Diversification of Escherichia coli expressing an SHV-type extended-spectrum beta-lactamase (ESBL) during a hospital outbreak: Emergence of an ESBL-hyperproducing strain resistant to expanded-spectrum cephalosporins

Twelve SHV-type extended-spectrum beta-lactamase (ESBL)-producing Escherich ia coli lac mutant isolates were recovered in October 1997 from 11 patients of the neonatal ward in a Warsaw hospital. The outbreak was clonal; howeve r, some of the isolates expressed a much higher level of resistance to seve ral beta-lactam antibiotics, including expanded-spectrum cephalosporins. Th is phenotype has been attributed to beta-lactamase hyperproduction correlat ing with the multiplication of ESBL gene copies, as was demonstrated for re presentative isolates.