The Effects and Genetic Mechanisms of Bacterial Species Interactions on Biofilm Formation.

Abstract

Bacteria can increase their survival in stressed environments by forming sessile biofilms on surfaces. Natural ecosystems are usually occupied by multiple species, which may interact with and therefore affect biofilm formation of an incoming species. This dissertation research explores the effects of species interactions and investigates genetic mechanisms of species interactions between an environmental strain Stenotrophomonas maltophilia and a water quality indicator species Escherichia coli on biofilm formation of E. coli. It was found that E. coli biofilm development was promoted in dynamic flow systems, but inhibited in static batch plates in mixed species culture compared with pure culture conditions. The opposite effects of co-culture on E. coli biofilm formation suggested that species interactions may have different impacts under different culture conditions. To enable the mechanistic study of species interactions, a separation method was developed to allow transcriptome analysis of mixed species communities. Transcriptomic responses of E. coli to S. maltophilia were analyzed to investigate genetic mechanisms of inhibited E. coli biofilm formation in static co-culture. Eighty-nine and 108 genes exhibited genetic responses of E. coli to S. maltophilia co-cultured in biofilm and suspensions, respectively. Several genes were involved with inhibited biofilm formation of E. coli in static co-culture. One highly up-regulated gene, fliA, was selected for a mechanistic study. It was found that the production of a major monomer of curli, CsgA, as well as cell aggregation were greatly repressed in E. coli with fliA overexpression. Knocking out fliA partially restored the inhibitive effect of co-culture on E. coli biofilm growth. Therefore, it was concluded that inhibited E. coli biofilm formation by interactions with S. maltophilia partially was caused by the induction of gene fliA to suppress curli production. Overall, this dissertation examined the effects of species interactions on biofilm formation of E. coli, highlighted the impact of environmental conditions on the effect, and revealed partial understanding of species interactions at a genetic level. This fundamental study contributes to understanding of biofilm formation in real environments with mixed species, and serves as a starting point towards the development of bacteriotherapy for pathogen control using indigenous species for environmental health.