To colonize surfaces, bacteria must adhere and proliferate. How those terms compete to shape the foundations of biofilm formation is barely understood. When bacteria grow between a coverslip and a gel pad, they form microcolonies, which develop initially in a plane before terraces establish at their center. In this configuration, we developed methods to probe adhesion at the scale of the microcolony and at the single cell level. Our results evidence that adhesion is stronger at the old pole. During surface colonization, we measured that cell elongation is able to rupture the adhesive bonds of surrounding bacteria regardless of their position in the microcolony. Finally, we show that the transition from 2D to 3D expansion is driven by the level of adhesion.