OXA-type beta-lactamases.

The OXA-type (oxacillin-hydrolysing) enzymes are widespread and have been mostly described in Enterobacteriaceae and in P. aeruginosa. They usually confer resistance to amino- and ureidopenicillin and possess high-level hydrolytic activity against cloxacillin, oxacillin, and methicillin. Their activities are weakly inhibited by clavulanic acid but sodium chloride (NaCl) possesses a strong inhibition activity. Oxacillin-hydrolysing b-lactamases belong to Ambler class D and thus possess an active serine site as classes A and C b-lactamases. Overall amino-acid identities between class D and class A or class C b-lactamases is about 16%. Until now, 24 Ambler class D enzymes, named OXA-1 to OXA-22, AmpS and LCR-1, have been characterised, either by sequence and/or by biochemical analyses, but for none of them a three dimensional structure is yet available. While some oxacillinases present a significant degree of amino-acid identity (for example, OXA-1 and OXA-4; OXA-10 (PSE-2) derivatives; OXA-2 and OXA-3), most of them are only weakly related (20% to 30% amino-acid identity). Oxacillinases usually display a restricted-spectrum phenotype. However extension of their spectrum towards oxyimino cephalosporins and/or imipenem has recently been observed mostly as a consequence of point mutations in OXA-2 or OXA-10 derivatives. Their frequent plasmid- and/or integron-location provide them a mean for a wide diffusion.