What is the difference between bactericidal and bacteriostatic antibiotics?
Question
I remember during our pharmacology review session, someone asked what sets bactericidal antibiotics apart from bacteriostatic ones. It got me thinking, so I looked into it. Turns out, the key difference is that one type kills bacteria outright, while the other just stops them from multiplying, leaving your immune system to handle the rest. Here’s a detailed breakdown of how both work and when they’re used.
Answer ( 1 )
Definition
Bactericidal drugs kill the bacterial cell, bacteriostatic drugs only stop its growth so the host immune system can clear it.
How they work
Cidal agents usually hit an essential structure like the cell wall or the DNA gyrase and cause irreversible damage. Static agents often block protein synthesis or metabolism in a reversible way. Remove the drug and the surviving cells can resume dividing.
Examples
Penicillins, cephalosporins, aminoglycosides and fluoroquinolones are classically bactericidal. Tetracyclines, macrolides, sulfonamides and chloramphenicol are typically bacteriostatic. Some, like linezolid, can swing either way depending on dose and bug.
Clinical use
In most infections either type works fine, but in life‑threatening situations such as endocarditis, meningitis or neutropenia we prefer cidal therapy because the immune backup is weak or too slow.
Lab testing
Minimum inhibitory concentration (MIC) tells you the static threshold. To prove killing you measure the minimum bactericidal concentration (MBC) which is the drug level that reduces the starting inoculum by 99.9 % in 24 h.
Key point
Remember that the distinction is operational, not absolute. A static drug can be cidal at high doses, and a cidal one can act static against some organisms.