Streptococcus mutans is one of the primary causative agents of dental caries in humans. S. mutans ferments dietary sugars in the mouth to produce organic acids. These acids lower local pH values resulting in demineralization of the tooth enamel, leading to caries. To survive acidic environments, S. mutans employs several adaptive mechanisms, including a shift from saturated to unsaturated fatty acids in membrane phospholipids. Evidence suggests that this shift requires de novo fatty acid and phospholipid synthesis; therefore, understanding these synthesis pathways is crucial for understanding how S. mutans adapts to low pH and causes caries. PlsX is an acyl- ACP:phosphate transacylase that links the fatty acid synthesis pathway to the phospholipid synthesis pathway, and is central to the movement of unsaturated fatty acids into the membrane. It has recently been discovered that plsX is not essential in S. mutans. This study explores how the loss of plsX affects the ability of S. mutans to alter its membrane fatty acid profile and survive at low pH. The plsX deletion mutant (ΔplsX) is not a fatty acid or phospholipid auxotroph, indicating that some alternative pathway is capable of carrying out the first step of phospholipid synthesis. Gas chromatography of fatty acid methyl esters (GC-FAME) indicates that deletion of plsX impacts the regulation of fatty acid synthesis, altering the length and saturation of fatty acids. Surprisingly, ΔplsX survives significantly longer than the parent strain, UA159, when subjected to an acid challenge of pH 2.5. This enhanced survival may be due to the increased F-ATPase activity observed at low pH. This enhanced F-ATPase activity may be due to the altered fatty acid profile, or may be part of a response to membrane stress. Supplementing ΔplsX with exogenous unsaturated fatty acids does not restore any wild-type phenotypes; however, incorporation of exogenous fatty acids is 2-fold greater in ΔplsX, compared to UA159. Exogenous oleic acid was observed to decrease survival in acid challenge for both ΔplsX and UA159. These results clearly indicate that the loss of plsX affects both the fatty acid synthesis pathway and the acid-adaptive response of S. mutans.