Mecillinam Resistance in Escherichia coli: Dissociation of Growth Inhibition and Morphologic Change

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Abstract
The resistance to 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.