SdiA of Salmonella enterica Is a LuxR Homolog That Detects Mixed Microbial Communities

Abstract

Proteins of the LuxR family detect the presence of N-acylhomoserine lactones (AHLs) and regulate transcription accordingly. When AHLs are synthesized by the same species that detects them, the system allows a bacterium to measure the population density of its own species, a phenomenon known as quorum sensing. The sdiA genes of Escherichia coli and Salmonella enterica serovar Typhimurium are predicted to encode LuxR homologs. However, these species do not appear to synthesize AHLs or any other molecule detected by SdiA. It has previously been demonstrated that overexpression of sdiA results in the activation of the ftsQAZ locus in E. coli and four other loci in Salmonella serovar Typhimurium. Here we report that transcriptional fusions to these five loci fall into two classes. The first class requires overexpression of sdiA for activation. The second class responds to sdiA expressed from its natural position in the chromosome if the appropriate AHLs are added to the culture. The only member of the second class is a series of Prck-luxCDABE fusions in Salmonella serovar Typhimurium. SdiA responds with highest sensitivity to AHLs that have a keto modification at the third carbon and an acyl chain length of 6 or 8 (half-maximal response between 1 and 5 nM). Growth of Salmonella in proximity to species known to synthesize these AHLs results in sdiA-dependent activation of the Prck-luxCDABE fusions. SdiA appears to be the first AHL receptor discovered that detects signals emanating exclusively from other species.

Future research trends in the major chemical language of bacteria

“…Both these bacteria are not reported to produce AHLs. As mentioned before, SdiA has been proposed to function in interspecies signaling and this is due to the observations that it binds to more than one type of AHL molecule and regulates target genes in an AHL dependent manner (Michael et al, 2001;Smith and Ahmer, 2003;Yao et al, 2006). SdiA also functions in mediating the effects of indole, a stationary phase signaling molecule of E. coli that may also serve as an interspecies signal as its levels can be changed by other bacteria in a mixed community .…”

A new repertoire of informations about the quorum sensing system in Salmonella enterica serovar Enteritidis PT4

“…AHLs: C6-N-hexanoyl homoserine lactone, C8-N-octanoyl homoserine lactone, C10-N-decanoyl homoserine lactone, and C12-N-dodecanoyl homoserine lactone. Michael et al (2001) showed that, depending on the AHL assessed, a concentration of 1 nM suffices to induce sdiA gene expression in Salmonella Typhimurium. Confirmation that a 100 nM concentration of different AHLs does not negatively affect the microorganism's growth is essential for studying phenotypes regulated by these AIs because it enables the detection of phenotypes stimulated by this signal.…”

The Different Strategies Used by Salmonella to Invade Host Cells

“…However, SdiA can detect and bind AHLs produced by others bacterial species (Michael et al, 2001). SdiA is a LuxR homologue and has two functional domains.…”

“…A genetic screening performed in S. Typhimurium to identify genes regulated by SdiA (suppressor of d ivision inhibition), a transcriptional regulator of quorum sensing, has suggested that rck belongs to a putative operon called the "rck operon" whose expression is activated by SdiA in an Acyl Homoserine Lactone (AHLs)-dependent manner (Figure 6) (Ahmer et al, 1998;Michael et al, 2001). …”