Reactive Oxygen Species Play an Important Role in the Bactericidal Activity of Quinolone Antibiotics

Proteomics in antibiotic resistance and tolerance research: Mapping the resistome and the tolerome of bacterial pathogens

Antibiotic resistance, the ability of a microbial pathogen to evade the effects of antibiotics thereby allowing them to grow under elevated drug concentrations, is an alarming health problem worldwide and has attracted the attention of scientists for decades. On the other hand, the clinical importance of persistence and tolerance as alternative mechanisms for pathogens to survive prolonged lethal antibiotic doses has recently become increasingly appreciated. Persisters and high-tolerance populations are thought to cause the relapse of infectious diseases, and provide opportunities for the pathogens to evolve resistance during the course of antibiotic therapy. Although proteomics and other omics methodology have long been employed to study resistance, its applications in studying persistence and tolerance are still limited. However, due to the growing interest in the topic and recent progress in method developments to study them, there have been some proteomic studies that yield fresh insights into the phenomenon of persistence and tolerance. Combined with the studies on resistance, these collectively guide us to novel molecular targets for the potential drugs for the control of these dangerous pathogens. In this review, we surveyed previous proteomic studies to investigate resistance, persistence, and tolerance mechanisms, and discussed emerging experimental strategies for studying these phenotypes with a combination of adaptive laboratory evolution and high-throughput proteomics. This article is protected by copyright. All rights reserved.

Organic acids and their salts potentiate the activity of selected antibiotics against Pseudomonas aeruginosa biofilms grown in a synthetic cystic fibrosis sputum medium

The failure of antibiotic therapy in respiratory tract infections in cystic fibrosis is partly due to the high tolerance observed in Pseudomonas aeruginosa biofilms. This tolerance is mediated by changes in bacterial metabolism linked to growth in biofilms, opening up potential avenues for novel treatment approaches based on modulating metabolism. The goal of the present study was to identify carbon sources that increase the inhibiting and/or eradicating activity of tobramycin, ciprofloxacin and ceftazidime against P. aeruginosa PAO1 biofilms grown in a synthetic cystic fibrosis sputum medium (SCFM2) and to elucidate their mode of action. After screening 69 carbon sources, several combinations of antibiotics + carbon sources that showed markedly higher anti-biofilm activity than antibiotics alone were identified. D,L-malic acid and sodium acetate could potentiate both biofilm inhibiting and eradicating activity of ciprofloxacin and ceftazidime, respectively, while citric acid could only potentiate biofilm inhibitory activity of tobramycin. The mechanisms underlying the increased biofilm eradicating activity of combinations ciprofloxacin/D,L-malic acid and ceftazidime/sodium acetate are similar but not identical. Potentiation of ceftazidime activity by sodium acetate was linked to increased metabolic activity, a functional TCA cycle, increased ROS production and high intracellular pH, whereas the latter was not required for D,L-malic acid potentiation of ciprofloxacin. Finally, our results indicate that the potentiation of antibiotic activity by carbon sources is strain dependent.

DNA glycosylases for 8-oxoguanine repair in Staphylococcus aureus

GO system is part of base excision DNA repair and is required for the correct repair of 8-oxoguanine (8-oxoG), one of the most abundant oxidative lesions. Due to the ability of 8-oxoG to mispair with A, this base is highly mutagenic, and its repair requires two enzymes: Fpg that removes 8-oxoG from 8-oxoG:C pairs, and MutY that excises the normal A from 8-oxoG:A mispairs. Here we characterize the properties of putative GO system DNA glycosylases from Staphylococcus aureus, an important human opportunistic pathogen that causes hospital infections and presents a serious health concern due to quick spread of antibiotic-resistant strains. In addition to Fpg and MutY from the reference NCTC 8325 strain (SauFpg1 and SauMutY), we have also studied an Fpg homolog from a multidrug-resistant C0673 isolate (SauFpg2), which is different from SauFpg1 in its sequence. Both SauFpg enzymes showed the highest activity at pH 7.0-9.0 and NaCl concentrations 25-75 mM (SauFpg1) or 50-100 mM (SauFpg2), whereas SauMutY was active at a broad pH range and had a salt optimum at ∼75 mM NaCl. Both SauFpg1 and SauFpg2 bound and cleaved duplexes containing 8-oxoG, 5-hydroxyuracil, 5,6-dihydrouracil or apurinic/apyrimidinic site paired with C, T, or G, but not with A. For SauFpg1 and SauFpg2, 8-oxoG was the best substrate tested, and 5,6-dihydrouracil was the worst one. SauMutY efficiently excised adenine from duplex substrates containing A:8-oxoG or A:G pairs. SauFpg enzymes were readily trapped on DNA by NaBH₄ treatment, indicating formation of a Schiff base reaction intermediate. Surprisingly, SauMutY was also trapped significantly better than its E. coli homolog. All three S. aureus GO glycosylases drastically reduced spontaneous mutagenesis when expressed in an fpg mutY E. coli double mutant. Overall, we conclude that S. aureus possesses an active GO system, which could possibly be targeted for sensitization of this pathogen to oxidative stress.

