Significance: Hydrogen sulfide (H ₂ S) is a highly toxic gas that interferes with cellular respiration; however, at low physiological amounts, it plays an important role in cell signaling. Remarkably, in bacteria, endogenously produced H ₂ S has been recently recognized as a general protective molecule, which renders multiple bacterial species highly resistant to oxidative stress and various classes of antibiotics. The mechanism of this phenomenon remains poorly understood. In this paper, we use Escherichia coli as a model system to elucidate its major enzymatic source of H ₂ S and establish the principle biochemical pathways that account for H ₂ S-mediated protection against reactive oxygen species. Understanding those mechanisms has far-reaching implications in preventing bacterial resistance and designing effective antimicrobial therapies.

Significance: The CydDC complex is involved in the assembly of cytochrome bd -I, a terminal oxidase of the respiratory chain required for growth under low oxygen conditions. It has been suggested that CydDC shuttles excessive l -cysteine from the cytoplasm to the periplasm, thereby maintaining bacterial redox homeostasis. Here, we demonstrate that the principle function of CydDC is in maintaining the reduced state of cytoplasmic l -cysteine, as opposed to exporting l -cysteine, thereby providing an important connection between sulfur metabolism, oxidative stress, and resistance to antibiotics. In particular, we establish the critical role of cytoplasmic l -cysteine in mediating the toxicity of aminoglycosides. Thus, understanding the mechanisms that control the bacterial redox state can lead to more effective strategies to counter bacterial resistance and tolerance.

Impaired lipopolysaccharide biosynthesis in Gram-negative bacteria results in the “deep rough” phenotype, which is characterized by increased sensitivity of cells to various hydrophobic compounds, including antibiotics novobiocin, actinomycin D, erythromycin, etc. The present study showed that E. coli mutants carrying deletions of the ADP-heptose biosynthesis genes became hypersensitive to a wide range of antibacterial drugs: DNA gyrase inhibitors, protein biosynthesis inhibitors (aminoglycosides, tetracycline), RNA polymerase inhibitors (rifampicin), and β-lactams (carbenicillin). In addition, it was found that inactivation of the gmhA, hldE, rfaD, and waaC genes led to dramatic changes in the redox status of cells: a decrease in the pool of reducing NADPH and ATP equivalents, the concentration of intracellular cysteine, a change in thiol homeostasis, and a deficiency in the formation of hydrogen sulfide. In “deep rough” mutants, intensive formation of reactive oxygen species was observed, which, along with a lack of reducing agents, such as reactive sulfur species or NADPH, leads to oxidative stress and an increase in the number of dead cells in the population. Within the framework of modern ideas about the role of oxidative stress as a universal mechanism of the bactericidal action of antibiotics, inhibition of the enzymes of ADP-heptose biosynthesis is a promising direction for increasing the effectiveness of existing antibiotics and solving the problem of multidrug resistance.

Counteraction of the origin and distribution of multidrug-resistant pathogens responsible for intra-hospital infections is a worldwide issue in medicine. In this brief review, we discuss the results of our recent investigations, which argue that many antibiotics, along with inactivation of their traditional biochemical targets, can induce oxidative stress (ROS production), thus resulting in increased bactericidal efficiency. As we previously showed, hydrogen sulfide, which is produced in the cells of different pathogens protects them not only against oxidative stress but also against bactericidal antibiotics. Next, we clarified the interplay of oxidative stress, cysteine metabolism, and hydrogen sulfide production. Finally, demonstrated that small molecules, which inhibit a bacterial enzyme involved in hydrogen sulfide production, potentiate bactericidal antibiotics including quinolones, beta-lactams, and aminoglycosides against bacterial pathogens in in vitro and in mouse models of infection. These inhibitors also suppress bacterial tolerance to antibiotics by disrupting the biofilm formation and substantially reducing the number of persister bacteria, which survive the antibiotic treatment. We hypothesise that agents which limit hydrogen sulfide biosynthesis are effective tools to counteract the origin and distribution of multidrug-resistant pathogens.

In cells of Escherichia coli, terminal oxidase bd-I encoded by the cydAB gene catalyzes the reduction of O₂ to water using hydroquinone as an electron donor. In addition to the cydAB operon, two other genes, cydC and cydD, encoding the heterodimeric ATP-binding cassette-type transporter are essential for the assembly of cytochrome bd-I. It was shown that inactivation of cytochrome bd-I by the introduction of cydB or cydD deletions into the E. coli chromosome leads to supersensitivity of the bacteria to antibiotics of the quinolone and beta-lactam classes. The sensitivity of these mutants to antibiotics is partially suppressed by introduction of a constitutively expressed gene katG under the control of the P_tet promoter into their genome. The increased level of hydrogen sulfide resulting from the introduction of the mstA gene, encoding 3-mercaptopyruvate sulfurtransferase, under the control of the P_tet promoter, leads to the same effect. These data demonstrate the important role of cytochrome bd-I in the defense of bacteria from oxidative stress and bactericidal antibiotics.

Biosynthesis of ribose-5-phosphate (R5P) underlies all biosynthetic processes associated with biomass growth. Actively dividing cells continuously require building blocks for genome replication, synthesis of ribosomes and other derivatives containing R5P as a carbohydrate backbone. The main source of R5P in the cell is the pentose phosphate pathway (PPP), which is an anabolic sensor designed to coordinate the level of pentose phosphates and reduced NADPH required for anabolic processes. This review is devoted to a comparative analysis of R5P biosynthesis pathways among different domains of microorganisms, the features of PPP regulation in bacterial cells depending on physiological conditions, as well as genetic modifications of PPP and their effect on cell viability. We emphasize that ribose metabolism is a factor in the consolidation of cellular homeostasis under conditions of intensive biomass growth and the discrepancy between the processes of ribose synthesis and consumption is marked by spontaneous cell death.