Induction of an enzyme is a temporary phenomenon in which the rate of enzyme synthesis is greatly increased in response to the presence of an inducer in the environment. Induction of beta-lactamase synthesis is important for the resistance of staphylococci to penicillins since the drug both induces synthesis of the enzyme and is hydrolysed by it. Similarly, some compounds both strongly induce and are hydrolysed by the chromosomally-determined beta-lactamases of gram-negative bacilli (e.g. amoxicillin and cefoxitin for Enterobacter cloacae). Other compounds (e.g. piperacillin and cefotaxime) although labile are poor inducers, so that in the case of these drugs the phenomenon of induction is not important but the presence of the enzyme is, since resistant mutants with genetically derepressed synthesis of the enzyme can emerge. Induction can also be important when a strong inducer is present with a poor inducer and antagonises the activity of the latter.