The PHP domain of PolX from Staphylococcus aureus aids high fidelity DNA synthesis through the removal of misincorporated deoxyribo-, ribo- and oxidized nucleotides

The X family is one of the eight families of DNA polymerases (dPols) and members of this family are known to participate in the later stages of Base Excision Repair. Many prokaryotic members of this family possess a Polymerase and Histidinol Phosphatase (PHP) domain at their C-termini. The PHP domain has been shown to possess 3'-5' exonuclease activity and may represent the proofreading function in these dPols. PolX from Staphylococcus aureus also possesses the PHP domain at the C-terminus, and we show that this domain has an intrinsic Mn²⁺ dependent 3'-5' exonuclease capable of removing misincorporated dNMPs from the primer. The misincorporation of oxidized nucleotides such as 8oxodGTP and rNTPs are known to be pro-mutagenic and can lead to genomic instability. Here, we show that the PHP domain aids DNA replication by the removal of misincorporated oxidized nucleotides and rNMPs. Overall, our study shows that the proofreading activity of the PHP domain plays a critical role in maintaining genomic integrity and stability. The exonuclease activity of this enzyme can, therefore, be the target of therapeutic intervention to combat infection by methicillin-resistant-Staphylococcus-aureus.

Decreased Antibiotic Susceptibility in Pseudomonas aeruginosa Surviving UV Irradition

Given its excellent performance against the pathogens, UV disinfection has been applied broadly in different fields. However, only limited studies have comprehensively investigated the response of bacteria surviving UV irradiation to the environmental antibiotic stress. Here, we investigated the antibiotic susceptibility of Pseudomonas aeruginosa suffering from the UV irradiation. Our results revealed that UV exposure may decrease the susceptibility to tetracycline, ciprofloxacin, and polymyxin B in the survival P. aeruginosa. Mechanistically, UV exposure causes oxidative stress in P. aeruginosa and consequently induces dysregulation of genes contributed to the related antibiotic resistance genes. These results revealed that the insufficient ultraviolet radiation dose may result in the decreased antibiotic susceptibility in the pathogens, thus posing potential threats to the environment and human health.

Bioenergetic Inhibitors: Antibiotic Efficacy and Mechanisms of Action in Mycobacterium tuberculosis

Development of novel anti-tuberculosis combination regimens that increase efficacy and reduce treatment timelines will improve patient compliance, limit side-effects, reduce costs, and enhance cure rates. Such advancements would significantly improve the global TB burden and reduce drug resistance acquisition. Bioenergetics has received considerable attention in recent years as a fertile area for anti-tuberculosis drug discovery. Targeting the electron transport chain (ETC) and oxidative phosphorylation machinery promises not only to kill growing cells but also metabolically dormant bacilli that are inherently more drug tolerant. Over the last two decades, a broad array of drugs targeting various ETC components have been developed. Here, we provide a focused review of the current state of art of bioenergetic inhibitors of Mtb with an in-depth analysis of the metabolic and bioenergetic disruptions caused by specific target inhibition as well as their synergistic and antagonistic interactions with other drugs. This foundation is then used to explore the reigning theories on the mechanisms of antibiotic-induced cell death and we discuss how bioenergetic inhibitors in particular fail to be adequately described by these models. These discussions lead us to develop a clear roadmap for new lines of investigation to better understand the mechanisms of action of these drugs with complex mechanisms as well as how to leverage that knowledge for the development of novel, rationally-designed combination therapies to cure TB.

