Quorum sensing in this species remains uncertain. Furthermore, luxS mutants do demonstrate an altered ability to colonize animal models. In the present study the mutant exhibited a decreased motility BIBW2992 consistent with previously published findings of luxS mutants in this species. The use of the FRET AI-2 assay in this manuscript provides, to our knowledge, the first truly quantifiable data concerning the physiological concentrations of AI-2 produced by C. jejuni grown in in vitro environments. While the ability of the organism to truly “sense” the compound is still unproven, this quantification data does provide much needed information concerning what range of potential concentrations of chemically synthesized AI-2 should be used to mimic natural production potentials. Based on our results we believe that concentrations between 0 and 40 uM are physiologically possible in logarithmic growth in MH media routinely used for C. jejuni growth. When comparing the results of the in vitro recombinant proteins enzymatic reactions we observe that while no homocysteine can be detected following incubation of SRH with the 1198AI2- enzyme we do see measurable synthesis of AI-2. This provides additional evidence of retention of low-level enzymatic activity of the 81116AI2- strain but also provides clues into the biological sensitivity of two of the assays utilized in detection of LuxS enzymatic activity, homocysteine formation using the Ellman reaction and AI-2 activity using the Vibrio harveyi assay. Given that homocysteine will be produced in the in vitro reaction in a stoichiometrically identical concentration to DPD, that then becomes AI-2, the ability to detect low levels of AI-2 in the absence of homocysteine suggest that the lower limit of detection of the Ellman reaction, as performed in this manuscript, is greater then that of the Vibrio bioluminescence assay. As has been described for other organisms, the absence of a fully functional luxS gene in C. jejuni does result in increased extracellular concentrations of SRH. The presence of increased SRH concentrations in the naturally occurring mutant strain suggest that despite measurable residual enzymatic activity of the mutant enzyme, the metabolic ability of the organism to recycle SRH into the SAM recyclingpathway is hindered. Thus,inadditiontothe phenotypic loss of AI-2 activity this strain has a phenotypic change in SAM recycling which precludes the ability to attribute any of the phenotypic characteristics observed directly to the loss of quorum sensing. In conclusion, we have fully described the molecular basis for the loss of AI-2 phenotype associated with a naturally occur luxS mutant of C. jejuni. Given that the amino acid responsible for this loss of function is highly conserved over a broad range of bacterial species, we speculate that homologous mutations in other bacterial species would also result in a loss of AI-2 phenotype.