Antibiotics
Penicillin
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Syphilis (Treponema pallidum):
- Drug of Choice: Penicillin G is the only effective treatment for all stages, especially neurosyphilis and syphilis in pregnancy.
- Regimen: Single IM dose of Benzathine Penicillin G for primary/secondary syphilis.
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Streptococcal Pharyngitis (Strep Throat - Group A Strep):
- Drug of Choice: Oral Penicillin V or Amoxicillin for 10 days.
- Key Goal: Prevent acute rheumatic fever. No known resistance of GAS to penicillin.
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Rheumatic Fever Prophylaxis:
- Drug of Choice: Long-acting IM Benzathine Penicillin G (e.g., every 3-4 weeks) for secondary prevention after an initial episode.
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Actinomycosis (Actinomyces israelii):
- Drug of Choice: High-dose IV Penicillin G for an extended period, followed by oral therapy.
- Buzzwords: "Sulfur granules," jaw/cervicofacial abscess.
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Leptospirosis (Leptospira spp.):
- Drug of Choice: IV Penicillin G is used for severe disease.
- Presentation: Flu-like illness, conjunctival suffusion, myalgias, often with animal/water exposure.
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Meningococcal Disease (Neisseria meningitidis):
- Drug of Choice: Penicillin G is effective if the isolate is susceptible (though ceftriaxone is often used empirically first).
Cephalosporins
Mnemonic
Cephalosporins are LAME: 1st–4th generation cephalosporins do not act against
- Listeria
- Atypical organisms (e.g., Chlamydia, Mycoplasma)
- No cell wall
- MRSA
- Enterococci (with the exception of ceftaroline, which does act against MRSA).
Nitrofurans
- Examples: nitrofurantoin
- Mechanism of action: reduced by bacterial nitroreductases to reactive metabolites → bind to bacterial ribosomes → impaired metabolism, impaired synthesis of protein, DNA, and RNA → cell death (bactericidal effect)
- Treatment and prophylaxis of acute uncomplicated UTIs (e.g., urethritis, cystitis)
Drug resistance
Mechanisms
Type of resistance | Example mechanisms | Example antibiotics affected |
---|---|---|
Increased drug efflux pump | • New membrane transport system | • Tetracyclines, fluoroquinolones |
Drug-inactivating enzyme | • Beta-lactamase • Acetyltransferase |
• Penicillins, cephalosporins • Aminoglycosides |
Decreased drug uptake | • Mutated porin protein | • Penicillins, fluoroquinolones |
Modified drug target | • Altered penicillin-binding protein • Altered ribosomal protein • Altered DNA gyrase |
• Penicillins • Aminoglycosides, macrolides • Fluoroquinolones |
Examples
1. MRSA (Methicillin-Resistant Staphylococcus aureus)
- Clues: Purulent skin/soft tissue infections (SSTIs) often looking like "spider bites" (CA-MRSA); hospital-acquired pneumonia, post-op infections, line infections (HA-MRSA). Post-influenza pneumonia.
- Mechanism: mecA gene (altered PBP2a).
- Tx:
- SSTI (CA): TMP-SMX, Doxycycline, Clindamycin (after I&D).
- Severe/Hospital (HA): Vancomycin, Linezolid, Daptomycin.
2. VRE (Vancomycin-Resistant Enterococci)
- Clues: UTI, bacteremia in hospitalized patients with prolonged antibiotic use (especially vancomycin/cephalosporins) or indwelling catheters.
- Mechanism: D-Ala-D-Ala to D-Ala-D-Lac/Ser.
- Tx: Linezolid, Daptomycin. (Nitrofurantoin/Fosfomycin for cystitis if susceptible).
3. Extended-Spectrum Beta-Lactamase (ESBL)-Producing Enterobacterales (E. coli, Klebsiella)
- Clues: Recurrent UTIs despite cephalosporins; hospital/LTCF-acquired infections (UTI, intra-abdominal).
- Mechanism: Beta-lactamases hydrolyzing most penicillins & cephalosporins.
- Tx: Carbapenems (Imipenem, Meropenem). (Fosfomycin/Nitrofurantoin for cystitis if susceptible).
4. CRE (Carbapenem-Resistant Enterobacterales - e.g., Klebsiella pneumoniae carbapenemase/KPC)
- Clues: Critically ill ICU patients; multiple prior antibiotics (including carbapenems); invasive devices. High mortality.
- Mechanism: Carbapenemase enzymes.
- Tx: Very limited; often combination therapy with newer agents (e.g., Ceftazidime-avibactam, Meropenem-vaborbactam) or Colistin/Tigecycline.
5. Drug-Resistant Streptococcus pneumoniae
- Clues: CAP, meningitis, otitis media failing first-line beta-lactams. Risk factors: age extremes, daycare, recent antibiotics.
- Mechanism: Altered PBPs (penicillin resistance); ribosomal methylation/efflux (macrolide resistance).
- Tx:
- CAP (non-meningeal): Respiratory Fluoroquinolones (Levofloxacin, Moxifloxacin), Vancomycin (if severe/highly resistant). Ceftriaxone (if lower-level penicillin resistance).
- Meningitis: Vancomycin + 3rd gen Cephalosporin (e.g., Ceftriaxone).
6. MDR-TB & XDR-TB (Multidrug/Extensively Drug-Resistant Tuberculosis)
- Clues: Prior TB treatment failure; from high-prevalence area; HIV co-infection; persistent TB symptoms despite standard therapy.
- MDR: Resistant to Isoniazid (INH) + Rifampin (RIF).
- XDR: MDR + resistance to a fluoroquinolone AND ≥1 injectable second-line drug.
- Mechanism: Chromosomal mutations (e.g., katG, rpoB).
- Tx: Complex, prolonged multi-drug regimens with second-line drugs (e.g., Bedaquiline, Linezolid, Fluoroquinolones) based on DST. Expert consultation mandatory.