The complex mechanism of resistance towards beta-lactam antibiotics in Gram-negative bacteria can be described by a simple equation in which the hydrolytic activity of the enzyme, the affinity to the penicillin binding protein in competition to the affinity to the beta-lactamase, the quantity of the enzyme and the ability of the drug to penetrate the outer membrane are considered. Depending on the characteristics of the drug, the beta-lactamase, and the microorganisms, the hydrolysis of the drug, its binding, or the reduced penetration can be the predominant factors.