Colonial architecture in mixed species assemblages affects AHL mediated gene expression.

Many bacterial species produce metabolites that accumulate in the extracellular environment and induce specific transcriptional responses in producing cells. This phenomenon, most often referred to as quorum sensing, is thought to constitute a self-cell-density sensing mechanism allowing bacterial populations to alter gene expression in response to increases in their own density. Quorum sensing systems involving N-acyl-L-homoserine lactone (AHL) production and response are the most intensively investigated example. In this study we have employed a novel technique, known as dielectrophoresis, to investigate the impact of colonial architecture on the induction of AHL mediated gene expression. Using dielectrophoresis, we constructed artificial mixed species microcolonies with specific architectures. In this way, we were able to show that approximately 1000 Escherichia coli cells layered over an immobilised cluster of approximately 500 AHL responsive cells alters the response of this cluster to AHLs supplied either exogenously or endogenously. These findings lend credence to the hypothesis that the accumulation of extracellular metabolites signifies generic crowding in mixed species assemblages.

Diversity and linkage of replication and mobilisation genes in Bacillus rolling circle-replicating plasmids from diverse geographical origins.

The prevalence, diversity and linkage of pTA-related plasmid rep and mob genes in 18 plasmids from three closely related species of Bacillus from diverse geographical locations have been investigated using PCR. pTA-related rep and mob sequences were both amplified from 13 of the 18 plasmids. For one plasmid, pBM9, only a pTA-related mob gene was amplified, whilst only a pTA-related rep gene was amplified from the Bacillus licheniformis plasmid pBL2. No products were amplified for either gene from two other B. licheniformis plasmids or from a larger 16-kb Bacillus subtilis plasmid, pBS6. Whilst simple gene detection suggests close linkage of both pTA-related rep and mob genes, on most of these plasmids, sequence analysis of amplified genes revealed more complex linkage relationships varying with geographical origin, plasmid size and bacterial host.