Greenwood and O'Grady compared ampicillin's effects on Escherichia coli and Proteus mirabilis isolated from urinary tract infections. Although conventional sensitivity testing showed both organisms appeared similarly susceptible, they responded markedly differently when exposed to ampicillin in dense cultures. Using a dilution system where cultures were continuously diluted while exposed to ampicillin, E. coli showed rapid lysis and opacity decline within 1-2 hours. Swarming strains of P. mirabilis initially increased in turbidity before slowly declining, while non-swarming P. mirabilis showed intermediate behavior. Electron microscopy revealed distinct morphological differences: E. coli underwent rapid lysis with penetration injuries at terminal sites, P. mirabilis predominantly formed spheroplasts with multiple cell-wall lesions, and non-swarming variants developed fusiform shapes before degenerating into a homogeneous mass. The results demonstrated that penicillin-induced lysis is not solely due to osmotic lysis of spheroplasts, but involves multiple lethal and non-lethal effects. Significant heterogeneity existed within bacterial populations, with individual cells showing varying susceptibility to ampicillin.

Key findings

• E. coli and P. mirabilis showed similar ampicillin sensitivity by conventional testing but exhibited dramatically different responses in dense cultures and morphological changes
• Swarming P. mirabilis strains produced numerous persisters and showed sustained growth in high ampicillin concentrations, while E. coli underwent rapid lysis
• E. coli lysis involved rapid cell dissolution with penetration injuries, whereas P. mirabilis primarily formed spheroplasts, indicating fundamentally different mechanisms of ampicillin action
• Non-swarming P. mirabilis variants displayed intermediate behavior and distinctive morphological changes including fusiform deformity and cell aggregation into homogeneous mass
• Individual cells within bacterial populations showed heterogeneous susceptibility to ampicillin, ranging from morphologically normal to completely destroyed