Indole contributes to tetracycline resistance via the outer membrane protein OmpN in Vibrio splendidus

As an interspecies and interkingdom signaling molecule, indole has recently received attention for its diverse effects on the physiology of both bacteria and hosts. In this study, indole increased the tetracycline resistance of Vibrio splendidus. The minimal inhibitory concentration of tetracycline was 10 μg/mL, and the OD₆₀₀ of V. splendidus decreased by 94.5% in the presence of 20 μg/mL tetracycline; however, the OD₆₀₀ of V. splendidus with a mixture of 20 μg/mL tetracycline and 125 μM indole was 10- or 4.5-fold higher than that with only 20 μg/mL tetracycline at different time points. The percentage of cells resistant to 10 μg/mL tetracycline was 600-fold higher in the culture with an OD₆₀₀ of approximately 2.0 (higher level of indole) than that in the culture with an OD₆₀₀ of 0.5, which also meant that the level of indole was correlated to the tetracycline resistance of V. splendidus. Furthermore, one differentially expressed protein, which was identified as the outer membrane porin OmpN using SDS-PAGE combined with MALDI-TOF/TOF MS, was upregulated. Consequently, the expression of the ompN gene in the presence of either tetracycline or indole and simultaneously in the presence of indole and tetracycline was upregulated by 1.8-, 2.54-, and 6.01-fold, respectively, compared to the control samples. The combined results demonstrated that indole enhanced the tetracycline resistance of V. splendidus, and this resistance was probably due to upregulation of the outer membrane porin OmpN.