Bacterial death from treatment with fluoroquinolones and other lethal stressors

A Review of Antibacterial Candidates with New Modes of Action

There is a lack of new antibiotics to combat drug-resistant bacterial infections that increasingly threaten global health. The current pipeline of clinical-stage antimicrobials is primarily populated by "new and improved" versions of existing antibiotic classes, supplemented by several novel chemical scaffolds that act on traditional targets. The lack of fresh chemotypes acting on previously unexploited targets (the "holy grail" for new antimicrobials due to their scarcity) is particularly unfortunate as these offer the greatest opportunity for innovative breakthroughs to overcome existing resistance. In recognition of their potential, this review focuses on this subset of high value antibiotics, providing chemical structures where available. This review focuses on candidates that have progressed to clinical trials, as well as selected examples of promising pioneering approaches in advanced stages of development, in order to stimulate additional research aimed at combating drug-resistant infections.

Reactive Oxygen Species (ROS)-Mediated Antibacterial Oxidative Therapies: Available Methods to Generate ROS and a Novel Option Proposal

The increasing emergence of multidrug-resistant (MDR) pathogens causes difficult-to-treat infections with long-term hospitalizations and a high incidence of death, thus representing a global public health problem. To manage MDR bacteria bugs, new antimicrobial strategies are necessary, and their introduction in practice is a daily challenge for scientists in the field. An extensively studied approach to treating MDR infections consists of inducing high levels of reactive oxygen species (ROS) by several methods. Although further clinical investigations are mandatory on the possible toxic effects of ROS on mammalian cells, clinical evaluations are extremely promising, and their topical use to treat infected wounds and ulcers, also in presence of biofilm, is already clinically approved. Biochar (BC) is a carbonaceous material obtained by pyrolysis of different vegetable and animal biomass feedstocks at 200-1000 °C in the limited presence of O2. Recently, it has been demonstrated that BC's capability of removing organic and inorganic xenobiotics is mainly due to the presence of persistent free radicals (PFRs), which can activate oxygen, H2O2, or persulfate in the presence or absence of transition metals by electron transfer, thus generating ROS, which in turn degrade pollutants by advanced oxidation processes (AOPs). In this context, the antibacterial effects of BC-containing PFRs have been demonstrated by some authors against Escherichia coli and Staphylococcus aureus, thus giving birth to our idea of the possible use of BC-derived PFRs as a novel method capable of inducing ROS generation for antimicrobial oxidative therapy. Here, the general aspects concerning ROS physiological and pathological production and regulation and the mechanism by which they could exert antimicrobial effects have been reviewed. The methods currently adopted to induce ROS production for antimicrobial oxidative therapy have been discussed. Finally, for the first time, BC-related PFRs have been proposed as a new source of ROS for antimicrobial therapy via AOPs.

Non-Canonical Aspects of Antibiotics and Antibiotic Resistance

The understanding of antibiotic resistance, one of the major health threats of our time, is mostly based on dated and incomplete notions, especially in clinical contexts. The "canonical" mechanisms of action and pharmacodynamics of antibiotics, as well as the methods used to assess their activity upon bacteria, have not changed in decades; the same applies to the definition, acquisition, selective pressures, and drivers of resistance. As a consequence, the strategies to improve antibiotic usage and overcome resistance have ultimately failed. This review gathers most of the "non-canonical" notions on antibiotics and resistance: from the alternative mechanisms of action of antibiotics and the limitations of susceptibility testing to the wide variety of selective pressures, lateral gene transfer mechanisms, ubiquity, and societal factors maintaining resistance. Only by having a "big picture" view of the problem can adequate strategies to harness resistance be devised. These strategies must be global, addressing the many aspects that drive the increasing prevalence of resistant bacteria aside from the clinical use of antibiotics.

Quorum-sensing agr system of Staphylococcus aureus primes gene expression for protection from lethal oxidative stress

The agr quorum-sensing system links Staphylococcus aureus metabolism to virulence, in part by increasing bacterial survival during exposure to lethal concentrations of H2O2, a crucial host defense against S. aureus. We now report that protection by agr surprisingly extends beyond post-exponential growth to the exit from stationary phase when the agr system is no longer turned on. Thus, agr can be considered a constitutive protective factor. Deletion of agr resulted in decreased ATP levels and growth, despite increased rates of respiration or fermentation at appropriate oxygen tensions, suggesting that Δagr cells undergo a shift towards a hyperactive metabolic state in response to diminished metabolic efficiency. As expected from increased respiratory gene expression, reactive oxygen species (ROS) accumulated more in the agr mutant than in wild-type cells, thereby explaining elevated susceptibility of Δagr strains to lethal H2O2 doses. Increased survival of wild-type agr cells during H2O2 exposure required sodA, which detoxifies superoxide. Additionally, pretreatment of S. aureus with respiration-reducing menadione protected Δagr cells from killing by H2O2. Thus, genetic deletion and pharmacologic experiments indicate that agr helps control endogenous ROS, thereby providing resilience against exogenous ROS. The long-lived 'memory' of agr-mediated protection, which is uncoupled from agr activation kinetics, increased hematogenous dissemination to certain tissues during sepsis in ROS-producing, wild-type mice but not ROS-deficient (Cybb-/-) mice. These results demonstrate the importance of protection that anticipates impending ROS-mediated immune attack. The ubiquity of quorum sensing suggests that it protects many bacterial species from oxidative damage.

