Bacteria that colonize the gastrointestinal tract must withstand bile salts, a key antimicrobial component of the gut environment. This study examined five putative transporters in Enterococcus faecalis for induction by bile salts and potential roles in survival and antibiotic resistance. Exposure of E. faecalis to physiologically relevant concentrations of bile salts led to differential transcriptional responses: ef2593 and ef0575 transcript levels rose by more than fivefold, while ef0420 increased approximately 12-fold when cells were exposed to 0.02% and 0.1% bile salts. In contrast, ef2592 and ef1814 did not show measurable changes in expression under the same conditions according to the abstract.
These transcriptional changes indicate that multiple transporter genes are responsive to bile salt stress. The pattern of induction suggests a coordinated or multi-component response enabling E. faecalis to adapt to increasing bile concentrations in the intestinal environment.
Functional studies investigated whether transporter expression modifies survival at higher bile salt concentrations. Overexpression constructs carrying efflux pump genes (EF2592, EF2593, EF0575, EF0420) introduced into wild-type MMH594 increased colony-forming units per milliliter (CFU/mL) compared with wild type when cultures were challenged with 0.3% bile salts. Additionally, pre-exposure of wild-type cells to 0.02% bile salts followed by challenge conferred increased survival when EF2593 or EF0420 were involved.
These results show that both experimentally driven overexpression and physiological pre-exposure can enhance tolerance, demonstrating that the bile salt response in E. faecalis involves multiple transporters and can raise the threshold of bile salt concentration the organism can withstand.
The investigators assessed the impact of transporter expression on antibiotic susceptibility using MIC assays. Overexpression of several transporters produced modest but notable increases in resistance (typically twofold) to clinically relevant antibiotics. Specific findings reported in the abstract include:
Although EF0575 overexpression did not change antibiotic susceptibility in MIC testing, a mutant lacking ef0575 displayed increased susceptibility to multiple antibiotics (daptomycin, linezolid, levofloxacin, and oritavancin), indicating that EF0575 contributes to baseline tolerance even if overexpression did not further elevate MICs in the conditions tested.
Collectively, these results indicate that expression of multiple ABC and MFS transporters can confer low-level, multidrug resistance phenotypes in E. faecalis.
To test transporter activity in a different host background, several E. faecalis transporters were expressed from plasmids in Lactococcus lactis with nisin-induced expression. In this heterologous system, overexpression increased MICs of multiple antibiotics; EF1814 produced the most pronounced effect, with reported increases of 4- to 16-fold in MICs for daptomycin, levofloxacin, linezolid, and vancomycin.
These heterologous expression data support the conclusion that the transporter proteins themselves can reduce antibiotic susceptibility across different Gram-positive hosts and are not strictly dependent on the native E. faecalis background to influence MICs.
The study links a host-associated environmental stressor—bile salts—with expression of ABC and MFS transport systems that both enhance bile tolerance and elevate antibiotic tolerance at low levels. Transport systems selected for survival during intestinal colonization may therefore have the unintended consequence of contributing to low-level multidrug resistance against clinically important antibiotics including daptomycin, vancomycin, levofloxacin, linezolid, and oritavancin.
By demonstrating that multiple transporters respond to bile salts and can modulate MICs, the findings identify these transporters as potential targets for strategies aimed at limiting intestinal colonization by E. faecalis and reducing the emergence or persistence of antibiotic tolerance.
The abstract summarizes key transcriptional, phenotypic, and heterologous-expression results but does not report full experimental details in this summary. Specific methods, quantitative datasets, statistical analyses, strain constructions, plasmid details, time courses, replicates, or any in vivo validation were not provided in the abstract and would require consultation of the full article for comprehensive evaluation. The abstract also does not provide detailed mechanistic insights into substrate specificity or transport kinetics for each transporter beyond their impact on MICs and bile salt survival.