Understanding LrgAB Regulation of Streptococcus mutans Metabolism

Abstract

Lack of LrgAB renders cariogenic Streptococcus mutans more sensitive to oxidative stress, as well as limits the capacity of this organism to re-uptake pyruvate upon starvation. This study was aimed at investigating the ecological and metabolic contribution of LrgAB to competitive fitness, using S. mutans strains, that either lack or overexpress lrgAB. These experiments revealed that impaired aerobic growth of the lrgAB mutant can be effectively restored by supplementation of pyruvate, and that perturbated expression of lrgAB significantly affects pyruvate flux and the conversion of pyruvate to acetyl-CoA by the Pdh pathway, verifying that LrgAB is closely linked to pyruvate catabolism. In vitro competition assays revealed that LrgAB plays an important role in S. mutans competition with H 2 O 2-producing S. gordonii, an interaction which can also be modulated by external pyruvate. However, no obvious competitive disadvantage was observed against S. gordonii by either the S. mutans lrgAB mutant or lrgAB overexpression strain in vivo using a mouse caries model. Organic acid analysis of mouse dental biofilms revealed that metabolites produced by the host and/or dental plaque microbiota could complement the deficiency of a lrgAB mutant, and favored S. mutans establishment compared to S. gordonii. Collectively, these results reinforce the importance of the oral microbiota and the metabolic environment in the oral cavity battleground, and highlight that pyruvate uptake through LrgAB may be crucial for interspecies competition that drives niche occupancy.

Spontaneous Mutants of Streptococcus sanguinis with Defects in the Glucose-PTS Show Enhanced Post-Exponential Phase Fitness

“…S. mutans possesses two holin/antiholin-like proteins, LrgAB, that are responsible for transporting pyruvate at the start of stationary phase. Research has suggested the existence of additional transporters or mechanisms for pyruvate to impact the growth of S. mutans (49). The genome of S. sanguinis lacks homologues of either gene, but S. sanguinis and other spxB-encoding species do release pyruvate in varying amounts to the surroundings, which can decrease the damaging effects of H 2 O 2 (21).…”

Testing of candidate probiotics to prevent dental caries induced by Streptococcus mutans in a mouse model

“…Although both smooth surface and sulcal caries in both BCC23 control groups were relatively low, they were greater than expected, based upon previous results with the mouse model that suggested autochthonous bacteria alone were poorly cariogenic when mice were fed the high sucrose diet (Ahn et al, 2020). It is unlikely that either BCC23 strain was directly responsible for the low levels of carious lesions as glycolysis and growth of S. sanguinis is repressed by prolonged acidification (Takahashi et al, 1997).…”

“…It was shown previously in this same mouse model that when mice underwent mock inoculations (i.e. no exposure to exogenous bacteria), total smooth surface and total sulcal E score were each 0.55 and with dentinal severity scores of zero (Ahn et al, 2020). Notably, DNA samples recovered from oral swabs and molar sonicates of mice from both groups were examined by qPCR for S. mutans DNA, and results were negative in all cases.…”

Murine Salivary Amylase Protects Against Streptococcus mutans-Induced Caries

“…Furthermore, S. mutans accounted for >50% of the total recovered bacteria from dental plaque of both WT and KO mice, indicating no significant differences in dental colonization by commensals as a whole. We have also recently shown that murine oral commensals did not produce any significant caries when mice were fed a highly cariogenic diet (Ahn et al, 2020). Although we cannot discount commensal streptococci accounting for a portion of the increased caries, it is unlikely that they would have accounted for a great majority of the observed difference.…”

“…At the same time, this dietary regimen represents a highly stringent test of the innate protective mechanisms associated with the oral cavity and, in particular, its hard tissues. Our rationale for using S. mutans UA159 is that it is widely used as a model in caries research, both in in vitro and animal studies, in which the genetic elements responsible for its colonization and virulence are being deciphered to better understand its pathogenesis and delineate potential therapeutic targets against mutans streptococci (Klein et al, 2010;Bowen et al, 2018;Ahn et al, 2020;Culp et al, 2021). The fact that the deletion of amylase expression resulted in a marked increase in caries answers our original question of whether its overriding effect is to either help promote or protect against caries.…”