Representative micrograph of Acanthamoeba castellanii interactions with Escehrichia coli K1, Enterobacter aerogenes and Aeromonas hydrophila . Assays were performed to determine bacterial association with and invasion of A. castellanii, as well as bacterial survival inside A. castellanii cysts as described in “Methods”. Finally, samples were stained using Gram staining and observed under a microscope (×400). Note that extracellular bacteria are observed in association assays only, suggesting that gentamicin is effective in killing extracellular bacteria in both invasion as well as intracellular survival assays. Arrows indicate bacteria.