Diversification of Escherichia coli Expressing an SHV-Type Extended-Spectrum β-Lactamase (ESBL) during a Hospital Outbreak: Emergence of an ESBL-Hyperproducing Strain Resistant to Expanded-Spectrum Cephalosporins

Twelve SHV-type extended-spectrum β-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 β-lactam antibiotics, including expanded-spectrum cephalosporins. This phenotype has been attributed to β-lactamase hyperproduction correlating with the multiplication of ESBL gene copies, as was demonstrated for representative isolates.