Biofilms are involved in 80% of human bacterial infections and are up to 1000 times more tolerant to antibiotics than their planktonic counterparts. To better understand the mechanism of bacteria-surface interactions, polydimethylsiloxane (PDMS) surfaces with microtopographic patterns were tested to study the effects of surface topography on bacterial adhesion and biofilm formation. The patterned PDMS surfaces were prepared by transferring complementary surface topography from a silicon wafer etched via photolithography to introduce 10 pm tall square shape features. The dimension of protruding square features and the distance between adjacent features were systematically varied. Escherichia coil RP437/pRSH103 (with constitutive expression of red fluorescent protein) was found to preferentially attach and form biofilms in valleys between protruding features even when the dimension of plateaus (top of the square features) is considerably larger than valleys. In addition, significant adhesion of E. coli on plateaus was only observed when the plateaus were bigger than 20 mu m x 20 mu m for face-up patterns and 40 mu m x 40 mu m for face-down patterns. This finding suggests that a threshold dimension may be essential for biofilm formation on flat surfaces without physical confinement.