Colonization of four mono-associated rats with E. coli strain MS15978. Faecal samples were plated on both BHI (◆) and BHI including ampicillin (■). From day 10 all rats were dosed with 1000 μg ampicillin each day to restore the population of pMR1 containing E. coli cells. The detection limit for detection of transformants was 100 cfu/g faeces. Vertical bars represent standard error of the mean (SEM).