Disruption of the (Acetoin Dehydrogenase) Operon Has Wide-Ranging Effects on Streptococcus Mutans Growth and Stress Response

The agent largely responsible for initiating dental caries, Streptococcus mutans, produces acetoin dehydrogenase that is encoded by the operon. The operon consists of the and genes (E1 dehydrogenase), (E2 lipoylated transacetylase), (E3 dihydrolipoamide dehydrogenase), and (lipoyl ligase). Evidence is presented that AdhC interacts with SpxA2, a redox-sensitive transcription factor functioning in cell wall and oxidative stress responses. In-frame deletion mutations of genes conferred oxygen-dependent sensitivity to slightly alkaline pH (pH 7.2-7.6), within the range of values observed in human saliva. Growth defects were also observed when glucose or sucrose served as major carbon sources. A deletion of the orthologous gene, gene of Streptococcus gordonii, did not result in pH sensitivity or defective growth in glucose and sucrose. The defects observed in mutants were partially reversed by addition of pyruvate. Unlike most 2-oxoacid dehydrogenases, the E3 AdhD subunit bears an N-terminal lipoylation domain nearly identical to that of E2 AdhC. Changing the lipoyl domains of AdhC and AdhD by replacing the lipoate attachment residue, lysine to arginine, caused no significant reduction in pH sensitivity but the mutation eliminating the lipoylation site resulted in an observable growth defect in glucose medium. The mutations were partially suppressed by a deletion of , encoding an NAD/NADH-sensing transcription factor that represses genes functioning in fermentation. double mutants show synthetic growth restriction at elevated pH and upon ampicillin treatment. These results suggest a role for Adh in stress management in S. mutans. Dental caries is often initiated by Streptococcus mutans, which establishes a biofilm and a low pH environment on tooth enamel surfaces. The current study has uncovered vulnerabilities of S. mutans mutant strains that are unable to produce the enzyme complex, acetoin dehydrogenase (Adh). Such mutants are sensitive to modest increases in pH to 7.2-7.6, within the range of human saliva, while a mutant of a commensal Streptococcal species is resistant. The S. mutans strains are also defective in carbohydrate utilization and are hypersensitive to a cell wall-acting antibiotic. The studies suggest that Adh could be a potential target for interfering with S. mutans colonization of the oral environment.