In vitro selection and characterization of mutants in TEM-1-producing Escherichia coli by ceftazidime and ceftibuten

The present work was undertaken to study the ability of ceftazidime and ceftibuten to select in vitro resistant mutants of Escherichia coli HB101 harboring bla(TEM-1) beta-lactamase gene. Minimum inhibitory concentrations (MICs) of ceftazidime and ceftibuten were increased by a factor of 32, overcoming in the case of ceftazidime the breakpoint for clinical resistance. Outer membrane protein analysis and PCR for bla(TEM) alleles revealed that ceftazidime and ceftibuten select for different resistance mechanisms. Ceftazidime created mutants that encode an extended-spectrum beta-lactamase (TEM-12) and exhibit decreased expression of OmpF. Ceftibuten was unable to select for extended-spectrum beta-lactamase expressing mutants but reduced the expression of two porins, OmpC and OmpF. The stability of ceftibuten to hydrolysis and the difference in the structure of these beta-lactam antibiotics could be responsible for the selection of different mechanisms of resistance.