Risk of type 2 diabetes and long-term antibiotic use in childhood: Evidence from the UK Biobank

AIMS

This study aimed to investigate the association between long-term use of antibiotics during childhood and the risk of type 2 diabetes mellitus (T2DM) using a prospective cohort from the UK Biobank.

METHODS

Participants in the UK Biobank who completed the online survey for digestive health were included in this prospective cohort study. A Cox regression model adjusted for sociodemographic characteristics, general health factors, mental health, lifestyle factors, comorbidities, and medication use was used to estimate the hazard ratio (HR) and confidence interval (CI) of the association between long-term use of antibiotics in the childhood and incident T2DM.

RESULTS

The final analyses included 152,992 participants and 22,133 of them received long-term/recurrent antibiotics as children or teenagers. During the follow-up, 3370 and 681 incident T2DM cases occurred in the non-exposed and exposed groups respectively. Long-term use of antibiotics in childhood was associated with an increased risk of T2DM, with an HR of 1.16 (95 % CI, 1.07-1.27) after adjusting for potential confounders. Results in the subgroup analyses and sensitivity analyses were highly consistent with the primary analyses.

CONCLUSIONS

Long-term use of antibiotics in childhood is associated with the risk of T2DM in middle and old age in the UK Biobank population.

Quorum-sensing agr system of Staphylococcus aureus primes gene expression for protection from lethal oxidative stress

The agr quorum-sensing system links Staphylococcus aureus metabolism to virulence, in part by increasing bacterial survival during exposure to lethal concentrations of H2O2, a crucial host defense against S. aureus. We now report that protection by agr surprisingly extends beyond post-exponential growth to the exit from stationary phase when the agr system is no longer turned on. Thus, agr can be considered a constitutive protective factor. Deletion of agr increased both respiration and fermentation but decreased ATP levels and growth, suggesting that Δagr cells assume a hyperactive metabolic state in response to reduced metabolic efficiency. As expected from increased respiratory gene expression, reactive oxygen species (ROS) accumulated more in the agr mutant than in wild-type cells, thereby explaining elevated susceptibility of Δagr strains to lethal H2O2 doses. Increased survival of wild-type agr cells during H2O2 exposure required sodA, which detoxifies superoxide. Additionally, pretreatment of S. aureus with respiration-reducing menadione protected Δagr cells from killing by H2O2. Thus, genetic deletion and pharmacologic experiments indicate that agr helps control endogenous ROS, thereby providing resilience against exogenous ROS. The long-lived "memory" of agr-mediated protection, which is uncoupled from agr activation kinetics, increased hematogenous dissemination to certain tissues during sepsis in ROS-producing, wild-type mice but not ROS-deficient (Nox2-/-) mice. These results demonstrate the importance of protection that anticipates impending ROS-mediated immune attack. The ubiquity of quorum sensing suggests that it protects many bacterial species from oxidative damage.

An Oxygen Supply Strategy for Sonodynamic Therapy in Tuberculous Granuloma Lesions Using a Catalase-Loaded Nanoplatform

Purpose

Tuberculosis (TB) is a chronic disease caused by Mycobacterium tuberculosis (MTB) that remains a major global health challenge. One of the main obstacles to effective treatment is the heterogeneous microenvironment of TB granulomas. This study aimed to investigate the potential of a hypoxic remission-based strategy to enhance the outcome of tuberculosis treatment when implemented in combination with ultrasound.

Methods

A PLGA nanoparticle (LEV@CAT-NPs) loaded with levofloxacin (LEV) and catalase (CAT) was fabricated by a double emulsification method, and its physical characteristics, oxygen production capacity, drug release capacity, and biosafety were thoroughly investigated. The synergistic therapeutic effects of ultrasound (US)-mediated LEV@CAT-NPs were evaluated using an experimental mouse model of subcutaneous tuberculosis granuloma induced by Bacille Calmette-Guérin (BCG) as a substitute for MTB.

Results

LEV@CAT-NPs exhibited excellent oxygen production capacity, biosafety, and biocompatibility. Histological analysis revealed that ultrasound-mediated LEV@CAT-NPs could effectively remove bacteria from tuberculous granulomas, significantly alleviate the hypoxia state, reduce the necrotic area and inflammatory cells within the granuloma, and increase the penetration of dyes in granuloma tissues. The combined treatment also reduced the serum levels of inflammatory cytokines (eg, TNF-α, IL-6, and IL-8), and significantly downregulated the expression of hypoxia-inducible factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF). These results suggested that the synergistic treatment of ultrasound-mediated LEV@CAT-NPs effectively eradicated the bacterial infection and reversed the hypoxic microenvironment of tuberculous granulomas, further promoting tissue repair.

Conclusion

This study provides a non-invasive and new avenue for treating refractory tuberculosis infections. The potential role of regulating hypoxia within infected lesions as a therapeutic target for infection deserves further exploration in future studies.

