Effects of soil pH and ionic strength (KCl) on adsorption of Streptococcus suis and Escherichia coli on Red soil colloids were studied, and mechanisms of their interactions analyzed from the aspects of surface physicochemical properties of the bacteria and soil colloids and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Results show that the adsorption isotherms of bacteria on Red soil colloids fitted well to the Freundlich equation (R²> 0.97). The partition coefficients (K_f) of S. suis adsorption on soil colloids were 4.5~6.1 times as high as those of E. coli. The K_f values of organic-matter-depleted colloids were 2.4~3.2 times as high as those of organic-matter-containing colloids. The bigger the specific surface areas or the lower the absolute zeta potential values, the higher the bacteria adsorption capacity, and the adsorbed bacteria were found at the secondary minimum, 90~100 nm apart from the colloids surface. With decreasing solution pH (9.0~4.0) or increasing IS (1~10 mmol L^-1), the interaction energy barrier between the Red soil colloids and the bacteria decreased and as a result more and more bacteria got adsorbed. This trend was in full agreement with the DLVO theory. The adsorption of S. suis on colloids decreased by 3.4%~5.6% under high IS (50~100 mmol L^-1), which, besides the DLVO effect, was also ascribed to some non-DLVO effects, like steric repulsion and hydrophobic interaction.