 The nature of the ESBL determinants will be investigated by molecular analysis using PCR and sequencing, as described previously 9, 15. The presence of ESBL determinants of the TEM-, SHV, CTX-M- and PER-type i. e. those known to be circulating in Italy 9, 15 will be investigated. In case of negative results with the available probes, the ESBL phenotype will be reconfirmed and, if positive, the nature of -lactamase deerminants will be studied by means of: i analytical isoelectric focusing IEF on crude extracts using previously described methods 14; ii DNA microarray technology, using a microarray for β-lactamase genes that has been developed in our laboratory, which includes probes for uncommon ESBL genes such as VEB, GES, BES, SFO, and TLA enzymes. In case of new ESBL genes undetected by the above methods an unlikely occurrence, In the case of production of ESBL but unaccounted for by the presence of known resistance determinants, the resistance determinants will be studied by means of a shotgun-cloning approach followed by mapping and sequencing. 