Gentamicin protection assay with rat rBCEC4 cells. Bacteria were grown overnight, inoculated and incubated for 1.5 h with rBCEC4 cells then treated with gentamicin for 30 min. E. coli K12 (non-invasive) and E. coli K1 (invasive) were used as controls. No bacteria were recovered at T0 following gentamicin treatment or following inoculation with E. coli K12. Results for T2 are presented as percent intracellular of the initial inoculation after 2 h. Strains without significant difference from E. coli K1 (p ≤ 0.18), 716(p ≤ 0.06) and 695(p ≤ 0.10), are designated by letter (a-c), respectively. All experimental strains had p values not significantly different from invasive (marked 'a'). Data are means ± standard errors from two independent assays performed in triplicate.