system
Given the following post, respond as if you're commenting on Reddit:
human
Why is tetracycline bacteriostatic whereas streptomycin is bactericidal?

Don't both of the antibiotics work by preventing translation by disrupting the 30S ribosomal subunit?
Why is tetracycline considered bacterriostatic while streptmocymin is considered bactericidal when they inhibit the same process?
gpt
Damn, this is an important question but where do you begin. Basically, the notion that whether an antibiotic is bacterio-static or -cidal is defined by its mechanism of action is incorrect. 

They are blanket terms describing what you see on your petri dish as a microbiologist. Bacteriostatic means no new colonies forming but existing colonies persist, and bactericidal means bacteria dying.

As you increase the concentration of any antibiotic starting from zero, that antibiotic will exhibit bacteriostatic effects (inhibit growth in-vitro), then bactericidal effects (kill bacteria in-vitro).

Before going on to the clinical definitions of bacteriostatic and bactericidal *agents*, here are some terms:

Minimum Bactericidal Concentration (MBC): Concentration at which 99.9% of the bacteria are killed (in-vitro)

Minimum Inhibitory Concentration (MIC): Concentration at which bacterial growth is inhibited (in-vitro)

A bacteriostatic agent is defined as one where MBC is much greater than MIC, meaning that an extremely large concentration is needed to achieve a bactericidal effect. A bactericidal agent is one where MBC is only so much greater than MIC, such that you *don't* need an extremely large concentration to achieve a bactericidal effect.

So in summary, as a generality, a bactericidal drug means "at a safe/practical concentration, this drug will exhibit bactericidal activity" and a bacteriostatic drug means "at a safe/practical concentration, this drug will exhibit bacteriostatic activity".