Streptococcus mutans is an etiologic agent of dental caries because of its ability to produce organic acids and survive prolonged exposure to the resulting low pH environment. In order to survive acidic conditions, S. mutans has evolved multiple adaptive mechanisms. Previously, we have shown that the organism shifts its membrane fatty acid profile to a long-chained, mono-unsaturated profile when grown at pH values of 5 as compared to 7. When the shift was blocked with antibiotics, the organism was rendered acid sensitive. Objective: to further clarify whether S. mutans requires mono-unsaturated membrane fatty acids for low pH survival. Methods: we identified and insertionally-inactivated, in strain UA159, a homologue to the S. pneumoniae gene fabM, which is responsible for the generation of unsaturated fatty acids. The defective strain was characterized physiologically. Results: the defective fabM strain, designated UR117, did not produce mono-unsaturated membrane fatty acids, as determined by GC-FAME (Avanti Polar Lipids, Inc.) Acid sensitivity assays demonstrated that the mutant strain was more sensitive to extreme acidification than the parental strain. In addition, the mutant strain exhibited altered glycolytic ability and could not acidify assay medium to the same extent as wild-type. The wild-type phenotype was restored via genetic complementation of the mutant or, nutritionally, through the addition of mono-unsaturated fatty acids to the growth media of the mutant. Conclusion: mono-unsaturated membrane fatty acids are necessary for low pH survival of S. mutans, and their absence can affect the activity of key metabolic pathways. This work was supported by grants from NIH/NIDCR, DE-11549 and Training Program T32 DE-07165.

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