Imipenem was patented in 1975 and approved for medical use in 1985. It was discovered via a lengthy trial-and-error search for a more stable version of the natural product thienamycin, which is produced by the bacterium "Streptomyces cattleya". Thienamycin has antibacterial activity, but is unstable in aqueous solution, so impractical to administer to patients. Imipenem has a broad spectrum of activity against aerobic and anaerobic, Gram-positive and Gram-negative bacteria. It is particularly important for its activity against "Pseudomonas aeruginosa" and the "Enterococcus" species. It is not active against MRSA, however.