A significant synergistic effect has been reported between microbial cultures and chemical surfactants in crude oil recovery processes. A number of attempts have been made to understand the synergistic mechanism. However, the existing studies only addressed the aspects of wettability alteration and interfacial tension (IFT) reduction, although there are many more contributors to the mechanism, such as emulsification, functional microbial activities, and diverse byproducts. In addition, the previous knowledge on synergistic oil recovery was based on indirect evidence from core-scale flooding and test tube experiments. To fully exploit the synergistic effects in the future, a new experimental system must be introduced (i) to verify the existing mechanisms in situ in porous media during the flooding process and (ii) to address the other potential contributors for a more comprehensive insight into the actual major contributors. Therefore, a visual pore-scale flooding experimental system was introduced in the present study to mimic the pore networks and conditions of a reservoir (55 degrees C, 10 MPa). This system enabled direct observations of fluid dynamics in pores during flooding. The final oil recovery using indigenous microorganisms (8.3%) and an anionic surfactant [sodium alcohol ether sulphate (AES)] (15.5%) was considerably enhanced (22.4%) when the two solutions were equally mixed, which indicated significant synergistic effects between them, whereas no such effects were observed with a nonionic surfactant [polyoxyethylene nonylphenol ether (OP10)]. The pore-scale and macroscale analyses were combined to reveal the synergistic mechanisms between the microbial culture and AES. The results show that the IFT reduction and wettability alteration, traditionally considered to be synergistic mechanisms, contributed to the oil recovery but were not the major contributors to the synergistic effects. In this case, the synergistic mechanisms include the following aspects: the anionic surfactant promotes microbial metabolism, such as biogas production (significantly enhanced from 0.0018 mL/mL medium to 0.0196 mL/mL medium), and the microbial culture, in turn, reduces the critical micelle concentration of the surfactant and enhances the emulsion effectiveness, including reducing the oil droplet sizes (from D-90 = 217 to 116 mu m) and increasing the stability of the emulsion system from several minutes to a few days. The two synergistic mechanisms reflect the mutually positive effects between the chemical surfactant and the microbial system; these mutual effects are especially essential for long-term and long-distance oil migration and final recovery in a real reservoir-scale flooding process.