Incorporation of Exogenous Fatty Acids Protects Enterococcus faecalis from Membrane-Damaging Agents

Abstract

Enterococcus faecalis is a commensal bacterium of the mammalian intestine that can persist in soil and aquatic systems and can be a nosocomial pathogen to humans. It employs multiple stress adaptation strategies in order to survive such a wide range of environments. Within this study, we sought to elucidate whether membrane fatty acid composition changes are an important component for stress adaptation. We noted that E. faecalis OG1RF was capable of changing its membrane composition depending upon growth phase and temperature. The organism also readily incorporated fatty acids from bile, serum, and medium supplemented with individual fatty acids, often dramatically changing the membrane composition such that a single fatty acid was predominant. Growth in either low levels of bile or specific individual fatty acids was found to protect the organism from membrane challenges such as high bile exposure. In particular, we observed that when grown in low levels of bile, serum, or the hostderived fatty acids oleic acid and linoleic acid, E. faecalis was better able to survive the antibiotic daptomycin. Interestingly, the degree of membrane saturation did not appear to be important for protection from the stressors examined here; instead, it appears that a specific fatty acid or combination of fatty acids is critical for stress resistance.

Exogenous Fatty Acids Protect Enterococcus faecalis from Daptomycin-Induced Membrane Stress Independently of the Response Regulator LiaR

“…The lengths of the fatty acid tails were significantly longer for cells grown with serum (P Ͻ 0.001) ( Table 2) as indicated by the ratio C 10 -C 17 /C 18 -C 20 . These findings are consistent with the composition of fatty acids in serum (32) and indicative of their incorporation by E. faecalis. As E. faecalis naturally inhabits the intestine, we wanted to examine the effects of physiological levels of bile (0.2% bovine bile) upon growth and membrane content, as it too is a source of fatty acids that can be utilized by the organism (32).…”

“…Further analysis confirmed that E. faecalis was able to incorporate exogenous fatty acids from these supplements into its membrane, thus altering the fatty acid composition of the membrane. Supplementation with specific fatty acids, such as oleic acid, a dominant fatty acid found in bile and serum, confirmed that growth in the presence of fatty acids provided tolerance to these stressors (32).…”

“…Previously, we demonstrated that growth in the presence of fatty acid sources impacted the generation time of E. faecalis OG1RF (32). Most notably, growth in the presence of saturated fatty acids significantly increased the generation time of OG1RF compared to that in unsupplemented cultures.…”

“…Our lab has recently discovered a previously unknown mechanism of environmentally induced tolerance to membrane-damaging agents (32). Specifically, we found that supplementing E. faecalis with bile or serum reduced susceptibility to high bile levels, sodium dodecyl sulfate (SDS), and daptomycin.…”

“…Cells were harvested at mid-log phase (OD 600 of Ϸ0.4), washed with 1ϫ PBS, centrifuged, and resuspended in the appropriate challenge medium, as performed previously (32). For antibiotic treatment, cells were resuspended in BHI medium containing 100 mM CaCl 2 and either 10 g/ml or 40 g/ml daptomycin as indicated in the text.…”