Mecillinam resistance in Escherichia coli: dissociation of growth inhibition and morphologic change.

The resistance of mecillinam of Escherichia coli strain RF292, which was isolated from a patient during relapse of septicemia, was investigated. Although strain RF292 underwent morphologic change at the same concentration of mecillinam as the wild-type strain (RF81) and a revertant strain (RF293), RF292 was 128-fold more resistant than RF81 or RF293 to growth inhibition by mecillinam. The resistance of RF292 was associated with a 15%-35% longer generation time and a 30%-35% smaller cell volume than RF81 or RF293. The latter two strains. When growing slowly under nutritionally deprived conditions, assumed a mecillinam-resistant phenotype. Thus, resistance of E. coli to growth inhibition by mecillinam can occur in association with a slow growth rate and a small cell volume of either hereditary or environmental origin.