Effect of sodium and potassium ions on intracellular pH and proton excretion in glycolyzing cells of Streptococcus mutans NCTC 10449 under strictly anaerobic conditions

Study on the mechanism of sodium ion inhibiting citric acid fermentation in Aspergillus niger

Excessive sodium significantly inhibits citric acid fermentation by Aspergillus niger during the recycling of citric acid wastewater. This study aimed to elucidate the inhibition mechanism at the interface of physiology and transcriptomics. The results showed that excessive sodium caused a 22.3 % increase in oxalic acid secretion and a 147.6 % increase in H+-ATPase activity at the 4 h fermentation compared to the control. Meanwhile, a 13.1 % reduction in energy charge level and a 15.2 % decline in NADH content were found, which implied the effects on carbon metabolism and redox balance. In addition, transcriptomic analysis revealed that excessive sodium altered the gene expression profiles related to ATPase, hydrolase, and oxidoreductase, as well as pathways like glyoxylate metabolism, and transmembrane transport. These findings gained insights into the metabolic regulation of A. niger response to environmental stress and provided theoretical guidance for the construction of sodium-tolerant A. niger for industrial application.

Beyond Homeostasis: Potassium and Pathogenesis during Bacterial Infections

Potassium is an essential mineral nutrient required by all living cells for normal physiological function. Therefore, maintaining intracellular potassium homeostasis during bacterial infection is a requirement for the survival of both host and pathogen. However, pathogenic bacteria require potassium transport to fulfill nutritional and chemiosmotic requirements, and potassium has been shown to directly modulate virulence gene expression, antimicrobial resistance, and biofilm formation. Host cells also require potassium to maintain fundamental biological processes, such as renal function, muscle contraction, and neuronal transmission; however, potassium flux also contributes to critical immunological and antimicrobial processes, such as cytokine production and inflammasome activation. Here, we review the role and regulation of potassium transport and signaling during infection in both mammalian and bacterial cells and highlight the importance of potassium to the success and survival of each organism.

Transcriptional profile of glucose-shocked and acid-adapted strains of Streptococcus mutans

The aciduricity of Streptococcus mutans is an important virulence factor of the organism, required to both out-compete commensal oral microorganisms and cause dental caries. In this study, we monitored transcriptional changes that occurred as a continuous culture of either an acid-tolerant strain (UA159) or an acid-sensitive strain (fabM::Erm) moved from steady-state growth at neutral pH, experienced glucose-shock and acidification of the culture, and transitioned to steady-state growth at low pH. Hence, the timing of elements of the acid tolerance response (ATR) could be observed and categorized as acute vs. adaptive ATR mechanisms. Modulation of branched chain amino acid biosynthesis, DNA/protein repair mechanisms, reactive oxygen species metabolizers and phosphoenolpyruvate:phosphotransferase systems occurred in the initial acute phase, immediately following glucose-shock, while upregulation of F1 F0 -ATPase did not occur until the adaptive phase, after steady-state growth had been re-established. In addition to the archetypal ATR pathways mentioned above, glucose-shock led to differential expression of genes suggesting a re-routing of resources away from the synthesis of fatty acids and proteins, and towards synthesis of purines, pyrimidines and amino acids. These adjustments were largely transient, as upon establishment of steady-state growth at acidic pH, transcripts returned to basal expression levels. During growth at steady-state pH 7, fabM::Erm had a transcriptional profile analogous to that of UA159 during glucose-shock, indicating that even during growth in rich media at neutral pH, the cells were stressed. These results, coupled with a recently established collection of deletion strains, provide a starting point for elucidation of the acid tolerance response in S. mutans.

Characterization of a glutamate transporter operon, glnQHMP, in Streptococcus mutans and its role in acid tolerance

Glutamate contributes to the acid tolerance response (ATR) of many Gram-negative and Gram-positive bacteria, but its role in the ATR of the oral bacterium Streptococcus mutans is unknown. This study describes the discovery and characterization of a glutamate transporter operon designated glnQHMP (Smu.1519 to Smu.1522) and investigates its potential role in acid tolerance. Deletion of glnQHMP resulted in a 95% reduction in transport of radiolabeled glutamate compared to the wild-type UA159 strain. The addition of glutamate to metabolizing UA159 cells resulted in an increased production of acidic end products, whereas the glnQHMP mutant produced less lactic acid than UA159, suggesting a link between glutamate metabolism and acid production and possible acid tolerance. To investigate this possibility, we conducted a microarray analysis with glutamate and under pH 5.5 and pH 7.5 conditions which showed that expression of the glnQHMP operon was downregulated by both glutamate and mild acid. We also measured the growth kinetics of UA159 and its glnQHMP-negative derivative at pH 5.5 and found that the mutant doubled at a much slower rate than the parent strain but survived at pH 3.5 significantly better than the wild type. Taken together, these findings support the involvement of the glutamate transporter operon glnQHMP in the acid tolerance response in S. mutans.

Intracellular and extracellular pHs of Streptococcus mutans after addition of acids: loading and efflux of a fluorescent pH indicator in streptococcal cells

A pH-sensitive fluorescent dye, 2', 7'-bis-(2-carboxyethyl)-5 and 6-carboxyfluorescein (BCECF), was used to determine intracellular pH (pH(in)). The efflux of BCECF loaded into oral streptococcal cells was determined after incubation of the cells at 35 degrees C for 20 min in the presence and absence of glucose. In the absence of glucose, the fluorescence of intracellular BCECF in Streptococcus mutans, Streptococcus sanguis, Streptococcus salivarius and Streptococcus sobrinus decreased only very slightly, indicating that the dye could be useful for pH(in) determination. In the presence of glucose, however, the fluorescence decreased by 57%. Thus, the pH(in) of S. mutans cells was measured by the BCECF method in the absence of glucose at various acidic pH levels by adding lactic, acetic and hydrochloric acids to the cell suspensions. The pH(in) was almost equal to the extracellular pH (pH(out)) for pH(out) values of between 8 and 5, indicating that protons permeated easily across the S. mutans cell membrane. For pH(out) between 5 and 4, pH(in) was constant at around 5, suggesting that the cell membrane was impermeable to protons, or that a cytoplasmic buffering system functioned. pH(in) decreased at pH(out) values of < 4. The constant pH(in) at acidic pH(out) levels could protect intracellular components, such as proteins, against acidification by sugar fermentation.

Mechanism of inhibition of acid production in Streptococcus mutans by sodium ions under strictly anaerobic conditions

Acids excreted and intracellular levels of glycolytic intermediates during glucose metabolism in streptococcus mutans NCTC 10449 under strictly anaerobic conditions were quantified in an attempt to understand the effect of sodium ions on bacterial acid production. In the presence of NaCl (0.15-0.30 M), the total amount of individual carboxylic acids excreted was inhibited by up to 31%. The intracellular level of fructose 1,6-bisphosphate increased by 58% and levels of 3-phosphoglycerate and pyruvate decreased by 46% and 12%, respectively. Sodium ions directly inhibited the activities of fructose 1,6-phosphate aldolase and triose phosphate isomerase. This indicated that the glycolytic enzymes responsible for the catalysis of fructose 1,6-bisphosphate to 3-phosphoglycerate were inhibited. However, in spite of the expected reduction in acid production intracellularly, the intracellular pH actually decreased in the presence of sodium ions. It is possible that the low intracellular pH inhibits the activity of the glycolytic enzymes involved in the breakdown of fructose 1,6-bisphosphate to 3-phosphoglycerate.