Phosphate starvation is accompanied by disturbance of intracellular cysteine homeostasis in Escherichia coli

Phosphate Limitation Enhances Heterologous Enzyme Production in Bacillus subtilis: Mechanistic Insights and Universal Applicability

Bacillus subtilis is one of the commonly used hosts for heterologous enzyme expression, depending on media rich in carbon, nitrogen, and phosphate sources for optimal growth and enzyme production. Interestingly, our investigation of maltotetraose-forming amylase, a key enzyme for efficient maltotetraose synthesis, revealed that phosphate limitation significantly enhances the growth rate and production of heterologous enzymes in recombinant B. subtilis. Under phosphate-limited conditions in a 15 L fermenter, the enzyme activity reached 679.15 U/mL, an improvement of 101% over the initial levels and a 12 h reduction in fermentation time. Transcriptomic analysis indicated that phosphate limitation promotes sustained enzyme production by upregulating protein synthesis and quality control pathways while optimizing energy utilization. This strategy was validated across various enzyme systems, highlighting its general applicability for enhancing heterologous protein expressions. These findings provide valuable insights for the industrial production of maltotetraose-forming amylase and other high-value enzymes, supporting the advancement of microbial fermentation technology.

Effect of H2S and cysteine homeostasis disturbance on ciprofloxacin sensitivity of Escherichia coli in cystine-free and cystine-fed minimal medium

Endogenous H2S has been proposed to be a universal defense mechanism against different antibiotics. Here, we studied the role of H2S transiently generated during ciprofloxacin (CF) treatment in M9 minimal medium with sulfate or produced by E. coli when fed with cystine. The cysM and mstA mutants did not produce H2S, while gshA generated more H2S in response to ciprofloxacin in cystine-free media. All mutants showed a reduced ability to maintain cysteine homeostasis under these conditions. We found no relationship between H2S generation, cysteine concentration and sensitivity to ciprofloxacin. Excess cysteine, which occurred during E. coli growth in cystine-fed media, triggered continuous H2S production, accelerated glutathione synthesis and cysteine export. This was accompanied by a twofold increase in ciprofloxacin tolerance in all strains except gshA, whose sensitivity increased 5-8-fold at high CF doses, indicating the importance of GSH in restoring the intracellular redox situation during growth in cystine-fed media.

Regulation of Cysteine Homeostasis and Its Effect on Escherichia coli Sensitivity to Ciprofloxacin in LB Medium

Cysteine and its derivatives, including H2S, can influence bacterial virulence and sensitivity to antibiotics. In minimal sulfate media, H2S is generated under stress to prevent excess cysteine and, together with incorporation into glutathione and export into the medium, is a mechanism of cysteine homeostasis. Here, we studied the features of cysteine homeostasis in LB medium, where the main source of sulfur is cystine, whose import can create excess cysteine inside cells. We used mutants in the mechanisms of cysteine homeostasis and a set of microbiological and biochemical methods, including the real-time monitoring of sulfide and oxygen, the determination of cysteine and glutathione (GSH), and the expression of the Fur, OxyR, and SOS regulons genes. During normal growth, the parental strain generated H2S when switching respiration to another substrate. The mutations affected the onset time, the intensity and duration of H2S production, cysteine and glutathione levels, bacterial growth and respiration rates, and the induction of defense systems. Exposure to chloramphenicol and high doses of ciprofloxacin increased cysteine content and GSH synthesis. A high inverse relationship between log CFU/mL and bacterial growth rate before ciprofloxacin addition was revealed. The study points to the important role of maintaining cysteine homeostasis during normal growth and antibiotic exposure in LB medium.