Loss of NADH Oxidase Activity in Streptococcus mutans Leads to Rex-Mediated Overcompensation in NAD+ Regeneration by Lactate Dehydrogenase.
Previous studies of the oral pathogen Streptococcus mutans have determined that this Gram-positive facultative anaerobe mounts robust responses to both acid and oxidative stresses. The water-forming NADH oxidase (Nox; encoded by nox) is thought to be critical for the regeneration of NAD(+), for use in glycolysis, and for the reduction of oxygen, thereby preventing the formation of damaging reactive oxygen species. In this study, the free NAD(+)/NADH ratio in a nox deletion strain (Δnox) was discovered to be remarkably higher than that in the parent strain, UA159, when the strains were grown in continuous culture. This unanticipated result was explained by significantly elevated lactate dehydrogenase (Ldh; encoded by ldh) activity and ldh transcription in the Δnox strain, which was mediated in part by the redox-sensing regulator Rex. cDNA microarray analysis of S. mutans cultures exposed to simultaneous acid stress (growth at a low pH) and oxidative stress (generated through the deletion of nox or the addition of exogenous oxygen) revealed a stress response synergistically heightened over that with either stress alone. In the Δnox strain, this elevated stress response included increased glucose phosphoenolpyruvate phosphotransferase system (PTS) activity, which appeared to be due to elevated manL transcription, mediated in part, like elevated ldh transcription, by Rex. While the Δnox strain does possess a membrane composition different from that of the parent strain, it did not appear to have defects in either membrane permeability or ATPase activity. However, the altered transcriptome and metabolome of the Δnox strain were sufficient to impair its ability to compete with commensal peroxigenic oral streptococci during growth under aerobic conditions.


OBJECTIVE
Streptococcus mutans is an oral pathogen whose ability to outcompete commensal oral streptococci is strongly linked to the formation of dental caries. Previous work has demonstrated that the S. mutans water-forming NADH oxidase is critical for both carbon metabolism and the prevention of oxidative stress. The results of this study show that upregulation of lactate dehydrogenase, mediated through the redox sensor Rex, overcompensates for the loss of nox. Additionally, nox deletion led to the upregulation of mannose and glucose transport, also mediated through Rex. Importantly, the loss of nox rendered S. mutans defective in its ability to compete directly with two species of commensal streptococci, suggesting a role for nox in the pathogenic potential of this organism.
Department of Microbiology and Immunology, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA.
J L
JL
Baker
Department of Microbiology and Immunology, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA.
A M
AM
Derr
Center for Oral Biology, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA.
R C
RC
Faustoferri
Department of Microbiology and Immunology, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA Center for Oral Biology, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA Robert_Quivey@urmc.rochester.edu.
R G
RG
Quivey
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D007770:L-Lactate Dehydrogenase / Q000235:genetics / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D009243:NAD / Q000378:metabolism*; D009247:NADH, NADPH Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D018384:Oxidative Stress; D013295:Streptococcus mutans / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D009097:Multienzyme Complexes; D009243:NAD; D007770:L-Lactate Dehydrogenase; C023107:NADH oxidase; D009247:NADH, NADPH Oxidoreductases