bacteriostatic
A bacteriostatic drug holds bacteria in check without killing them; it stops them dividing and multiplying, but the organisms remain alive. Think of it as pressing pause on the infection: the bacterial population stops growing, and the body's own immune system is then left to clear the stalled invaders.
Many protein-synthesis inhibitors are bacteriostatic, including macrolides, tetracyclines and clindamycin, as is the antifolate trimethoprim. By starving the bacterium of new proteins or essential metabolites, they freeze its life cycle. Crucially, if the drug is stopped too early or fails to reach the site, the suppressed bacteria can simply resume growing, so a competent immune system is usually assumed.
The label is relative, not absolute: a bacteriostatic drug at higher concentration, or against a more susceptible organism, may kill. Because of their reliance on host defences, bacteriostatic agents are generally avoided as sole therapy in settings where the immune system cannot help, such as meningitis, endocarditis or severe neutropenia, where bactericidal drugs are preferred.
Doxycycline, a bacteriostatic tetracycline, is enough to treat mild acne in an otherwise healthy person because the immune system handles the rest.
Bacteriostatic therapy leans on a working immune system to finish the clearance.
Combining a bacteriostatic drug with a bactericidal one that needs actively growing cells (such as a beta-lactam) can in theory reduce killing, since stalled bacteria are harder to kill; in practice this antagonism is uncommon.