AUTHOR=Ahn Sang-Joon , Desai Shailja , Blanco Loraine , Lin Min , Rice Kelly C. 

TITLE=Acetate and Potassium Modulate the Stationary-Phase Activation of lrgAB in Streptococcus mutans

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.00401

DOI=10.3389/fmicb.2020.00401

ISSN=1664-302X

ABSTRACT=<p>Fluctuating environments force bacteria to constantly adapt and optimize the uptake of substrates to maintain cellular and nutritional homeostasis. Our recent findings revealed that LrgAB functions as a pyruvate uptake system in <italic>Streptococcus mutans</italic>, and its activity is modulated in response to glucose and oxygen levels. Here, we show that the composition of the growth medium dramatically influences the magnitude and pattern of <italic>lrgAB</italic> activation. Specifically, tryptone (T) medium does not provide a preferred environment for stationary phase <italic>lrgAB</italic> activation, which is independent of external pyruvate concentration. The addition of pyruvate to T medium can elicit P<italic>lrgA</italic> activation during exponential growth, enabling the cell to utilize external pyruvate for improvement of cell growth. Through comparison of the medium composition and a series of GFP quantification assays for measurement of P<italic>lrgA</italic> activation, we found that acetate and potassium (K<sup>+</sup>) play important roles in eliciting P<italic>lrgA</italic> activation at stationary phase. Of note, supplementation of pooled human saliva to T medium induced <italic>lrgAB</italic> expression at stationary phase and in response to pyruvate, suggesting that LrgAB is likely functional in the oral cavity. High concentrations of acetate inhibit cell growth, while high concentrations of K<sup>+</sup> negatively regulate <italic>lrgAB</italic> activation. qPCR analysis also revealed that growth in T medium (acetate/K<sup>+</sup> limited) significantly affects the expression of genes related to the catabolic pathways of pyruvate, including the Pta/AckA pathway (acetate metabolism). Lastly, stationary phase <italic>lrgAB</italic> expression is not activated when <italic>S. mutans</italic> is cultured in T medium, even in a strain that overexpresses <italic>lytST</italic>. Taken together, these data suggest that <italic>lrgAB</italic> activation and pyruvate uptake in <italic>S. mutans</italic> are connected to acetate metabolism and potassium uptake systems, important for cellular and energy homeostasis. They also suggest that these factors need to be implemented when planning metabolic experiments and analyzing data in <italic>S. mutans</italic> studies that may be sensitive to stationary growth phase.</p>