Combating Antibiotic Tolerance Through Activating Bacterial Metabolism

The emergence of antibiotic tolerance enables genetically susceptible bacteria to withstand the killing by clinically relevant antibiotics. As is reported, an increasing body of evidence sheds light on the critical and underappreciated role of antibiotic tolerance in the disease burden of bacterial infections. Considering this tense situation, new therapeutic strategies are urgently required for combating antibiotic tolerance. Herein, we provide an insightful illustration to distinguish between antibiotic resistance and tolerance, and highlight its clinical significance and complexities of drug-tolerant bacteria. Then, we discuss the close relationship between antibiotic tolerance and bacterial metabolism. As such, a bacterial metabolism-based approach was proposed to counter antibiotic tolerance. These exogenous metabolites including amino acids, tricarboxylic acid cycle (TCA cycle) metabolites, and nucleotides effectively activate bacterial metabolism and convert the tolerant cells to sensitive cells, and eventually restore antibiotic efficacy. A better understanding of molecular mechanisms of antibiotic tolerance particularly in vivo would substantially drive the development of novel strategies targeting bacterial metabolism.

N-acetylcysteine blocks SOS induction and mutagenesis produced by fluoroquinolones in Escherichia coli

BACKGROUND

Fluoroquinolones such as ciprofloxacin induce the mutagenic SOS response and increase the levels of intracellular reactive oxygen species (ROS). Both the SOS response and ROS increase bacterial mutagenesis, fuelling the emergence of resistant mutants during antibiotic treatment. Recently, there has been growing interest in developing new drugs able to diminish the mutagenic effect of antibiotics by modulating ROS production and the SOS response.

OBJECTIVES

To test whether physiological concentrations of N-acetylcysteine, a clinically safe antioxidant drug currently used in human therapy, is able to reduce ROS production, SOS induction and mutagenesis in ciprofloxacin-treated bacteria without affecting antibiotic activity.

METHODS

The Escherichia coli strain IBDS1 and its isogenic mutant deprived of SOS mutagenesis (TLS-) were treated with different concentrations of ciprofloxacin, N-acetylcysteine or both drugs in combination. Relevant parameters such as MICs, growth rates, ROS production, SOS induction, filamentation and antibiotic-induced mutation rates were evaluated.

RESULTS

Treatment with N-acetylcysteine reduced intracellular ROS levels (by ∼40%), as well as SOS induction (by up to 75%) and bacterial filamentation caused by subinhibitory concentrations of ciprofloxacin, without affecting ciprofloxacin antibacterial activity. Remarkably, N-acetylcysteine completely abolished SOS-mediated mutagenesis.

CONCLUSIONS

Collectively, our data strongly support the notion that ROS are a key factor in antibiotic-induced SOS mutagenesis and open the possibility of using N-acetylcysteine in combination with antibiotic therapy to hinder the development of antibiotic resistance.

A multilayered repair system protects the mycobacterial chromosome from endogenous and antibiotic-induced oxidative damage

Oxidative damage to DNA is a threat to the genomic integrity and coding accuracy of the chromosomes of all living organisms. Guanine is particularly susceptible to oxidation, and 8-oxo-dG (OG), when produced in situ or incorporated by DNA polymerases, is highly mutagenic due to mispairing with adenine. In many bacteria, defense against OG depends on MutT enzymes, which sanitize OG in the nucleotide pool, and the MutM/Y system, which counteracts OG in chromosomal DNA. In Escherichia coli, antibiotic lethality has been linked to oxidative stress and the downstream consequences of OG processing. However, in mycobacteria, the role of these systems in genomic integrity and antibiotic lethality is not understood, in part because mycobacteria encode four MutT enzymes and two MutMs, suggesting substantial redundancy. Here, we definitively probe the role of OG handling systems in mycobacteria. We find that, although MutT4 is the only MutT enzyme required for resistance to oxidative stress, this effect is not due to OG processing. We find that the dominant system that defends against OG-mediated mutagenesis is MutY/MutM1, and this system is dedicated to in situ chromosomal oxidation rather than correcting OG incorporated by accessory polymerases (DinB1/DinB2/DinB3/DnaE2). In addition, we uncover that mycobacteria resist antibiotic lethality through nucleotide sanitization by MutTs, and in the absence of this system, accessory DNA polymerases and MutY/M contribute to antibiotic-induced lethality. These results reveal a complex, multitiered system of OG handling in mycobacteria with roles in oxidative stress resistance, mutagenesis, and antibiotic lethality.

