Penicillinaseresistant

Penicillinaseresistant refers to penicillin antibiotics that are resistant to hydrolysis by penicillinase, the beta-lactamase enzyme produced by many bacteria. The term is commonly used for penicillinase-resistant penicillins, a subset of beta-lactam antibiotics known as isoxazolyl penicillins.

These agents were developed to treat infections caused by penicillinase-producing staphylococci, particularly Staphylococcus aureus, by maintaining activity in the presence of the penicillinase enzyme. Their activity is primarily against Gram-positive cocci, including methicillin-susceptible Staphylococcus aureus and various streptococci, with limited efficacy against many Gram-negative bacteria.

Examples include methicillin, nafcillin, oxacillin, cloxacillin, and flucloxacillin. The term is often used to distinguish these drugs from other penicillins that are readily inactivated by penicillinase.

Resistance can arise when bacteria acquire other beta-lactamases or modify penicillin-binding proteins. Meticillin-resistant Staphylococcus aureus (MRSA) is resistant to methicillin and other penicillinase-resistant penicillins due to the mecA gene, which limits their usefulness. These agents are also less effective against organisms producing broad-spectrum beta-lactamases (such as ESBLs and AmpC) and against non-susceptible Gram-negative bacteria.

In modern clinical practice, penicillinase-resistant penicillins are largely supplanted for many infections by other agents, especially where MRSA is a concern. They may still be used for certain methicillin-susceptible staphylococcal and some streptococcal infections, but their role is more limited than in the past. See also beta-lactamase, MRSA, and isoxazolyl penicillins.