Trimethoprim is a bacteriostatic antibiotic that inhibits bacterial growth by interfering with the synthesis of folic acid. This antibiotic binds to the 30S ribosomal subunit and inhibits protein synthesis and cell division. Trimethoprim inhibits the growth of bacteria that produce benzalkonium chloride such as Pseudomonas aeruginosa. Trimethoprim is used in combination with polymyxin B or colistin against gram-negative bacteria that are resistant to other antibiotics.