Temperature-sensitive 6-aminopenicillanic acid-resistant mutants of Escherichia coli

Wild-type Escherichia coli cells became spherical in the presence of low concentrations of 6-aminopenicillanic acid (APA). Higher concentrations of APA caused inhibition of cell division (filamentation) and lysis. Spontaneous temperature-sensitive APA-resistant mutants were isolated and characterized. These mutants also possessed increased resistance to mecillinam but not to benzylpenicillin, ampicillin, and cephaloridine. They formed round or pleomorphic cells at the nonpermissive temperature and possessed thermolabile penicillin-binding protein 2 activity. Temperature-resistant revertants of these mutants had normal rod-shaped morphology, normal levels of sensitivity to APA and mecillinam, and thermostable penicillin-binding protein 2 activity. The mutation conferring APA resistance cotransduced with lip and mapped, therefore, close to 14 min on the E. coli linkage map. APA appears to be more specific than mecillinam as a selective agent for the isolation of penicillin-binding protein 2 mutants.