Hi Adouane, I've used a protocol similar to the one used in this paper and it worked well:

Article Genetic Determinants Involved in the Susceptibility of Pseud...

I increased the nitrocefin to 100uM for stronger signal. Also, depending on your organism, you may need to incubate your strain with a subinhibitory dose of inducer b-lactam (cefoxitin or imipenem) to see the b-lactamase activity.

Hope this helps,

Andrew