Corals under thermal stress alone showed signs of bleaching, but the changes in zooxanthellae density and metabolic rates were less severe than those under synergistic thermal stress and bacterial challenge. Among the bacteria examined, Sulfitobacter sp. had a greater capacity to enhance and accelerate the bleaching process under thermal stress. The mechanisms by which bacteria affect corals are not yet understood. Direct actions include infection and production of toxic substances by bacteria; indirect effects due to changes in the bacterial community under the influence of added bacteria must also be considered. Our results suggest that bacterial challenges combined with thermal stress can synergistically lead to negative outcomes in corals.