One acidifying (ST1) and two texturing strains (ST2 and ST3) of Streptococcus thermophilus were used as pure or co-cultures to identify and understand their effects on the structuring of acid milk gels and on the appearance of texture defects, i.e., syneresis and graininess. Symbiosis between specific texturing and acidifying strains reduced acidification time and increased exocellular polysaccharide (EPS) content. The texturing strain could simultaneously produce low and high molar mass EPS and their distribution in mass and/or in number were influenced by the proportion of acidifying to texturing strain used. The results of this study suggest that the high molar mass EPS contributes to acid gel firmness, but less so compared with the acidification rate. The ability of strain ST3 to prevent texture defects, specifically graininess, did not depend on the acidification kinetics or final EPS content, but rather on the structural properties of EPS and/or the bacterial chain morphology.