The proofreading activity of Pfprex from Plasmodium falciparum can prevent mutagenesis of the apicoplast genome by oxidized nucleotides

The DNA polymerase module of the Pfprex enzyme (PfpPol) is responsible for duplication of the genome of the apicoplast organelle in the malaria parasite. We show that PfpPol can misincorporate oxidized nucleotides such as 8oxodGTP opposite dA. This event gives rise to transversion mutations that are known to lead to adverse physiological outcomes. The apicoplast genome is particularly vulnerable to the harmful effects of 8oxodGTP due to very high AT content (~ 87%). We show that the proofreading activity of PfpPol has the unique ability to remove the oxidized nucleotide from the primer terminus. Due to this property, the proofreading domain of PfpPol is able to prevent mutagenesis of the AT-rich apicoplast genome and neutralize the deleterious genotoxic effects of ROS generated in the apicoplast due to normal metabolic processes. The proofreading activity of the Pfprex enzyme may, therefore, represent an attractive target for therapeutic intervention. Also, a survey of DNA repair pathways shows that the observed property of Pfprex constitutes a novel form of dynamic error correction wherein the repair of promutagenic damaged nucleotides is concomitant with DNA replication.

Inhibitors of DNA Glycosylases as Prospective Drugs

DNA glycosylases are enzymes that initiate the base excision repair pathway, a major biochemical process that protects the genomes of all living organisms from intrinsically and environmentally inflicted damage. Recently, base excision repair inhibition proved to be a viable strategy for the therapy of tumors that have lost alternative repair pathways, such as BRCA-deficient cancers sensitive to poly(ADP-ribose)polymerase inhibition. However, drugs targeting DNA glycosylases are still in development and so far have not advanced to clinical trials. In this review, we cover the attempts to validate DNA glycosylases as suitable targets for inhibition in the pharmacological treatment of cancer, neurodegenerative diseases, chronic inflammation, bacterial and viral infections. We discuss the glycosylase inhibitors described so far and survey the advances in the assays for DNA glycosylase reactions that may be used to screen pharmacological libraries for new active compounds.

A Novel Screening Strategy Reveals ROS-Generating Antimicrobials That Act Synergistically against the Intracellular Veterinary Pathogen Rhodococcus equi

Rhodococcus equi is a facultative intracellular pathogen that causes infections in foals and many other animals such as pigs, cattle, sheep, and goats. Antibiotic resistance is rapidly rising in horse farms, which makes ineffective current antibiotic treatments based on a combination of macrolides and rifampicin. Therefore, new therapeutic strategies are urgently needed to treat R. equi infections caused by antimicrobial resistant strains. Here, we employed a R. equi mycoredoxin-null mutant strain highly susceptible to oxidative stress to screen for novel ROS-generating antibiotics. Then, we used the well-characterized Mrx1-roGFP2 biosensor to confirm the redox stress generated by the most promising antimicrobial agents identified in our screening. Our results suggest that different combinations of antibacterial compounds that elicit oxidative stress are promising anti-infective strategies against R. equi. In particular, the combination of macrolides with ROS-generating antimicrobial compounds such as norfloxacin act synergistically to produce a potent antibacterial effect against R. equi. Therefore, our screening approach could be applied to identify novel ROS-inspired therapeutic strategies against intracellular pathogens.

Pyrophosphate hydrolysis is an intrinsic and critical step of the DNA synthesis reaction

DNA synthesis by DNA polymerases (dPols) is central to duplication and maintenance of the genome in all living organisms. dPols catalyze the formation of a phosphodiester bond between the incoming deoxynucleoside triphosphate and the terminal primer nucleotide with the release of a pyrophosphate (PPi) group. It is believed that formation of the phosphodiester bond is an endergonic reaction and PPi has to be hydrolyzed by accompanying pyrophosphatase enzymes to ensure that the free energy change of the DNA synthesis reaction is negative and it can proceed in the forward direction. The fact that DNA synthesis proceeds in vitro in the absence of pyrophosphatases represents a long-standing conundrum regarding the thermodynamics of the DNA synthesis reaction. Using time-resolved crystallography, we show that hydrolysis of PPi is an intrinsic and critical step of the DNA synthesis reaction catalyzed by dPols. The hydrolysis of PPi occurs after the formation of the phosphodiester bond and ensures that the DNA synthesis reaction is energetically favorable without the need for additional enzymes. Also, we observe that DNA synthesis is a two Mg2+ ion assisted stepwise associative SN2 reaction. Overall, this study provides deep temporal insight regarding the primary enzymatic reaction responsible for genome duplication.

