Core42: Principles of antimicrobial treatment - bactericidal, bacteriostatic antibiotics and antibiotic combinations

💊 High-yield / 要点：Bactericidal antibiotics kill bacteria; bacteriostatic antibiotics inhibit growth and rely on host immunity. Combinations are used for synergy, broader spectrum, or resistance prevention.

Bactericidal vs bacteriostatic / 殺菌性と静菌性

Type	Meaning	Required concentration	Clinical point
Bactericidal	Kills microorganisms within a short time	MBC at infection site	Preferred in immunocompromised patients or severe infections
Bacteriostatic	Inhibits microbial growth	MIC at infection site	Depends on host immune defense for clearance

Examples / 例

Bactericidal	Bacteriostatic
β-lactams, glycopeptides, aminoglycosides, fluoroquinolones, metronidazole	Chloramphenicol, lincosamides, macrolides, tetracyclines

Antibiotic combinations / 抗菌薬併用

• Using more than one antibiotic can:
  • Produce synergy and improve efficacy.
  • Extend antimicrobial spectrum.
  • Prevent resistance development.

Purpose	Examples
Synergy	Sulfonamide + trimethoprim; β-lactam/glycopeptide + aminoglycoside
Broader spectrum	Cephalosporin + metronidazole for mixed abdominal infection
Prevent resistance	Antituberculous combination therapy

Combination caution / 併用時の注意

• Combining bacteriostatic + bactericidal drugs is generally avoided.
• Reason: bacteriostatic drugs stop bacterial division, which can make some bactericidal drugs less effective.
• Exception:
  • If targets/locations differ, e.g. intracellular + extracellular coverage.
  • Example: streptomycin + doxycycline for zoonoses such as plague or brucellosis.

Remember / 覚え方

• Cidal = kills
• Static = stops growth
• MBC for cidal, MIC for static
• Combination = synergy/spectrum/resistance