Antibiotics (Antibacterials)

Used to prevent and treat bacterial infections.

Two main mechanisms of action:

• Bacteriostatic → Inhibits bacterial growth.

• Bactericidal → Kills bacteria directly.
  ❗ Do not combine bacteriostatic and bactericidal drugs — they cancel each other out.

1. Sulfonamides (Sulfa Drugs)

• Mechanism: Folic acid synthesis inhibitor → competes with PABA (para-aminobenzoic acid).

• Effect: Causes folate deficiency → inhibits bacterial DNA synthesis.

• Type: Bacteriostatic.

• Examples: Sulfadiazine, Sulfamethoxazole.

• Key tip: “Sulfa–” prefix → think folate synthesis inhibition.

2. Fluoroquinolones

• Mechanism: DNA synthesis inhibitor (directly).

• Type: Bactericidal.

• Examples: Ciprofloxacin, Levofloxacin.

• Tip: “–floxacin” suffix → DNA synthesis blocker.

3. Penicillins

• Mechanism: Cell wall synthesis inhibitor.

• Type: Bactericidal.

• Class: β-lactam antibiotics (contain β-lactam ring).

• Cross-reactivity: Allergic with Cephalosporins.

• Examples:

  • Penicillin G: IV only (acid-sensitive).

  • Penicillin V: Oral (acid-stable).

  • Amoxicillin: Broad-spectrum.

  • Augmentin: Amoxicillin + Clavulanic acid (β-lactamase resistant).

  • Methicillin / Dicloxacillin: β-lactamase-resistant forms.

  • Ampicillin: Best gram-negative coverage.

  • Carbenicillin: Effective vs. Pseudomonas.

• Tip: “–cillin” suffix = Penicillin family.

4. Cephalosporins

• Mechanism: Cell wall synthesis inhibitor.

• Type: Bactericidal.

• Class: β-lactam antibiotic.

• Generations: 1st → 5th (increasing gram-negative coverage).

• Prefix: “Cef–” or “Ceph–”.

• Example: Cephalexin (Keflex).

5. Monobactams

• Mechanism: Cell wall synthesis inhibitor.

• Type: Bactericidal.

• Example: Aztreonam.

• Tip: “–am” suffix (from monobactam).

6. Carbapenems

• Mechanism: Cell wall synthesis inhibitor.

• Type: Bactericidal.

• Example: Imipenem.

• Tip: “–penem” suffix = Carbapenem family.

7. Tetracyclines

• Mechanism: Protein synthesis inhibitor (binds 30S ribosomal subunit).

• Type: Bacteriostatic.

• Examples: Tetracycline, Doxycycline, Minocycline.

• Note: Broadest antimicrobial spectrum.

• Tip: “–cycline” suffix → 30S inhibitor.

8. Macrolides

• Mechanism: Protein synthesis inhibitor (binds 50S ribosomal subunit).

• Type: Bacteriostatic.

• Examples: Erythromycin, Clarithromycin, Azithromycin.

• Mnemonic: “Mac likes to throw mice” → Macrolides → “–thromycin”.

9. Lincosamides

• Mechanism: Protein synthesis inhibitor (50S subunit).

• Type: Bacteriostatic.

• Example: Clindamycin.

• Mnemonic: “Link also hides mice” → Lincosamides → “–mycin” (without “thro”).

Antibiotic Prophylaxis in Dentistry

When Required (AHA Guidelines):

1. Prosthetic heart valve.

2. History of infective endocarditis.

3. Heart transplant with valvulopathy.

4. Congenital heart disease (certain cases).

Also indicated for immunocompromised patients:

• Organ transplant, chemotherapy, neutropenia, etc.

Dental Procedures Needing Prophylaxis:

• Extractions, periodontal surgery, implants, or any invasive procedure with bleeding.

Prophylaxis Regimens

1️⃣ First Choice:

• Amoxicillin 2 g PO, 1 hr before procedure.

• Children: 50 mg/kg PO, 1 hr before.

2️⃣ If Penicillin Allergy:

• Clindamycin 600 mg PO, 1 hr before.

• Children: 20 mg/kg PO, 1 hr before.

3️⃣ If Oral Not Possible:

• Ampicillin 2 g IV/IM, 30 min before.

• Children: 50 mg/kg IV/IM, 30 min before.

4️⃣ Prosthetic Joint Prophylaxis:

• Cephalexin (Keflex) 2 g PO, 1 hr before.

When NOT Needed

• Cardiac pacemaker.

• Rheumatic fever without valvular dysfunction.

• Mitral valve prolapse without regurgitation.

• Non-invasive dental procedures (restorations, RCT, impressions).

Antibiotic Side Effects

Effect	Drug
Pseudomembranous colitis	Clindamycin
Superinfection	Broad-spectrum drugs
Aplastic anemia	Chloramphenicol
Liver damage	Tetracycline
Allergic cholestatic hepatitis	Erythromycin

Drug Interactions

• Bactericidal + Bacteriostatic → Decrease effectiveness.

• Penicillin + Probenecid → ↓ Renal clearance.

• Tetracycline + Milk/Antacids → ↓ Absorption (Chelation with Ca²⁺).

• Broad-spectrum antibiotics + Anticoagulants → ↑ Coumarin effect (↓ Vitamin K).

• Antibiotics + Oral contraceptives → ↓ Effectiveness.

• Macrolides (Erythromycin) → Inhibit metabolism of Digoxin, Seldane.

Drug concentration facts:

• Clindamycin → Bone.

• Tetracycline → Gingival crevicular fluid.

Antivirals & Antifungals

Antivirals (–vir):

• Acyclovir, Valacyclovir → Treat Herpes infections.

Antifungals (–azole):

• Fluconazole, Ketoconazole → Treat Candidiasis.

• Clotrimazole (Mycelex) → Found in troche (lozenge form).

High-Yield Summary

• β-lactam antibiotics: Penicillins, Cephalosporins, Monobactams, Carbapenems.

• Broadest spectrum: Tetracyclines.

• 30S inhibitor: Tetracyclines.

• 50S inhibitor: Macrolides, Lincosamides.

• Drug allergy link: Penicillin ↔ Cephalosporins.

• Prophylaxis 1st line: Amoxicillin 2g, 1 hr before.