Mechanisms, Indications, and Clinical Considerations of Infectious Disease Medications

Exam Tip: When a patient presents with a penicillin allergy, clinicians often look toward Macrolides or Clindamycin as alternatives, but always screen for cross-reactivity in $\beta$-lactam hypersensitivity.

Glycopeptides

Vancomycin is a heavy-duty antibiotic used primarily for MRSA (Methicillin-resistant Staphylococcus aureus). Because it can be toxic, clinicians must measure plasma vancomycin levels and monitor renal function to avoid nephrotoxicity. It should be administered slowly to avoid complications, and it is contraindicated in patients with a history of deafness or certain blood disorders like thrombocytopenia.

Protein Synthesis Inhibitors

These drugs target the bacterial ribosome, preventing the translation of mRNA into essential proteins.

Macrolides and Tetracyclines

Macrolides bind to the 50S ribosomal subunit.

• Erythromycin: Often used when a patient has a penicillin allergy. It carries risks of arrhythmias (specifically Long QT syndrome) and gastrointestinal distress.
• Azithromycin: Used for URTI, SSTI, UTI, and STIs.

Tetracyclines, specifically Doxycyclin, bind to the 30S ribosome to inhibit bacterial growth. It is used for Chlamydia and Pelvic Inflammatory Disease (PID).

Clinical Alert: Doxycyclin is contraindicated in pregnancy, breastfeeding, and children under 12 years of age due to potential effects on bone and tooth development.

Aminoglycosides and Clindamycin

Gentamicin (an Aminoglycoside) binds to the 30S ribosome and alters cell membrane permeability. It is used for septicemia, meningitis, and CNS infections. However, it is highly associated with nephrotoxicity and ototoxicity (damage to hearing/balance).

Clindamycin is used for staphylococcal bone and joint infections, such as osteomyelitis. Prolonged use requires monitoring of blood counts.

DNA and RNA Synthesis Inhibitors

These medications target the genetic machinery of the bacteria, preventing them from replicating or transcribing essential instructions.

DNA Synthesis Inhibition

• Antifolates:
  • Nitrofurantoin: Used for UTIs (cystitis) because it concentrates in the urine. It is contraindicated in pregnancy (near labor) and patients with pulmonary or hepatic disease.
  • Trimethoprim: A second-line treatment for UTI. It works by inhibiting dihydrofolate reductase, which interferes with DNA synthesis.
• Fluoroquinolones: Ciprofloxacin inhibits bacterial replication, transcription, and repair. It is used for severe UTIs and pyelonephritis. A major warning for this class is the risk of irreversible collagen damage in tendons and cardiac abnormalities.

RNA Synthesis Inhibition

Rifamycins, such as Rifampicin, bind to the beta subunit of RNA to inhibit RNA synthesis. This makes it a cornerstone treatment for Tuberculosis. Side effects include nausea and potential liver function alterations.

Other Mechanisms: DNA Damage

Nitroimidazoles like Metronidazole kill anaerobic microbes by causing direct DNA damage. It is the standard treatment for Clostridioides difficile.

Summary Table of Key Antibiotics