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 Escherichia coli lac mutant isolates were recovered in October 1997 from 11 patients of the neonatal ward in a Warsaw hospital. The outbreak was clonal; however, some of the isolates expressed a much higher level of resistance to several beta-lactam antibiotics, including expanded-spectrum cephalosporins. This phenotype has been attributed to beta-lactamase hyperproduction correlating with the multiplication of ESBL gene copies, as was demonstrated for representative isolates.