Morphological changes induced by imipenem and meropenem at sub-inhibitory concentrations in Acinetobacter baumannii

Sub-inhibitory concentrations of imipenem and meropenem were evaluated for their ability to induce morphological changes with six strains of Acinetobacter baumannii isolated from patients with nosocomial pneumonia. Three strains were susceptible and three were resistant to carbapenems. The strains were grown in the presence of 0 (controls), 0.25×, 0.5× and 1× the MIC of both carbapenems for 4 h, and then examined after Gram's stain. Cells ≥ 3 µm in size (spheroplasts) were considered to be altered. Both carbapenems induced significant numbers of spheroplasts compared to controls. Imipenem had more effect against susceptible strains, while meropenem had a greater effect against resistant strains.