Evolution of YacG to safeguard DNA gyrase from external perturbation

Cells have evolved strategies to safeguard their genome integrity. We describe a mechanism to counter double strand breaks in the chromosome that involves the protection of an essential housekeeping enzyme from external agents. YacG is a DNA gyrase inhibitory protein from Escherichia coli that protects the bacterium from the cytotoxic effects of catalytic inhibitors as well as cleavage-complex stabilizers of DNA gyrase. By virtue of blocking the primary DNA binding site of the enzyme, YacG prevents the accumulation of double strand breaks induced by gyrase poisons. It also enables the bacterium to resist the growth-inhibitory property of novobiocin. Gyrase poison-induced oxidative stress upregulates YacG production, probably as a cellular response to counter DNA damage. YacG-mediated protection of the genome is specific for gyrase targeting agents as the protection is not observed from the action of general DNA damaging agents. YacG also intensifies the transcription stress induced by rifampicin substantiating the importance of gyrase activity during transcription. Although essential for bacterial survival, DNA gyrase often gets entrapped by external inhibitors and poisons, resulting in cell death. The existence of YacG to specifically protect an essential housekeeping enzyme might be a strategy adopted by bacteria for competitive fitness advantage.

Analysis of Regulatory Mechanism of AcrB and CpxR on Colistin Susceptibility Based on Transcriptome and Metabolome of Salmonella Typhimurium

With the increasing and inappropriate use of colistin, the emerging colistin-resistant isolates have been frequently reported during the last few decades. Therefore, new potential targets and adjuvants to reverse colistin resistance are urgently needed. Our previous study has confirmed a marked increase of colistin susceptibility (16-fold compared to the wild-type Salmonella strain) of cpxR overexpression strain JSΔacrBΔcpxR::kan/pcpxR (simplified as JSΔΔ/pR). To searching for potential new drug targets, the transcriptome and metabolome analysis were carried out in this study. We found that the more susceptible strain JSΔΔ/pR displayed striking perturbations at both the transcriptomics and metabolomics levels. The virulence-related genes and colistin resistance-related genes (CRRGs) were significantly downregulated in JSΔΔ/pR. There were significant accumulation of citrate, α-ketoglutaric acid, and agmatine sulfate in JSΔΔ/pR, and exogenous supplement of them could synergistically enhance the bactericidal effect of colistin, indicating that these metabolites may serve as potential adjuvants for colistin therapy. Additionally, we also demonstrated that AcrB and CpxR could target the ATP and reactive oxygen species (ROS) generation, but not proton motive force (PMF) production pathway to potentiate antibacterial activity of colistin. Collectively, these findings have revealed several previously unknown mechanisms contributing to increased colistin susceptibility and identified potential targets and adjuvants for potentiating colistin treatment of Salmonella infections. IMPORTANCE Emergence of multidrug-resistant (MDR) Gram-negative (G-) bacteria have led to the reconsideration of colistin as the last-resort therapeutic option for health care-associated infections. Finding new drug targets and strategies against the spread of MDR G- bacteria are global challenges for the life sciences community and public health. In this paper, we demonstrated the more susceptibility strain JSΔΔ/pR displayed striking perturbations at both the transcriptomics and metabolomics levels and revealed several previously unknown regulatory mechanisms of AcrB and CpxR on the colistin susceptibility. Importantly, we found that exogenous supplement of citrate, α-ketoglutaric acid, and agmatine sulfate could synergistically enhance the bactericidal effect of colistin, indicating that these metabolites may serve as potential adjuvants for colistin therapy. These results provide a theoretical basis for finding potential new drug targets and adjuvants.

Enhancement of β-Lactam-Mediated Killing of Gram-Negative Bacteria by Lysine Hydrochloride

Widespread bacterial resistance among Gram-negative bacteria is rapidly depleting our antimicrobial arsenal. Adjuvants that enhance the bactericidal activity of existing antibiotics provide a way to alleviate the resistance crisis, as new antimicrobials are becoming increasingly difficult to develop. The present work with Escherichia coli revealed that neutralized lysine (lysine hydrochloride) enhances the bactericidal activity of β-lactams in addition to increasing bacteriostatic activity. When combined, lysine hydrochloride and β-lactam increased expression of genes involved in the tricarboxylic acid (TCA) cycle and raised reactive oxygen species (ROS) levels; as expected, agents known to mitigate bactericidal effects of ROS reduced lethality from the combination treatment. Lysine hydrochloride had no enhancing effect on the lethal action of fluoroquinolones or aminoglycosides. Characterization of a tolerant mutant indicated involvement of the FtsH/HflkC membrane-embedded protease complex in lethality enhancement. The tolerant mutant, which carried a V86F substitution in FtsH, exhibited decreased lipopolysaccharide levels, reduced expression of TCA cycle genes, and reduced levels of ROS. Lethality enhancement by lysine hydrochloride was abolished by treating cultures with Ca2+ or Mg2+, cations known to stabilize the outer membrane. These data, plus damage observed by scanning electron microscopy, indicate that lysine stimulates β-lactam lethality by disrupting the outer membrane. Lethality enhancement of β-lactams by lysine hydrochloride was also observed with Acinetobacter baumannii and Pseudomonas aeruginosa, thereby suggesting that the phenomenon is common among Gram-negative bacteria. Arginine hydrochloride behaved in a similar way. Overall, the combination of lysine or arginine hydrochloride and β-lactam offers a new way to increase β-lactam lethality with Gram-negative pathogens. IMPORTANCE Antibiotic resistance among Gram-negative pathogens is a serious medical problem. The present work describes a new study in which a nontoxic nutrient increases the lethal action of clinically important β-lactams. Elevated lethality is expected to reduce the emergence of resistant mutants. The effects were observed with significant pathogens (Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa), indicating widespread applicability. Examination of tolerant mutants and biochemical measurements revealed involvement of endogenous reactive oxygen species in response to outer membrane perturbation. These lysine hydrochloride-β-lactam data support the hypothesis that lethal stressors can stimulate the accumulation of ROS. Genetic and biochemical work also revealed how an alteration in a membrane protease, FtsH, abolishes lysine stimulation of β-lactam lethality. Overall, the work presents a method for antimicrobial enhancement that should be safe, easy to administer, and likely to apply to other nutrients, such as arginine.

