Corrigendum: All Three Endogenous Quinone Species of Escherichia coli Are Involved in Controlling the Activity of the Aerobic/Anaerobic Response Regulator ArcA.

[This corrects the article DOI: 10.3389/fmicb.2016.01339.].

RodZ and PgsA Play Intertwined Roles in Membrane Homeostasis of Bacillus subtilis and Resistance to Weak Organic Acid Stress.

Weak organic acids like sorbic and acetic acid are widely used to prevent growth of spoilage organisms such as Bacilli. To identify genes involved in weak acid stress tolerance we screened a transposon mutant library of Bacillus subtilis for sorbic acid sensitivity. Mutants of the rodZ (ymfM) gene were found to be hypersensitive to the lipophilic weak organic acid. RodZ is involved in determining the cell's rod-shape and believed to interact with the bacterial actin-like MreB cytoskeleton. Since rodZ lies upstream in the genome of the essential gene pgsA (phosphatidylglycerol phosphate synthase) we hypothesized that expression of the latter might also be affected in rodZ mutants and hence contribute to the phenotype observed. We show that both genes are co-transcribed and that both the rodZ::mini-Tn10 mutant and a conditional pgsA mutant, under conditions of minimal pgsA expression, were sensitive to sorbic and acetic acid. Both strains displayed a severely altered membrane composition. Compared to the wild-type strain, phosphatidylglycerol and cardiolipin levels were lowered and the average acyl chain length was elongated. Induction of rodZ expression from a plasmid in our transposon mutant led to no recovery of weak acid susceptibility comparable to wild-type levels. However, pgsA overexpression in the same mutant partly restored sorbic acid susceptibility and fully restored acetic acid sensitivity. A construct containing both rodZ and pgsA as on the genome led to some restored growth as well. We propose that RodZ and PgsA play intertwined roles in membrane homeostasis and tolerance to weak organic acid stress.

All Three Endogenous Quinone Species of Escherichia coli Are Involved in Controlling the Activity of the Aerobic/Anaerobic Response Regulator ArcA.

The enteron Escherichia coli is equipped with a branched electron transfer chain that mediates chemiosmotic electron transfer, that drives ATP synthesis. The components of this electron transfer chain couple the oxidation of available electron donors from cellular metabolism (e.g., NADH, succinate, lactate, formate, etc.) to the reduction of electron acceptors like oxygen, nitrate, fumarate, di-methyl-sulfoxide, etc. Three different quinones, i.e., ubiquinone, demethyl-menaquinone and menaquinone, couple the transfer of electrons between the dehydrogenases and reductases/oxidases that constitute this electron transfer chain, whereas, the two-component regulation system ArcB/A regulates gene expression, to allow the organism to adapt itself to the ambient conditions of available electron donors and acceptors. Here, we report that E. coli can grow and adjust well to transitions in the availability of oxygen, with any of the three quinones as its single quinone. In all three 'single-quinone' E. coli strains transitions in the activity of ArcB are observed, as evidenced by changes in the level of phosphorylation of the response regulator ArcA, upon depletion/readmission of oxygen. These results lead us to conclude that all quinol species of E. coli can reduce (i.e., activate) the sensor ArcB and all three quinones oxidize (i.e., de-activate) it. These results also confirm our earlier conclusion that demethyl-menaquinone can function in aerobic respiration.

Intracellular pH Response to Weak Acid Stress in Individual Vegetative Bacillus subtilis Cells.

Intracellular pH (pHi) critically affects bacterial cell physiology. Hence, a variety of food preservation strategies are aimed at perturbing pHi homeostasis. Unfortunately, accurate pHi quantification with existing methods is suboptimal, since measurements are averages across populations of cells, not taking into account interindividual heterogeneity. Yet, physiological heterogeneity in isogenic populations is well known to be responsible for differences in growth and division kinetics of cells in response to external stressors. To assess in this context the behavior of intracellular acidity, we have developed a robust method to quantify pHi at single-cell levels in Bacillus subtilis Bacilli spoil food, cause disease, and are well known for their ability to form highly stress-resistant spores. Using an improved version of the genetically encoded ratiometric pHluorin (IpHluorin), we have quantified pHi in individual B. subtilis cells, cultured at an external pH of 6.4, in the absence or presence of weak acid stresses. In the presence of 3 mM potassium sorbate, a decrease in pHi and an increase in the generation time of growing cells were observed. Similar effects were observed when cells were stressed with 25 mM potassium acetate. Time-resolved analysis of individual bacteria in growing colonies shows that after a transient pH decrease, long-term pH evolution is highly cell dependent. The heterogeneity at the single-cell level shows the existence of subpopulations that might be more resistant and contribute to population survival. Our approach contributes to an understanding of pHi regulation in individual bacteria and may help scrutinizing effects of existing and novel food preservation strategies. IMPORTANCE: This study shows how the physiological response to commonly used weak organic acid food preservatives, such as sorbic and acetic acids, can be measured at the single-cell level. These data are key to coupling often-observed single-cell heterogeneous growth behavior upon the addition of weak organic acid food preservatives. Generally, these data are gathered in the form of plate counting of samples incubated with the acids. Here, we visualize the underlying heterogeneity in cellular pH homeostasis, opening up avenues for mechanistic analyses of the heterogeneity in the weak acid stress response. Thus, microbial risk assessment can become more robust, widening the scope of use of these well-known weak organic acid food preservatives.

