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Changes in bacterial ultrastructure after antibiotic exposure and during the postantibiotic effect (PAE) have been demonstrated by electron microscopy (EM). However, EM is qualitative and subject to individual interpretation. In contrast, flow cytometry gives qualitative and quantitative information. The sizes and nucleic acid contents ofEscherichia coli andPseudomonas aeruginosa were studied during antimicrobial exposure as well as during the PAE period by staining the organisms with propidium iodide and analyzing them with flow cytometry and fluorescence microscopy. The effects of ampicillin, ceftriaxone, ciprofloxacin, gentamicin, and rifampin were studied forE. coli , whereas forP. aeruginosa imipenem and ciprofloxacin were investigated. After exposure ofE. coli to ampicillin, ceftriaxone, and ciprofloxacin, filamentous organisms were observed by fluorescence microscopy. These changes in morphology were reflected by increased forward light scatter (FSC) and nucleic acid content as measured by flow cytometry. For the β-lactams the extent of filamentation increased in a dose-dependent manner after drug removal, resulting in formation of distinct subpopulations of bacteria. These changes peaked at 20 to 35 min, and bacteria returned to normal after 90 min after drug removal. In contrast, the subpopulations induced by ciprofloxacin did not return to normal until &gt;180 min after the end of the classically defined PAE. Rifampin resulted in formation of small organisms with low FSC, whereas no distinctive characteristics were noted after gentamicin exposure. ForP. aeruginosa an identifiable subpopulation of large globoid cells and increased nucleic acid content was detected after exposure to imipenem. These changes persisted past the PAE, as defined by viability counting. Swollen organisms with increased FSC were detected after ciprofloxacin exposure, even persisting during bacterial growth. In summary, for β-lactam antibiotics and ciprofloxacin, the PAE is characterized by dynamic formation of enlarged cell populations of increased nucleic acid content, whereas rifampin induces a decrease in size and nucleic acid content in the organisms. Flow cytometry is an ideal method for future studies of bacterial phenotypic characteristics during the PAE.

Characteristics and Dynamics of Bacterial Populations during Postantibiotic Effect Determined by Flow Cytometry