Interactions between clay minerals and microbes are ubiquitous in natural environments. It is widely reported that some bacteria may greatly influence the decomposition and transformation of clay minerals by reducing structural Fe(Ⅲ) in Fe- rich smectites. However, the Fe-rich nontronite is not the most common type of smectite. Most natural smectite is montmorillonite bearing poor Fe. The question remains to be answered whether there is other ways of interaction between microbes and Fe-poor smectites except for the Fe(Ⅲ) reduction. The authors therefore studied the effect of Bacillus mucilaginosus on the local structure transformation of montmorillonite. Experiments were conducted under aerobic atmosphere at 35℃ and 1 atm. The mineral samples were bentonite containing about 90% Ca-montmorillonite and 10% cristobalite, collected from Jianping District of Liaoning Province. The aerobic bacteria named Bacillus mucilaginosus 3027, kindly donated by Chinese Academy of Agricultural Science, was inoculated into suspension of sterilized mineral powder and cultured for 20 days. During this period, 3.0 mL solutions were extracted regularly for pH and Si concentration analysis. After the experiments, minerals were collected and characterized by FTIR, XANES, XRD and SEM. Abiotic controls with only sterilized bentonite were conducted simultaneously. Initially, the pH of biotic solutions decreased, probably caused by acidic metabolites produced by bacteria during their growth and metabolism. Meanwhile, Si concentration in the solutions increased, suggesting the release of Si from the tetrahedral sheets of solid mineral. These factors might later trigger a local structure distortion in montmorillonite, as indicated by the FTIR spectrum. The local structure change was also implied by reduced symmetry of coordinate octahedral geometry, as observed from the broadened edge crest in the Fe K-edge XANES. More importantly, newly formed α-quartz was detected in SR-XRD patterns, possibly formed by the Si released from montmorillonite. Furthermore, partial layer and edge curling were observed by SEM.