"Sinorhizobium meliloti" is a bacterium of great agroeconomic importance because of its ability to fix atmospheric nitrogen in symbiotic association with alfalfa ("Medicago sativa") roots, and it is often used in model studies. We investigated the effects of exopolysaccharides (EPSs) in cell-to-cell and cell-to-surface interactions in "S. meliloti." The microtiter plate assay, a quantitative spectrophotometric method, is used to study biofilm formation by bacterial adherence to an abiotic surface. It consists in staining biofilms grown in microtiter plates. Here, we describe two microbiology laboratory classes designed for undergraduate students of Experimental Biological Chemistry, in which they learn about biofilm forming capacity by observing the behavior of both wild-type and mutant strains of "S. meliloti."