Methacycline is a tetracycline antibiotic. Similar to other tetracyclines, it has a wide spectrum of antimicrobial action. It is active against most Gram-positive bacteria (pneumococci, streptococci, staphylococci) and Gram-negative bacteria (E. coli, salmonella, shigella, etc.), and towards agents causing onithosis, psittacosis, trachoma, and some Protozoa. Like other tetracyclines, the general usefulness of methacycline has been reduced with the onset of bacterial resistance. Methacycline inhibits the binding of aminoacyl-tRNA to the mRNA-ribosome complex. Methacycline inhibits cell growth by inhibiting translation. It binds to the 16S part of the 30S ribosomal subunit and prevents the amino-acyl tRNA from binding to the A site of the ribosome. Methacycline is mostly used for the treatment of acute bacterial exacerbations of chronic bronchitis.