Carbon Monoxide Gas Is Not Inert, but Global, in Its Consequences for Bacterial Gene Expression, Iron Acquisition, and Antibiotic Resistance.

AIMS: Carbon monoxide is a respiratory poison and gaseous signaling molecule. Although CO-releasing molecules (CORMs) deliver CO with temporal and spatial specificity in mammals, and are proven antimicrobial agents, we do not understand the modes of CO toxicity. Our aim was to explore the impact of CO gas per se, without intervention of CORMs, on bacterial physiology and gene expression. RESULTS: We used tightly controlled chemostat conditions and integrated transcriptomic datasets with statistical modeling to reveal the global effects of CO. CO is known to inhibit bacterial respiration, and we found expression of genes encoding energy-transducing pathways to be significantly affected via the global regulators, Fnr, Arc, and PdhR. Aerobically, ArcA-the response regulator-is transiently phosphorylated and pyruvate accumulates, mimicking anaerobiosis. Genes implicated in iron acquisition, and the metabolism of sulfur amino acids and arginine, are all perturbed. The global iron-related changes, confirmed by modulation of activity of the transcription factor Fur, may underlie enhanced siderophore excretion, diminished intracellular iron pools, and the sensitivity of CO-challenged bacteria to metal chelators. Although CO gas (unlike H2S and NO) offers little protection from antibiotics, a ruthenium CORM is a potent adjuvant of antibiotic activity. INNOVATION: This is the first detailed exploration of global bacterial responses to CO, revealing unexpected targets with implications for employing CORMs therapeutically. CONCLUSION: This work reveals the complexity of bacterial responses to CO and provides a basis for understanding the impacts of CO from CORMs, heme oxygenase activity, or environmental sources. Antioxid. Redox Signal. 24, 1013-1028.

Compartment-specific pH monitoring in Bacillus subtilis using fluorescent sensor proteins: a tool to analyze the antibacterial effect of weak organic acids.

The internal pH (pHi) of a living cell is one of its most important physiological parameters. To monitor the pH inside Bacillus subtilis during various stages of its life cycle, we constructed an improved version (IpHluorin) of the ratiometric, pH-sensitive fluorescent protein pHluorin by extending it at the 5' end with the first 24 bp of comGA. The new version, which showed an approximate 40% increase in fluorescence intensity, was expressed from developmental phase-specific, native promoters of B. subtilis that are specifically active during vegetative growth on glucose (PptsG) or during sporulation (PspoIIA, PspoIIID, and PsspE). Our results show strong, compartment-specific expression of IpHluorin that allowed accurate pHi measurements of live cultures during exponential growth, early and late sporulation, spore germination, and during subsequent spore outgrowth. Dormant spores were characterized by an pHi of 6.0 ± 0.3. Upon full germination the pHi rose dependent on the medium to 7.0-7.4. The presence of sorbic acid in the germination medium inhibited a rise in the intracellular pH of germinating spores and inhibited germination. Such effects were absent when acetic was added at identical concentrations.

Challenges and advances in systems biology analysis of Bacillus spore physiology; molecular differences between an extreme heat resistant spore forming Bacillus subtilis food isolate and a laboratory strain.

Bacterial spore formers are prime organisms of concern in the food industry. Spores from the genus Bacillus are extremely stress resistant, most notably exemplified by high thermotolerance. This sometimes allows surviving spores to germinate and grow out to vegetative cells causing food spoilage and possible intoxication. Similar issues though more pending toward spore toxigenicity are observed for the anaerobic Clostridia. The paper indicates the nature of stress resistance and highlights contemporary molecular approaches to analyze the mechanistic basis of it in Bacilli. A molecular comparison between a laboratory strain and a food borne isolate, very similar at the genomic level to the laboratory strain but generating extremely heat resistant spores, is discussed. The approaches cover genome-wide genotyping, proteomics and genome-wide expression analyses studies. The analyses aim at gathering sufficient molecular information to be able to put together an initial framework for dynamic modelling of spore germination and outgrowth behaviour. Such emerging models should be developed both at the population and at the single spore level. Tools and challenges in achieving the latter are succinctly discussed.