In vitro study of the susceptibility of Escherichia coli to mecillinam

Sir, There is considerable concern about the increasing levels of antimicrobial resistance. Results from a recent UK survey showed that, for enterobacteria, the major urinary tract pathogens, ampicillin resistance is now around 40% and resistance to trimethoprim approaches 30%. Whereas the development of totally new antimicrobial agents takes several years, the re-evaluation of older antibiotics, which are not widely used, can be conducted more rapidly. As part of two studies of uncomplicated urinary tract infections (UTI) in women we investigated the susceptibility pattern of all the Escherichia coli isolates to mecillinam. A total of 574 isolates from six centres in five countries (UK (two), Sweden, France, Portugal and Greece) were tested against the following antibiotics: ampicillin, amoxycillin/ clavulanic acid, mecillinam, sulphonamides, trimethoprim, sulphamethoxazole/trimethoprim (not by UK centre II), nitrofurantoin and ciprofloxacin. The in vitro susceptibility was determined primarily by agar diffusion using Rosco Neosensitabs (Rosco, Taastrup, Denmark) on Danish Blood Agar (Statens Serum Institut (SSI), Copenhagen, Denmark). Because this medium tends to give false resistant results for mecillinam, such ‘resistant’ isolates were retested on Isosensitest agar using 25 g discs (Oxoid, Basingstoke, UK) and/or Etest (AB Biodisk, Solna, Sweden). The susceptibility was determined using the guidelines from Rosco and the guidelines issued by the Swedish Reference Group for Antibiotics (SRGA). The percentage of resistance to the eight antibiotics is shown in the Table. Pivmecillinam, the orally administered form of mecillinam, is an effective treatment for UTI including bacteriuria during pregnancy. The observation that a high percentage of E. coli remain susceptible to mecillinam suggests that this antibiotic may have a valuable role in the treatment of uncomplicated UTI in primary care.