If your assay doesn't interfere with agar, you can mix E.coli cells in agar (1.5% w/v) medium when it is ~@45C and add the agar to the wells then let it solidify this way the cells get immobilized.

You can coat the well plates with poly-L-lysin. Cells will attach forming a monolayer more or less homogeneous.

Hi, what kind of assay? There are several protocols and conditions to immobilize bacteria to 96 well plates. If you want to perform a Whole cell Elisa technique, maybe you can try using the protocol refered in the attached article. Briefly consist in washing the cells with coatting buffer 7.4, suspended to a final OD578 of 0.5. A and a o/n at 37C. Nevertheless that not ensures that all the cells will be intact. We used a similar protocol for neisseria. If you need the bacteria alive for some hours, maybe you can try the Thavasi Renga Thavasi suggestion. You can look for a bactericidal assay conditions against the meningococcus.