Antibiotics
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I now have rich textbook content on antibiotic classification. Here is a thorough overview:
Antibiotics - Overview & Classification
An antibiotic (antimicrobial agent) is a substance that kills or inhibits the growth of bacteria. They are broadly grouped by their mechanism of action, spectrum of activity, and whether they are bactericidal (kill bacteria) or bacteriostatic (inhibit growth).
Types of Antimicrobial Therapy
Before choosing an antibiotic, clinicians consider three therapeutic strategies (Textbook of Family Medicine 9e, p. 254):
| Type | Definition |
|---|---|
| Empiric (Presumptive) | Started before culture results, based on likely pathogens |
| Targeted (Precise) | Narrowed after culture identifies the organism (de-escalation) |
| Prophylactic (Preventive) | Given before a procedure or in defined high-risk situations |
Classification by Mechanism of Action
1. Beta-Lactams - Disrupt Cell Wall Synthesis
(inhibit peptidoglycan cross-links)
The largest and most widely used class. Further sub-classified:
| Sub-class | Examples |
|---|---|
| Penicillins (narrow-spectrum) | Penicillin V, Penicillin G, Amoxicillin, Ampicillin |
| Antistaphylococcal penicillins | Oxacillin, Nafcillin, Dicloxacillin, Methicillin |
| Extended-spectrum penicillins | Ticarcillin, Piperacillin |
| Beta-lactamase inhibitor combinations | Amoxicillin-clavulanate, Piperacillin-tazobactam, Ampicillin-sulbactam |
| Carbapenems | Ertapenem, Meropenem, Doripenem, Imipenem |
| Monobactams | Aztreonam (gram-negative only; safe in penicillin allergy) |
| 1st-gen cephalosporins | Cephalexin, Cefazolin |
| 2nd-gen cephalosporins | Cefoxitin, Cefuroxime |
| 3rd-gen cephalosporins | Ceftriaxone, Cefotaxime, Ceftazidime, Cefixime, Cefpodoxime |
| 5th-gen cephalosporins | Ceftaroline (MRSA-active) |
Key note: Carbapenems cover most aerobic and anaerobic bacteria but have gaps including Stenotrophomonas maltophilia, Burkholderia cepacia, Legionella, and MRSA. Cross-reactivity risk between penicillins and cephalosporins is low (≤3-5%).
2. Glycopeptides - Disrupt Cell Wall Synthesis
(steric hindrance of backbone glycan formation)
| Agent | Key Use |
|---|---|
| Vancomycin | MRSA, severe gram-positive infections, C. diff (oral) |
| Teicoplanin | Similar spectrum to vancomycin |
3. Lipopeptides - Disrupt Cell Membrane
(create pores/holes in the membrane)
| Agent | Key Use |
|---|---|
| Daptomycin | MRSA, VRE, skin/soft tissue gram-positive infections |
4. Protein Synthesis Inhibitors
| Sub-class | Target | Examples |
|---|---|---|
| Macrolides | 50S ribosome | Azithromycin, Clarithromycin, Erythromycin |
| Lincosamides | 50S ribosome | Clindamycin |
| Chloramphenicol | 50S ribosome | Chloramphenicol (rarely used - aplastic anemia risk) |
| Oxazolidinones | 50S ribosome | Linezolid, Tedizolid |
| Aminoglycosides | 30S ribosome | Gentamicin, Tobramycin, Amikacin, Streptomycin |
| Tetracyclines | 30S ribosome | Doxycycline, Minocycline, Tetracycline |
| Glycylcyclines | 30S ribosome | Tigecycline |
5. DNA/Nucleic Acid Synthesis Inhibitors
| Sub-class | Mechanism | Examples |
|---|---|---|
| Fluoroquinolones | Inhibit DNA gyrase (topoisomerase II/IV) | Ciprofloxacin, Levofloxacin, Moxifloxacin, Norfloxacin |
| Nitroimidazoles | Disrupt DNA synthesis via nitro radical | Metronidazole (anaerobes and protozoa) |
| Rifamycins | Inhibit RNA polymerase | Rifampin (TB, leprosy, MRSA decolonization) |
6. Folate Synthesis Inhibitors
(inhibit nucleotide synthesis)
| Agent | Mechanism |
|---|---|
| Trimethoprim-Sulfamethoxazole (TMP-SMX) | Sequential blockade of folate pathway (synergistic combination) |
Bactericidal vs. Bacteriostatic
| Bactericidal (kill bacteria) | Bacteriostatic (inhibit growth) |
|---|---|
| Beta-lactams | Tetracyclines |
| Aminoglycosides | Macrolides |
| Fluoroquinolones | Clindamycin |
| Vancomycin | Trimethoprim-sulfamethoxazole |
| Metronidazole | Chloramphenicol |
| Daptomycin | Linezolid |
Spectrum of Activity at a Glance
| Antibiotic | Gram + | Gram - | Anaerobes | Special Coverage |
|---|---|---|---|---|
| Penicillin G | +++ | + | + | Streptococci, syphilis |
| Amox-clavulanate | +++ | ++ | ++ | Beta-lactamase producers |
| Piperacillin-tazobactam | +++ | +++ | +++ | Pseudomonas |
| Carbapenems | +++ | +++ | +++ | Broad (not MRSA) |
| Aztreonam | - | +++ | - | Gram-negatives only |
| Vancomycin | +++ | - | - | MRSA, VRE |
| Ciprofloxacin | + | +++ | - | Pseudomonas, GI pathogens |
| Moxifloxacin | ++ | ++ | ++ | Respiratory pathogens, atypicals |
| Metronidazole | - | - | +++ | Anaerobes, C. diff, parasites |
| TMP-SMX | ++ | ++ | - | UTI, PCP, MRSA (community) |
Key Practical Principles
- De-escalate once culture results return - always narrow the spectrum to reduce resistance and C. diff risk.
- Nearly 100% oral bioavailability drugs (no need for IV): TMP-SMX, fluoroquinolones, metronidazole, chloramphenicol.
- Monitor vancomycin and aminoglycosides with serum drug levels and renal function.
- Penicillin allergy cross-reactivity with cephalosporins and carbapenems is low (≤3-5%) - allergy history and reaction type (IgE-mediated vs. non-IgE) must be documented.
Sources: Textbook of Family Medicine 9e (Table 15-2, pp. 254-255)
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