Imipenem belongs to the subgroup of carbapenems. It is derived from a compound called thienamycin, which is produced by the bacterium Streptomyces cattleya. 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. Imipenem and other drugs in the carbapenem class are typically restricted in use, to avoid widespread bacterial resistance. Imipenem acts as an antimicrobial through inhibiting cell wall synthesis of various Gram-positive and Gram-negative bacteria. It remains very stable in the presence of beta-lactamase (both penicillinase and cephalosporinase) produced by some bacteria, and is a strong inhibitor of beta-lactamases from some Gram-negative bacteria that are resistant to most beta-lactam antibiotics.