Autoinducer 2-based quorum sensing response of Escherichia coli to sub-therapeutic tetracycline exposure
Autoinducer 2 (AI-2) is a quorum sensing signal employed by bacteria to coordinate their response to environmental stresses. The objective of this study was to determine the relationship between presence of AI-2 molecules, exposure to sub-therapeutic tetracycline, the expression of genes associated with the conjugal transfer of antibiotic resistance plasmids, and the conjugal transfer of these plasmids in Escherichia coli. The studies showed that AI-2 activity increased in Tets E. coli in the presence of tetracycline (2 ????g/mL) under both batch and continuous culture conditions. The presence of AI-2 molecules induced tetracycline tolerance development in Tets E. coli. The studies showed that the survival rates of Tets E. coli exposed to AI-2 molecules were significantly higher compared to the cells not exposed to AI-2 molecules or cells that were exposed to only LB (Lauria-Bertani) broth. Molecular analyses using real-time PCR indicate that the expression of at least one conjugation-associated gene (trbC) is increased 9-fold in cells exposed to AI-2 molecules in the presence of sub-therapeutic tetracycline compared to its negative controls. The transconjugation frequency of the plasmid RP4 carrying the tet(A) gene increased between 10-100 fold in the presence of AI-2 molecules. In companion studies, AI-2-like activity was detected in fish, tomatoes, cantaloupes, carrots and milk samples. Interestingly, ground beef and poultry meat contained substances that appear to inhibit AI-2 activity. Collectively, these results highlight the potential importance of bacterial quorum sensing signals such as AI-2 in the response of bacterial cells to environmental stimuli and the possible role of quorum sensing signals in the quality and safety of foods.