Tetracyclines are growth inhibitors (bacteriostatic) rather than killers of the infectious agent (bacteriocidal) and are only effective against multiplying microorganisms. They are short-acting and passively diffuse through porin channels in the bacterial membrane. They inhibit protein synthesis by binding reversibly to the bacterial 30S ribosomal subunit and preventing the aminoacyl tRNA from binding to the A site of the ribosome. They also bind to some extent the bacterial 50S ribosomal subunit and may alter the cytoplasmic membrane causing intracellular components to leak from bacterial cells.