**5. Conclusion**

There are several benefits and limitations using either phenotypic or molecular methods for the detection of resistance mechanisms in Gram negative pathogens. Phenotypic tests require bacteria in pure culture from a clinical sample thus needing 24-48h to obtain a final result. Molecular techniques on the other hand, can be performed directly with clinical speciments reducing significantly the procedure time.

The detection of low-level resistance is by definition problematic using phenotypic tests thus interpretation problems may appear. In such cases, molecular techniques are an option for clarifying the possible involvement of any known resistance mechanism.

Moreover, genetic detection gives a definite answer for the presence or not of specific resistance determinants within a study isolate (a specific beta-lactamase for example) whereas this is not possible with the phenotypic tests which provide only general information about the resistance mechanisms involved.

Genetic assays however, present also some major limitations: (i) It is possible to screen exclusively for known mechanisms and for one gene at the time (unless a multiplex PCR assay [85-88] can be applied) and; (ii) their cost is high and becomes higher when screening for multiple resistance determinants.

Consequently, the combined and rational use of the available methodologies seems to be the optimal solution for the cost-effective detection of resistance mechanisms in Gram negative pathogens by the Clinical Microbiology laboratory.
