Human U937 macrophages were used in a gentamicin protection assay with mid-log phase bacteria. Intracellular bacteria were enumerated after 24 h (T24) and 48 h (T48). Results are presented as percent intracellular of the inoculum. Data are means ± standard errors of two independent experiments performed in triplicate. E. coli K1 (persists), E. coli K12 (killed), and C. koseri strain 319 (replicates) were used as controls.