Indole Reverses Intrinsic Antibiotic Resistance by Activating a Novel Dual-Function Importer

Abstract

Bacterial antibiotic resistance modulation by small signaling molecules is an emerging mechanism that has been increasingly reported in recent years. Several studies indicate that indole, an interkingdom signaling molecule, increases bacterial antibiotic resistance. However, the mechanism through which indole reduces antibiotic resistance is largely unknown. In this study, we demonstrated a novel mechanism for indole-mediated reversal of intrinsic antibiotic resistance in Lysobacter. This reversal was facilitated by a novel BtuD-associated dual-function importer that can transfer both vitamin B12 and antibiotics. Indole stimulated btuD overexpression and promoted efficient absorption of extracellular vitamin B12; meanwhile, the weak selectivity of the importer caused cells to take up excessive doses of antibiotics that resulted in cell death. Consistently, btuD deletion and G48Y/K49D substitution led to marked reductions in the uptake of both antibiotics and vitamin B12. This novel mechanism is common across multiple bacterial species, among which the Q-loop amino acid of BtuD proteins is Glu (E) instead of Gln (Q). Interestingly, the antibiotic resistance of Lysobacter spp. can be restored by another small quorum sensing signaling factor, 13-methyltetradecanoic acid, designated LeDSF, in response to bacterial population density. This work highlights the mechanisms underlying dynamic regulation of bacterial antibiotic resistance by small signaling molecules and suggests that the effectiveness of traditional antibiotics could be increased by coupling them with appropriate signaling molecules. IMPORTANCE Recently, signaling molecules were found to play a role in mediating antibiotic resistance. In this study, we demonstrated that indole reversed the intrinsic antibiotic resistance (IRAR) of multiple bacterial species by promoting the expression of a novel dual-function importer. In addition, population-dependent behavior induced by 13-methyltetradecanoic acid, a quorum sensing signal molecule designated LeDSF, was involved in the IRAR process. This study highlights the dynamic regulation of bacterial antibiotic resistance by small signaling molecules and provides direction for new therapeutic strategies using traditional antibiotics in combination with signaling molecules.

5-Methylindole Potentiates Aminoglycoside Against Gram-Positive Bacteria Including Staphylococcus aureus Persisters Under Hypoionic Conditions

“…Our approach differs from earlier reports in several aspects Han et al, 2011;Vega et al, 2012Vega et al, , 2013Hu et al, 2015;Kwan et al, 2015;Wang et al, 2019). First, 5M-induced aminoglycoside potentiation is achieved within a couple of minutes.…”

“…First, 5M-induced aminoglycoside potentiation is achieved within a couple of minutes. In contrast, bacterial eradication by a combined treatment with indole and antibiotics under conventional conditions takes a couple of hours Han et al, 2017;Wang et al, 2019). Second, salts are able to abolish the potentiating effect.…”

“…For instance, it has been shown to increase the antibiotic tolerance of Escherichia coli cells Han et al, 2011;Vega et al, 2013), likely by activating stress responserelated pathways (Vega et al, 2012) and/or upregulating the expression of antibiotic-efflux pump genes (Hirakawa et al, 2005). Conversely, several studies have revealed that indole is able to reduce the antibiotic tolerance of E. coli Kwan et al, 2015) and Lysobacter enzymogenes (Han et al, 2017;Wang et al, 2019), highlighting its potential as an adjuvant for antibiotic potentiation. In particular, halogenated indoles are even able to directly eradicate bacterial persister cells and biofilms .…”

Interspecies and Intraspecies Signals Synergistically Regulate Lysobacter enzymogenes Twitching Motility

“…Subsequently, 1 M exogenous LeDSF was added to the culture medium, and obvious twitching activity was observed in individual cells, confirming our speculation. Moreover, under conditions in which the amount of LeDSF in the environment was insufficient to stimulate the quorum-sensing activities of the bacteria (0.1 M), the single cells exhibited no twitching motility (37). Conversely, when LeDSF was present at greater than 1 M, single Lysobacter cells displayed obvious, regular twitching motility, though an increase in the concentration of LeDSF did not enhance the speed of bacterial movement ( Fig.…”

“…Site-specific mutations in the qseC gene also resulted in antibiotic susceptibility (36). Besides, this antibiotic resistance reversal was facilitated by a novel BtuD-associated importer which can transfer both vitamin B12 and antibiotics (37). So we wanted to know whether other characteristics of L. enzymogenes are also related to indole.…”