Future research trends in the major chemical language of bacteria

Microbiology was revolutionized in the 1990’s by the discovery that many different bacterial species coordinate their behavior when they form a group. In fact, bacteria are now considered multicellular organisms capable of communicating and changing behavior in relation to their cell-density; since 1994 this has been called quorum sensing. This group behavior ensures survival and propagation of the community in many natural environments. Bacterial intercellular communication is mediated by different chemical signals that are synthesized by bacteria which are then either secreted or diffused in the external environment. Bacteria are then able to detect the type and concentration of the signal resulting in regulation of gene expression and, consequently, a synchronized response by the community. The predominant signalling molecules produced by Gram-negative bacteria are N-acyl derivatives of homoserine lactone (AHLs) which have been shown to be produced by over seventy bacterial species. In this essay we discuss the importance of quorum sensing via AHLs and highlight current and future trends in this important field of research

XerR, a negative regulator of XccR in Xanthomonas campestris pv. campestris, relieves its repressor function in planta

We previously reported that XccR, a LuxR-type regulator of Xanthomonas campestris pv. campestris (Xcc), activates the downstream proline iminopeptidase virulence gene (pip) in response to certain host plant factor(s). In this report, we further show that the expression of the xccR gene was repressed in the culture medium by an NtrC-type response regulator, which we named XerR (XccR expression-related, repressor), and that this repression was relieved when the bacteria were grown in planta. Such a regulatory mechanism is reinforced by the observations that XerR directly bound to the xccR promoter in vitro, and that mutations at the phosphorylation-related residues of XerR resulted in the loss of its repressor function. Furthermore, the expression level of xccR increased even in XerR-overexpressing Xcc cells when they were vacuum infiltrated into cabbage plants. We also preliminarily characterized the host factor(s) involved in the above mentioned interactions between Xcc and the host plant, showing that a plant material(s) with molecular weight(s) less than 1 kDa abolished the binding of XerR to the xccR promoter, while the same material enhanced the binding of XccR to the luxXc box in the pip promoter. Taken together, our results implicate XerR in a new layer of the regulatory mechanism controlling the expression of the virulence-related xccR/pip locus and provide clues to the identification of plant signal molecules that interact with XerR and XccR to enhance the virulence of Xcc