Penicillins, Cephalosporins & UTI Antibiotics
When a vignette puts you on a primary-care infection, the choice almost always comes down to a beta-lactam or a UTI agent. Penicillins (amoxicillin, amoxicillin-clavulanate, dicloxacillin) cover strep pharyngitis, otitis media, dental infections, and skin/soft-tissue infections; add the clavulanate for beta-lactamase coverage in bite wounds and sinusitis. Cephalosporins layer broader gram-negative coverage by generation. For uncomplicated cystitis, the first-line agents are nitrofurantoin (avoid in CrCl <30 and at term), TMP-SMX (avoid in 3rd trimester and sulfa allergy), and fosfomycin (single-dose). Pyelonephritis steps up to a fluoroquinolone or ceftriaxone. The exam pattern is matching the syndrome, the patient (pregnancy, renal function, allergy), and the local resistance to the right first-line drug.
📖 Overview
Penicillins (amoxicillin, amoxicillin-clavulanate) and cephalosporins (cephalexin) are first-line for most primary care infections — streptococcal pharyngitis, otitis media, sinusitis, cellulitis, and uncomplicated UTI. Nitrofurantoin and TMP-SMX are UTI-specific workhorses with distinct pregnancy trimester rules that get tested constantly.
⚙️ Mechanism of Action
Beta-lactams: inhibit penicillin-binding proteins → block cell wall cross-linking → bacterial lysis. Nitrofurantoin: multiple intracellular targets, concentrated in urine. TMP-SMX: sequential block of folate synthesis (dihydrofolate reductase + dihydropteroate synthase).
💎 Board Pearls
- ⚠️ True PCN allergy (anaphylaxis) → avoid all PCNs and 1st-gen cephalosporins; 2nd+ gen usually OK.
- 🦴 Amoxicillin-clavulanate = bite wounds, chronic sinusitis, recurrent otitis.
- 🤰 Pregnancy-safe UTI abx: nitrofurantoin (avoid 1st trim + near term), amox, cephalexin.
- 🚫 Nitrofurantoin contraindicated: CrCl <30 (insufficient urinary concentration).
- 🚫 TMP-SMX avoided in 1st trimester (NTDs) + near term (kernicterus risk).
- 🧪 TMP-SMX → hyperkalemia, ↑ warfarin effect, SJS/TEN risk.
- 🐾 Amoxicillin-clavulanate first-line for dog/cat/human bite wounds.
💊 Drugs in This Class
- Amoxicillin — AmoxilStrep throat, otitis media, sinusitis, CAP (child), UTI in pregnancy, dental prophylaxis, H. pylori (combo).
- Amoxicillin-Clavulanate — AugmentinSinusitis (if abx indicated), chronic otitis, aspiration pneumonia, dog/cat/human bites, diabetic foot infection.
- Cephalexin — KeflexCellulitis/SSTI, UTI in pregnancy, strep pharyngitis alternative, dental prophylaxis (if PCN allergy non-severe).
- Trimethoprim-Sulfamethoxazole — Bactrim, Septra, TMP-SMXUncomplicated UTI, MRSA SSTI, PJP/PCP prophylaxis, traveler's diarrhea.
- Nitrofurantoin — Macrobid, MacrodantinUncomplicated cystitis (lower UTI only) — NOT for pyelonephritis.
Practice Questions
Which of the following is considered FIRST-LINE oral therapy for uncomplicated cystitis in a pregnant patient at 14 weeks gestation?
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