Membrane biofouling was investigated in a laboratory-scale membrane bioreactor (MBR) to determine how microbial community structure influences the fouling process and the potential for quorum sensing. Differences in microbial communities of the mixed liquor and the membrane fouling layer were evaluated using pyrosequencing. During continuous MBR operation, macroscopic engineering parameters such as total cell concentration and organic removal rate were maintained at constant levels, however, microbial community structure was greatly changed showing a dynamic shift in the dominant bacterial species. The microbial composition in the membrane-biocake was very different from that in the mixed liquor and specific microbial groups such as the genera Enterobacter and Dyella were found to be closely associated with the initial and late biofouling stages, respectively. In parallel, in a culture-dependent study, a total of 61 isolates were obtained from the MBR. Both approaches indicate that Enterobacter cancerogenus is a dominant community member and that it is likely a contributor to membrane fouling in this particular MBR system. Furthermore, a bioassay with Agrobacterium tumefaciens A136 (Ti-)(pCF218)(pCF372) reporter strain demonstrated that this E. cancerogenus-like isolate can produce N-acyl homoserine lactone-type quorum sensing autoinducers, although an in-situ experiment would be needed to prove that QS mechanism is operative under actual operational conditions. (C) 2011 Elsevier B.V. All rights reserved.