Ascorbate and Antibiotics, at Concentrations Attainable in Urine, Can Inhibit the Growth of Resistant Strains of Escherichia coli Cultured in Synthetic Human Urine

There are conflicting reports on the antibacterial activity of ascorbate; all at concentrations much higher than the typical in human plasma, but that can be reached in urine. The effect of 10 mM ascorbate (in itself not inhibitory) along with antibiotics, was tested both in Mueller-Hinton broth (MHb) and in synthetic human urine (SHU), against resistant isolates of Escherichia coli from lower urinary infections. The activity of nitrofurantoin and sulfamethoxazole was higher in SHU than in MHb; minimal inhibitory concentrations (MICs) in SHU with ascorbate were below typical urinary concentrations. For other antibiotics, MICs were the same in MHb vs. SHU, with no effect of ascorbate in MHb; but in SHU with ascorbate, MICs of ciprofloxacin and gentamicin also went below reported urinary concentrations, with a lesser effect with norfloxacin and trimethoprim, and none with ampicillin. The effect of ascorbate was independent of oxygen and not related to the susceptibility of each strain to oxidative stress. Ascorbate oxidizes during incubation in SHU, and bacterial growth partially prevented oxidation. These results suggest that 10 mM ascorbate can enhance the inhibitory activity of antibiotics upon resistant strains in urine. Clinical experimentation with ascorbate-antibiotic combinations against urinary infections caused by resistant bacteria is warranted.

Salmonella antimicrobials inherited and the non-inherited resistance: mechanisms and alternative therapeutic strategies

Salmonella spp. is one of the most important foodborne pathogens. Typhoid fever and enteritis caused by Salmonella enterica are associated with 16-33 million infections and 500,000 to 600,000 deaths annually worldwide. The eradication of Salmonella is becoming increasingly difficult because of its remarkable capacity to counter antimicrobial agents. In addition to the intrinsic and acquired resistance of Salmonella, increasing studies indicated that its non-inherited resistance, which commonly mentioned as biofilms and persister cells, plays a critical role in refractory infections and resistance evolution. These remind the urgent demand for new therapeutic strategies against Salmonella. This review starts with escape mechanisms of Salmonella against antimicrobial agents, with particular emphasis on the roles of the non-inherited resistance in antibiotic failure and resistance evolution. Then, drug design or therapeutic strategies that show impressive effects in overcoming Salmonella resistance and tolerance are summarized completely, such as overcoming the barrier of outer membrane by targeting MlaABC system, reducing persister cells by limiting hydrogen sulfide, and applying probiotics or predatory bacteria. Meanwhile, according to the clinical practice, the advantages and disadvantages of above strategies are discussed. Finally, we further analyze how to deal with this tricky problems, thus can promote above novel strategies to be applied in the clinic as soon as possible. We believed that this review will be helpful in understanding the relationships between tolerance phenotype and resistance of Salmonella as well as the efficient control of antibiotic resistance.

Identification of a Small Molecule Compound Active against Antibiotic-Tolerant Staphylococcus aureus by Boosting ATP Synthesis

Antibiotic tolerance poses a threat to current antimicrobial armamentarium. Bacteria at a tolerant state survive in the presence of antibiotic treatment and account for persistence, relapse and recalcitrance of infections. Antibiotic treatment failure may occur due to antibiotic tolerance. Persistent infections are difficult to treat and are often associated with poor prognosis, imposing an enormous burden on the healthcare system. Effective strategies targeting antibiotic-tolerant bacteria are therefore highly warranted. In this study, small molecule compound SA-558 was identified to be effective against Staphylococcus aureus that are tolerant to being killed by conventional antibiotics. SA-558 mediated electroneutral transport across the membrane and led to increased ATP and ROS generation, resulting in a reduction of the population of antibiotic-tolerant bacteria. In a murine chronic infection model, of which vancomycin treatment failed, we demonstrated that SA-558 alone and in combination with vancomycin caused significant reduction of MRSA abundance. Our results indicate that SA-558 monotherapy or combinatorial therapy with vancomycin is an option for managing persistent S. aureus bacteremia infection and corroborate that bacterial metabolism is an important target for counteracting antibiotic tolerance.

