• Bacteriostatic antimicrobials : If the structural bonds between antimicrobials and bacteria are weak, the antimicrobials only inhibit the growth of bacteria and do not kill the bacteria. This is called the “bacteriostatic effect” and bacteria can continue to proliferate once antimicrobials that have a bacteriostatic effect are removed.
• Bactericidal antimicrobials : If the structural bonds between antimicrobials and bacteria are irreversible, the antimicrobials completely kill the bacteria. This is called the “bactericidal effect.”

Bacteria are covered by cell walls, which are not present in mammalian cells, and some antimicrobials hinder each stage of cell wall synthesis, showing antibacterial effects. Beta-lactam antimicrobials and glycopeptides are included in this group.

1. Beta-lactam antimicrobials : These are antimicrobials that have the lactam ring in the chemical structure. Beta-lactam antimicrobials are the most widely used antimicrobials in clinical practice.
   • ① Penicillins : These antimicrobials were first used for the treatment of staphylococcal infections, and resistant strains have emerged owing to increased usage. Beta-lactamase inhibitors such as clavulanic acid and tazobactam are used in combination with beta-lactam antimicrobials.
   • ② Cephalosporins : Cephalosporins are classified into generations through their antimicrobial spectrum from the first to the fifth. The progress in the generation does not imply an increase in the strength of the antimicrobials.
   • ③ Monobactams : Monobactams are a drug comprising exclusively of the beta-lactam ring and has a good antibacterial effect against Gram-negative bacteria. In addition, this drug does not show cross-hypersensitiveness.
   • ④ Carbapenems : Carbapenems have a broad antimicrobial spectrum and are effective against most Gram-positive and negative bacteria.
2. Glycopeptide : Glycopeptides, including vancomycin and teicoplanin, that show a narrow spectrum antimicrobial effect are effective only against Gram-positive bacteria.

The cell membrane performs selective active transport as a permeability barrier, thus balancing the ions and high-density molecules between the intracellular and extracellular space. Some antimicrobials, including polymyxin, and some antifungal agents, change the function of the cell membranes of bacteria that are involved in active transport, resulting in bacterial death. These antimicrobials have neuro- and nephrotoxicity when administered in high doses; therefore, caution must be taken when using them.

Ribosomal protein synthesis in cytoplasm is essential for cell survival and the ribosome is a good target for antimicrobials, including aminoglycosides, tetracyclines, macrolides, lincosamides, and phenicols, because of the different ribosome compositions of bacteria and human cells.

Antimicrobials, including quinolone and rifampicin, have antimicrobial effects by interfering with DNA replication, transcription, and RNA synthesis, which are necessary processes for bacterial growth.

Folic acid, also known as vitamin B9, is an essential substance for nucleic acid synthesis, and human cells and bacteria obtain folic acid through different ways. Human cells ingest folic acid as an external food because they cannot biosynthesize it, while bacteria have their own biosynthesis process. Sulfonamide and trimethoprim are representative antimicrobial activities by impeding the folic acid biosynthesis process.