on older exams.

Outpatient CAP (comorbidities) → Amoxicillin/Clav + Macrolide OR Respiratory FQ alone

Need to cover atypicals (Mycoplasma, Legionella) plus typical bacteria. Respiratory FQ = levofloxacin or moxifloxacin (NOT cipro — cipro has poor pneumococcal coverage).

Inpatient CAP → Ceftriaxone + Azithromycin (or respiratory FQ alone)

Ceftriaxone covers S. pneumoniae and gram-negatives. Azithromycin adds atypical coverage. This is the most-tested empiric regimen for inpatient CAP.

Meningitis — age group?

Neonatal meningitis → Ampicillin + Cefotaxime (or Gentamicin)

Ampicillin covers Listeria and GBS. Cefotaxime covers E. coli. NOT ceftriaxone in neonates — displaces bilirubin from albumin → kernicterus risk.

Standard meningitis → Ceftriaxone + Vancomycin + Dexamethasone

Ceftriaxone for S. pneumoniae and N. meningitidis. Vancomycin for resistant pneumococcus. Dexamethasone BEFORE or WITH first antibiotic dose — reduces inflammation and improves outcomes. Give steroids late = no benefit.

Elderly/IC meningitis → Ceftriaxone + Vancomycin + Ampicillin

Add ampicillin for Listeria. Listeria is the bug boards love for elderly and immunocompromised meningitis. Cephalosporins do NOT cover Listeria.

MRSA skin infection — severity?

Mild MRSA SSTI → I&D + TMP-SMX or Doxycycline

Incision and drainage is the #1 intervention for abscess. Oral antibiotics are adjunctive. TMP-SMX and doxycycline both have community MRSA coverage.

Severe MRSA → IV Vancomycin (or Daptomycin if not PNA)

Daptomycin is inactivated by surfactant — never use for pneumonia. If the question says MRSA pneumonia, the answer is vancomycin (or linezolid). Daptomycin for bloodstream/skin only.

Uncomplicated UTI — pregnant?

Uncomplicated UTI → Nitrofurantoin or TMP-SMX

First-line. Avoid FQs for simple UTI — save them for complicated infections. Nitrofurantoin only works in the bladder (concentrates in urine, no systemic levels).

UTI in pregnancy → Nitrofurantoin (1st/2nd tri) or Cephalexin

TMP-SMX is teratogenic in 1st trimester (folate antagonist) and causes kernicterus near term. FQs and tetracyclines are contraindicated. Nitrofurantoin is safe but avoid near term (hemolysis in G6PD-deficient neonates).

Hospital-acquired PNA / VAP — risk for MDR?

HAP (low risk) → Pip/Tazo or Cefepime or Levofloxacin

Monotherapy with a broad-spectrum agent. All cover Pseudomonas. Pick based on local antibiogram.

HAP/VAP (high MDR risk) → Anti-pseudomonal beta-lactam + FQ (or aminoglycoside) + MRSA coverage

Triple therapy: pip/tazo or meropenem (beta-lactam) + cipro or tobramycin (double Pseudomonas coverage) + vancomycin or linezolid (MRSA). Two anti-pseudomonal agents from different classes = double coverage.

The Quiz

5 patients. 5 antibiotics questions. Board-style vignettes. Don't prescribe anything that'll hurt them.