Quinolone Antibiotics: Resistance and Therapy

The clinical application of quinolone antibiotics is particularly extensive. In addition to their high efficiency in infectious diseases, the treatment process brings multiple hidden dangers or side effects. In this regard, drug resistance becomes a major challenge and is almost unavoidable in the clinical application of quinolones. Both genetic and phenotypic variations contribute to bacterial survival resistance under antibiotic therapy. This review is focusing on the drug discovery history, compound structure, and bactericidal mechanism of quinolone antibiotics. Recent studies bring a more in-depth insight into the research progress of quinolone antibiotics in the causes of death, drug resistance formation, and closely related SOS response after disease treatment at this stage. Combined with the latest clinical studies, we summarize the clinical application of quinolone antibiotics and further lay a theoretical foundation for the mechanism study of resistant or sensitive bacteria in response to quinolone treatment.

RecA inactivation as a strategy to reverse the heteroresistance phenomenon in clinical isolates of Escherichia coli

RecA inhibition could be an important strategy to combat antimicrobial resistance because of its key role in the SOS response, DNA repair and homologous recombination contributing to bacterial survival. This study evaluated the impact of RecA inactivation on heteroresistance in clinical isolates of Escherichia coli and their corresponding recA-deficient isogenic strains to multiple classes of antimicrobial agents. A high frequency (>30%) of heteroresistance was observed in this collection of clinical isolates. Deletion of the recA gene led to a marked reduction in heteroresistant subpopulations, especially against quinolones or β-lactams. The molecular basis of heteroresistance was associated with an increase in copy number of plasmid-borne resistance genes (bla_TEM-1B) or tandem gene amplifications (qnrA1). Of note, in the absence of the recA gene, the increase in copy number of resistance genes was suppressed. This makes the recA gene a promising target for combating heteroresistance.

Optical Methods for Label-Free Detection of Bacteria

Pathogenic bacteria are the leading causes of food-borne and water-borne infections, and one of the most serious public threats. Traditional bacterial detection techniques, including plate culture, polymerase chain reaction, and enzyme-linked immunosorbent assay are time-consuming, while hindering precise therapy initiation. Thus, rapid detection of bacteria is of vital clinical importance in reducing the misuse of antibiotics. Among the most recently developed methods, the label-free optical approach is one of the most promising methods that is able to address this challenge due to its rapidity, simplicity, and relatively low-cost. This paper reviews optical methods such as surface-enhanced Raman scattering spectroscopy, surface plasmon resonance, and dark-field microscopic imaging techniques for the rapid detection of pathogenic bacteria in a label-free manner. The advantages and disadvantages of these label-free technologies for bacterial detection are summarized in order to promote their application for rapid bacterial detection in source-limited environments and for drug resistance assessments.

An effective antimicrobial complex of nanoscale β-cyclodextrin and ciprofloxacin conjugated to a cell adhesive dipeptide

Today, various drug delivery systems (DDS) are utilized to carry and deliver the desired drugs to the targeted action area to reduce potential side effects and negative interactions. Nanomaterials are an excellent candidate for the delivery of potent drugs, as they enhance pharmacokinetic and pharmacodynamic properties. Herein, we present a new ciprofloxacin (CPFX) delivery system based on a polymeric nanocarrier (β-cyclodextrin) conjugated to a cell-adhesive dipeptide structure. Cyclodextrin (CD) is an inexpensive, easily accessible, biodegradable, and biocompatible material. Also, the conjugation of cysteine-arginine (CR) dipeptide to the CPFX/β-CD particles is carried out to enhance cell adhesion growth. Through accurate analysis, the drug content and release for a final product have been estimated to be ca. 32%. Overall, the antimicrobial effects of CPFX were considerably raised through a low dose of CPFX. The growth zone inhibition of CPFX/β-CD-CR particles on the staphylococcus aureus and the Escherichia coli bacterial cells was 5.5 ± 0.2 cm and 3.5 ± 0.2 cm, respectively. Hence, this therapeutic nano bioconjugate is an excellent candidate to be applied in antimicrobial applications with the minimum incorporated CPFX.

Redox Impact on Bacterial Macromolecule: A Promising Avenue for Discovery and Development of Novel Antibacterials

Antibiotic resistance in bacteria has remained a serious public health concern, resulting in substantial deaths and morbidity each year. Factors such as mutation and abuse of currently available antibiotics have contributed to the bulk of the menace. Hence, the introduction and implementation of new therapeutic strategies are imperative. Of these strategies, data supporting the role of reactive oxygen species (ROS) in bacterial lethality are intriguing, with several antimicrobials, including antibiotics such as fluoroquinolones, β-lactams, and aminoglycosides, as well as natural plant compounds, being remarkably implicated. Following treatment with ROS-inducing antimicrobials, ROS such as O₂^•-, ^•OH, and H₂O₂ generated in bacteria, which the organism is unable to detoxify, damage cellular macromolecules such as proteins, lipids, and nucleic acids and results in cell death. Despite the unique mechanism of action of ROS-inducing antibacterials and significant studies on ROS-mediated means of bacterial killing, the field remains a topical one, with contradicting viewpoints that require frequent review. Here, we appraised the antibacterial agents (antibiotics, natural and synthetic compounds) implicated in ROS generation and the safety concerns associated with their usage. Further, background information on the sources and types of ROS in bacteria, the mechanism of bacterial lethality via oxidative stress, as well as viewpoints on the ROS hypothesis undermining and solidifying this concept are discussed.

