It’s All in the Mix: Physiological Effects of Fatty Acid Supplementation on Enterococcus faecalis

Enterococcus faecalis is a Gram-positive bacterium that normally exists as an intestinal commensal in humans. Additionally, it can survive in the environment for extended periods and cause infections in immunocompromised hosts, making E. faecalis a leading cause of nosocomial infections. Previous work noted that specific fatty acids found in serum contribute to tolerance of membrane damaging agents, including the drug daptomycin. However, not all fatty acids found in serum were able to induce such protection. Herein, we measured a wide array of physiological responses after supplementation with combinations of protective (i.e., induced daptomycin tolerance) and non-protective fatty acids. When supplementing cells with either non-protective fatty acid, palmitic acid or stearic acid, there was a significant increase in generation time and severely distorted morphology. Both physiological defects were rescued when these cultures were supplemented with one of the protective fatty acids identified in serum, oleic acid or linoleic acid. Similarly, membrane fluidity decreased with growth in either palmitic or stearic acid alone but returned to basal levels when supplemented in combination with a protective fatty acid. While cell envelope charge has been associated with tolerance to daptomycin in other Gram-positive bacteria, we concluded that it does not correlate with the fatty acid induced protection we observed. Further, daptomycin tolerance could be induced when cultures were supplemented with non-protective fatty acids in combination with a protective fatty acid. Beyond physiology, we measured changes in transcription after protective fatty acid supplementation, noting upregulation in a gene encoding for a glycoside hydrolase as well as the known daptomycin genetic resistance gene gdpD. Combined, we conclude that a single protective fatty acid is able to alleviate negative growth defects and protect cells from daptomycin.