Pretreatment with probiotics prevents enteroinvasive Escherichia coli (EIEC) induced decrease in transepithelial resistance (TER) of polarised HT29/cl.19A cell monolayers. HT29/cl.19A cells were exposed to medium, EIEC followed by Streptococcus thermophilus and Lactobacillus acidophilus (EIEC+ST/LA), EIEC and ST/LA given simultaneously (EIEC/ST/LA), EIEC alone, ST/LA followed by EIEC (ST/LA+EIEC), or ST/LA alone on the apical side. Preparations of living (A), antibiotic killed (a) (B), heat inactivated (h) (B), or spent medium (SM) from ST and LA cultures (B) were tested in these experiments. TER was measured at the times indicated. Resistance values of cell free inserts were subtracted from each measurement. Data are means (SEM), n=8. *p<0.05, **p<0.01, ***p<0.001 compared with uninfected controls (by ANOVA). Similar findings were observed in Caco-2 cells.