Fluoroquinolone heteroresistance, antimicrobial tolerance, and lethality enhancement

With tuberculosis, the emergence of fluoroquinolone resistance erodes the ability of treatment to interrupt the progression of MDR-TB to XDR-TB. One way to reduce the emergence of resistance is to identify heteroresistant infections in which subpopulations of resistant mutants are likely to expand and make the infections fully resistant: treatment modification can be instituted to suppress mutant enrichment. Rapid DNA-based detection methods exploit the finding that fluoroquinolone-resistant substitutions occur largely in a few codons of DNA gyrase. A second approach for restricting the emergence of resistance involves understanding fluoroquinolone lethality through studies of antimicrobial tolerance, a condition in which bacteria fail to be killed even though their growth is blocked by lethal agents. Studies with Escherichia coli guide work with Mycobacterium tuberculosis. Lethal action, which is mechanistically distinct from blocking growth, is associated with a surge in respiration and reactive oxygen species (ROS). Mutations in carbohydrate metabolism that attenuate ROS accumulation create pan-tolerance to antimicrobials, disinfectants, and environmental stressors. These observations indicate the existence of a general death pathway with respect to stressors. M. tuberculosis displays a variation on the death pathway idea, as stress-induced ROS is generated by NADH-mediated reductive stress rather than by respiration. A third approach, which emerges from lethality studies, uses a small molecule, N-acetyl cysteine, to artificially increase respiration and additional ROS accumulation. That enhances moxifloxacin lethality with M. tuberculosis in culture, during infection of cultured macrophages, and with infection of mice. Addition of ROS stimulators to fluoroquinolone treatment of tuberculosis constitutes a new direction for suppressing the transition of MDR-TB to XDR-TB.

Moxifloxacin-Mediated Killing of Mycobacterium tuberculosis Involves Respiratory Downshift, Reductive Stress, and Accumulation of Reactive Oxygen Species

Moxifloxacin is central to treatment of multidrug-resistant tuberculosis. Effects of moxifloxacin on the Mycobacterium tuberculosis redox state were explored to identify strategies for increasing lethality and reducing the prevalence of extensively resistant tuberculosis. A noninvasive redox biosensor and a reactive oxygen species (ROS)-sensitive dye revealed that moxifloxacin induces oxidative stress correlated with M. tuberculosis death. Moxifloxacin lethality was mitigated by supplementing bacterial cultures with an ROS scavenger (thiourea), an iron chelator (bipyridyl), and, after drug removal, an antioxidant enzyme (catalase). Lethality was also reduced by hypoxia and nutrient starvation. Moxifloxacin increased the expression of genes involved in the oxidative stress response, iron-sulfur cluster biogenesis, and DNA repair. Surprisingly, and in contrast with Escherichia coli studies, moxifloxacin decreased expression of genes involved in respiration, suppressed oxygen consumption, increased the NADH/NAD⁺ ratio, and increased the labile iron pool in M. tuberculosis. Lowering the NADH/NAD⁺ ratio in M. tuberculosis revealed that NADH-reductive stress facilitates an iron-mediated ROS surge and moxifloxacin lethality. Treatment with N-acetyl cysteine (NAC) accelerated respiration and ROS production, increased moxifloxacin lethality, and lowered the mutant prevention concentration. Moxifloxacin induced redox stress in M. tuberculosis inside macrophages, and cotreatment with NAC potentiated the antimycobacterial efficacy of moxifloxacin during nutrient starvation, inside macrophages, and in mice, where NAC restricted the emergence of resistance. Thus, NADH-reductive stress contributes to moxifloxacin-mediated killing of M. tuberculosis, and the respiration stimulator (NAC) enhances lethality and suppresses the emergence of drug resistance.

Reactive metabolic byproducts contribute to antibiotic lethality under anaerobic conditions

Understanding how bactericidal antibiotics kill bacteria remains an open question. Previous work has proposed that primary drug-target corruption leads to increased energetic demands, resulting in the generation of reactive metabolic byproducts (RMBs), particularly reactive oxygen species, that contribute to antibiotic-induced cell death. Studies have challenged this hypothesis by pointing to antibiotic lethality under anaerobic conditions. Here, we show that treatment of Escherichia coli with bactericidal antibiotics under anaerobic conditions leads to changes in the intracellular concentrations of central carbon metabolites, as well as the production of RMBs, particularly reactive electrophilic species (RES). We show that antibiotic treatment results in DNA double-strand breaks and membrane damage and demonstrate that antibiotic lethality under anaerobic conditions can be decreased by RMB scavengers, which reduce RES accumulation and mitigate associated macromolecular damage. This work indicates that RMBs, generated in response to antibiotic-induced energetic demands, contribute in part to antibiotic lethality under anaerobic conditions.

