Dissemination of cephalosporin-resistantSerratia marcescens strains producing a plasmidic SHV type beta-lactamase in Greek hospitals

The resistance to third generation cephalosporins in nineSerratia marcescens strains isolated in Greek hospitals was studied. Eight of the strains transferred resistance toEscherichia coli by means of large plasmids that encoded for an extended-spectrum -lactamase. Hybridization, isoelectric focusing and hydrolysis studies showed that the enzyme resembled the SHV-5 -lactamase. In the eight isolates that possessed the SHV type enzyme, cephalosporinase expression was inducible, whereas the remaining strain was a cephalosporinase hyperproducing strain. Introduction of a plasmid coding for the regulatoryampD gene in the latter strain eliminated -lactamase production and rendered the strain susceptible to cephalosporins.