Effects of Low Concentrations of Fatty Acids on Escherichia coli Depend on the Kind of Culture Medium and Incubation Temperature

Fatty acids play important, yet different roles in bacterial physiology, specifically their growth. On the one hand, these compounds may be used by bacteria to produce storage materials or serve as a carbon source. On the other hand, due to their influence on biolog-ical membranes, fatty acids have been reported to exhibit antimicrobial effects. It was in-dicated that the length of the fatty acid chain is crucial for specific effects on bacterial cells. However, the literature still contains reports of contradictory findings in studies on these molecules. Here, we present results showing to what degree short- and medium-chain fatty acids (butyric acid (butanoic acid, C4:0); caproic acid (hexanoic acid, C6:0); caprylic acid (octanoic acid, C8:0)), used at relatively low concentrations (in a range of g/mL, spe-cifically 0.25-16 g/mL, contrary to previously reported mg/mL which revealed inhibitory effects on bacterial growth) affect growth of Escherichia coli K-12 (MG1655 laboratory strain) depending on various conditions. In rich medium (LB) positive effects of all tested fatty acids on E. coli growth were observed, while temperature of incubation (growth at 25oC and 37oC was assessed) modulated these effects. In contrast, a slight but significant growth inhibition by fatty acids was observed in a minimal medium (M9) supplemented with glucose. Nonetheless, in minimal medium containing acetate, the effects of these compounds varied, being either positive or negative depending on their concentrations. No measurable bacterial growth was observed in the case of the presence of any tested fatty acids when primary carbon source (glucose or acetate) was removed from a minimal medium before addition of butyric acid, caproic acid or caprylic acid. These results indi-cated that effects of fatty acids on E. coli cells depend on growth conditions of bacterial cultures, and may suggest that previously reported discrepancies between results ob-tained by various groups might arise, at least partially, from using various cultivation procedures used for different experiments.