A broadly applicable, stress-mediated bacterial death pathway regulated by the phosphotransferase system (PTS) and the cAMP-Crp cascade

Recent work indicates that killing of bacteria by diverse antimicrobial classes can involve reactive oxygen species (ROS), as if a common, self-destructive response to antibiotics occurs. However, the ROS-bacterial death theory has been challenged. To better understand stress-mediated bacterial death, we enriched spontaneous antideath mutants of Escherichia coli that survive treatment by diverse bactericidal agents that include antibiotics, disinfectants, and environmental stressors, without a priori consideration of ROS. The mutants retained bacteriostatic susceptibility, thereby ruling out resistance. Surprisingly, pan-tolerance arose from carbohydrate metabolism deficiencies in ptsI (phosphotransferase) and cyaA (adenyl cyclase); these genes displayed the activity of upstream regulators of a widely shared, stress-mediated death pathway. The antideath effect was reversed by genetic complementation, exogenous cAMP, or a Crp variant that bypasses cAMP binding for activation. Downstream events comprised a metabolic shift from the TCA cycle to glycolysis and to the pentose phosphate pathway, suppression of stress-mediated ATP surges, and reduced accumulation of ROS. These observations reveal how upstream signals from diverse stress-mediated lesions stimulate shared, late-stage, ROS-mediated events. Cultures of these stable, pan-tolerant mutants grew normally and were therefore distinct from tolerance derived from growth defects described previously. Pan-tolerance raises the potential for unrestricted disinfectant use to contribute to antibiotic tolerance and resistance. It also weakens host defenses, because three agents (hypochlorite, hydrogen peroxide, and low pH) affected by pan-tolerance are used by the immune system to fight infections. Understanding and manipulating the PtsI-CyaA-Crp–mediated death process can help better control pathogens and maintain beneficial microbiota during antimicrobial treatment.

Dual Effect: High NADH Levels Contribute to Efflux-Mediated Antibiotic Resistance but Drive Lethality Mediated by Reactive Oxygen Species

In light of the antibiotic crisis, emerging strategies to sensitize bacteria to available antibiotics should be explored. Several studies on the mechanisms of killing suggest that bactericidal antibiotic activity is enforced through the generation of reactive oxygen species (ROS-lethality hypothesis). Here, we artificially manipulated the redox homeostasis of the model opportunistic pathogen Pseudomonas aeruginosa using specific enzymes that catalyze either the formation or oxidation of NADH. Increased NADH levels led to the activation of antibiotic efflux pumps and high levels of antibiotic resistance. However, higher NADH levels also resulted in increased intracellular ROS and amplified antibiotic killing. Our results demonstrate that growth inhibition and killing activity are mediated via different mechanisms. Furthermore, the profound changes in bioenergetics produced low-virulence phenotypes characterized by reduced interbacterial signaling controlled pathogenicity traits. Our results pave the way for a more effective infection resolution and add an antivirulence strategy to maximize chances to combat devastating P. aeruginosa infections while reducing the overall use of antibiotics. IMPORTANCE The emergence of antibiotic resistance has become one of the major threats to public health. A better understanding of antimicrobial killing mechanisms promises to uncover new ways to resensitize bacteria to commonly used antibiotics. In this context, there is increasing evidence that the metabolic status of the cell plays a fundamental role in reactive oxygen species (ROS)-mediated cell death. In this work, we artificially manipulated the redox balance in Pseudomonas aeruginosa by the expression of two orthologous enzymes. We found that the increase of intracellular NADH concentrations leads to higher antibiotic resistance but also generates a burst in the production of ROS that amplified antimicrobial killing. Our work suggests that the combination of bactericidal antibiotics with agents that disturb the cellular redox homeostasis could significantly enhance antibiotic killing via sensitization of pathogens to currently available antibiotics.

Antimicrobials Functioning through ROS-Mediated Mechanisms: Current Insights

Antibiotic resistance and infections caused by multidrug-resistant bacteria are global health concerns. Reducing the overuse and misuse of antibiotics is the primary step toward minimizing the antibiotic resistance crisis. Thus, it is imperative to introduce and implement novel antimicrobial strategies. Recently, several alternative antimicrobials targeting oxidative stress in bacteria have been studied and shown to be promising. Oxidative stress occurs when bacterial cells fail to detoxify the excessive reactive oxygen species (ROS) accumulated in the cells. Bacteria deploy numerous defense mechanisms against oxidative stress. The oxidative stress response is not essential for the normal growth of bacteria, but it is crucial for their survival. This toxic oxidative stress is created by the host immune response or antimicrobials generating ROS. ROS possess strong oxidation potential and cause serious damage to nucleic acids, lipids, and proteins. Since ROS-based antimicrobials target multiple sites in bacteria, these antimicrobials have attracted the attention of several researchers. In this review, we present recent ROS-based alternative antimicrobials and strategies targeting oxidative stress which might help in mitigating the problem of antibiotic resistance and dissemination.

Effect of RecA inactivation and detoxification systems on the evolution of ciprofloxacin resistance in Escherichia coli

Background

Suppression of SOS response and overproduction of reactive oxygen species (ROS) through detoxification system suppression enhance the activity of fluoroquinolones.

Objectives

To evaluate the role of both systems in the evolution of resistance to ciprofloxacin in an isogenic model of Escherichia coli.

