In vivo mecillinam resistance in Escherichia coli after pivmecillinam treatment of a urinary tract infection.

Pivmecillinam (amdinocillin pivoxil) is the recommended first choice antibiotic used to treat urinary tract infections (UTIs) in Denmark. Yet, in laboratory settings the frequency of mutation to mecillinam resistance has been found to be very high. Treatment of UTI has a good clinical response, but low bacteriological cure rates. Prevalence of mecillinam resistance in Escherichia coli remains low in spite of many years of use. We describe occurrence of in vivo mecillinam resistance in a clinical isolate of extended-spectrum β-lactamase producing E. coli following pivmecillinam treatment. The only identified phenotypic differences in MEC-R compared to the original MEC-S isolate were full-length lipopolysaccharides with O-antigen (O25), mecillinam resistance, and a lower minimum inhibitory concentration for ceftazidime. Whether the serotype change and mutation in fhlA is the cause of the mecillinam resistance is unknown. Further studies are required to elucidate this possible mechanism. We continue to recommend the use of pivmecillinam as first-line treatment for UTI, yet mecillinam treatment for other foci should be investigated.