Methods

Single-gene deletion mutants of E. coli BW25113 (wild-type) (ΔrecA, ΔkatG, ΔkatE, ΔsodA, ΔsodB), double-gene (ΔrecA-ΔkatG, ΔrecA-ΔkatE, ΔrecA-ΔsodA, ΔrecA-ΔsodB, ΔkatG-ΔkatE, ΔsodB-ΔsodA) and triple-gene (ΔrecA-ΔkatG-ΔkatE) mutants were included. The response to sudden high ciprofloxacin pressure was evaluated by mutant prevention concentration (MPC). The gradual antimicrobial pressure response was evaluated through experimental evolution and antibiotic resistance assays.

Results

For E. coli BW25113 strain, ΔkatE, ΔsodB and ΔsodB/ΔsodA mutants, MPC values were 0.25 mg/L. The ΔkatG, ΔsodA, ΔkatG/katE and ΔrecA mutants showed 2-fold reductions (0.125 mg/L). The ΔkatG/ΔrecA, ΔkatE/ΔrecA, ΔsodA/ΔrecA, ΔsodB/ΔrecA and ΔkatG/ΔkatE/ΔrecA strains showed 4–8-fold reductions (0.03–0.06 mg/L) relative to the wild-type. Gradual antimicrobial pressure increased growth capacity for ΔsodA and ΔsodB and ΔsodB/ΔsodA mutants (no growth in 4 mg/L) compared with the wild-type (no growth in the range of 0.5–2 mg/L). Accordingly, increased growth was observed with the mutants ΔrecA/ΔkatG (no growth in 2 mg/L), ΔrecA/ΔkatE (no growth in 2 mg/L), ΔrecA/ΔsodA (no growth in 0.06 mg/L), ΔrecA/ΔsodB (no growth in 0.25 mg/L) and ΔrecA/ΔkatG/ΔkatE (no growth in 0.5 mg/L) compared with ΔrecA (no growth in the range of 0.002–0.015 mg/L).

Conclusions

After RecA inactivation, gradual exposure to ciprofloxacin reduces the evolution of resistance. After suppression of RecA and detoxification systems, sudden high exposure to ciprofloxacin reduces the evolution of resistance in E. coli.

Synergistic Antibacterial and Antibiofilm Activity of the MreB Inhibitor A22 Hydrochloride in Combination with Conventional Antibiotics against Pseudomonas aeruginosa and Escherichia coli Clinical Isolates

In the era of antibiotic resistance, the bacterial cytoskeletal protein MreB is presented as a potential target for the development of novel antimicrobials. Combined treatments of clinical antibiotics with anti-MreB compounds may be promising candidates in combating the resistance crisis, but also in preserving the potency of many conventional drugs. This study aimed to evaluate the synergistic antibacterial and antibiofilm activities of the MreB inhibitor A22 hydrochloride in combination with various antibiotics. The minimum inhibitory concentration (MIC) values of the individual compounds were determined by the broth microdilution method against 66 clinical isolates of Gram-negative bacteria. Synergy was assessed by the checkerboard assay. The fractional inhibitory concentration index was calculated for each of the A22-antibiotic combination. Bactericidal activity of the combinations was evaluated by time-kill curve assays. The antibiofilm activity of the most synergistic combinations was determined by crystal violet stain, methyl thiazol tetrazolium assay, and confocal laser scanning microscopy analysis. The combined cytotoxic and hemolytic activity was also evaluated toward human cells. According to our results, Pseudomonas aeruginosa and Escherichia coli isolates were resistant to conventional antibiotics to varying degrees. A22 inhibited the bacterial growth in a dose-dependent manner with MIC values ranging between 2 and 64 μg/mL. In combination studies, synergism occurred most frequently with A22-ceftazidime and A22-meropemen against Pseudomonas aeruginosa and A22-cefoxitin and A22-azithromycin against Escherichia coli. No antagonism was observed. In time-kill studies, synergism was observed with all expected combinations. Synergistic combinations even at the lowest tested concentrations were able to inhibit biofilm formation and eradicate mature biofilms in both strains. Cytotoxic and hemolytic effects of the same combinations toward human cells were not observed. The findings of the present study support previous research regarding the use of MreB as a novel antibiotic target. The obtained data expand the existing knowledge about the antimicrobial and antibiofilm activity of the A22 inhibitor, and they indicate that A22 can serve as a leading compound for studying potential synergism between MreB inhibitors and antibiotics in the future.

Oxygen and Metabolism: Digesting Determinants of Antibiotic Susceptibility in the Gut

Microbial metabolism is a major determinant of antibiotic susceptibility. Environmental conditions that modify metabolism, notably oxygen availability and redox potential, can directly fine-tune susceptibility to antibiotics. Despite this, relatively few studies have discussed these modifications within the gastrointestinal tract and their implication on in vivo drug activity and the off-target effects of antibiotics in the gut. In this review, we discuss the environmental and biogeographical complexity of the gastrointestinal tract in regard to oxygen availability and redox potential, addressing how the heterogeneity of gut microhabitats may modify antibiotic activity in vivo. We contextualize the current literature surrounding oxygen availability and antibiotic efficacy and discuss empirical treatments. We end by discussing predicted patterns of antibiotic activity in prominent microbiome taxa, given gut heterogeneity, oxygen availability, and polymicrobial interactions. We also propose additional work required to fully elucidate the role of oxygen metabolism on antibiotic susceptibility in the context of the gut.