Lophirones B and C induce oxidative cellular death pathway in Acinetobacter baumannii by inhibiting DNA gyrase

We evaluated the inactivation of DNA gyrase on the oxidative stress response and sensitivity of A. baumannii to lophirones B and C. The sensitivity of parental and the mutant strains of A. baumannii to lophirones B and C was determined using minimum inhibitory concentration (MIC) and time-kill sensitivity. Inactivation of sodB, katG, recA enhanced the sensitivity of A. baumannii to lophirones B and C. Furthermore, this inactivation increased the accumulation of superoxide anion radical and hydrogen peroxide in lophirones B and C-treated A. baumannii, which was reversed in the presence of thiourea. Inactivation of gyrA stalled lophirones B and C-mediated ROS accumulation in A. baumannii. In addition, lophirones B and C raised the Fe²⁺ contents of A. baumannii. Dipyridyl (Fe chelator) reversed the sensitivity of A. baumannii to lophirones B and C. Lophirones significantly lowered the NAD+/NADH ratio of A. baumannii. The results of this study revealed that the impact of DNA gyrase in lophirones B and C-mediated ROS accumulation, Fe²⁺ release and cell death.

Contributions of reactive oxygen species, oxidative DNA damage and glutathione depletion to the sensitivity of Acinetobacter baumannii to 2-(2-nitrovinyl) furan

2-(2-nitrovinyl) furan is a broad-spectrum antibacterial agent with activity against Gram-positive and Gram-negative bacteria. In this study, the contributions of reactive oxygen species, oxidative DNA damage and glutathione depletion to its activity against Acinetobacter baumannii was investigated. Inactivation of sodB, katG and recA lowered the minimum inhibitory concentration of 2-(2-nitrovinyl) furan. Furthermore, the inactivation increased the superoxide anion radical and hydrogen peroxide contents of 2-(2-nitrovinyl) furan-treated A. baumannii. Antioxidant (thiourea) reversed the elevated levels of superoxide anion radical and hydrogen peroxide. In addition, thiourea lowered the susceptibility of A. baumannii to 2-(2-nitrovinyl) furan. 2-(2-nitrovinyl) furan depleted reduced glutathione (GSH) contents of parental, sodB, katG and recA strains of A. baumannii. NAD+/NADH ratio parental, sodB, katG and recA strains of A. baumannii exposed to 2-(2-nitrovinyl) furan increased significantly. Inactivation of type-I NADH dehydrogenase lowered the reactive oxygen species generation in 2-(2-nitrovinyl) furan-treated A. baumannii. It is evident from this study that 2-(2-nitrovinyl) furan stimulates respiratory chain activity of A. baumannii leading to enhanced ROS generation, which depletes GSH and reacts with Fe²⁺ to produce hydroxyl radical that damage DNA.

Inhibition of inflammasome activation by a clinical strain of Klebsiella pneumoniae impairs efferocytosis and leads to bacterial dissemination

Klebsiella pneumoniae is a Gram-negative bacterium responsible for severe cases of nosocomial pneumonia. During the infectious process, both neutrophils and monocytes migrate to the site of infection, where they carry out their effector functions and can be affected by different patterns of cell death. Our data show that clinical strains of K. pneumoniae have dissimilar mechanisms for surviving within macrophages; these mechanisms include modulation of microbicidal mediators and cell death. The A28006 strain induced high IL-1β production and pyroptotic cell death in macrophages; by contrast, the A54970 strain induced high IL-10 production and low IL-1β production by macrophages. Pyroptotic cell death induced by the A28006 strain leads to a significant increase in bacterial sensitivity to hydrogen peroxide, and efferocytosis of the pyroptotic cells results in efficient bacterial clearance both in vitro and in vivo. In addition, the A54970 strain was able to inhibit inflammasome activation and pyroptotic cell death by inducing IL-10 production. Here, for the first time, we present a K. pneumoniae strain able to inhibit inflammasome activation, leading to bacterial survival and dissemination in the host. The understanding of possible escape mechanisms is essential in the search for alternative treatments against multidrug-resistant bacteria.

Antibiotic efficacy-context matters

Antibiotic lethality is a complex physiological process, sensitive to external cues. Recent advances using systems approaches have revealed how events downstream of primary target inhibition actively participate in antibiotic death processes. In particular, altered metabolism, translational stress and DNA damage each contribute to antibiotic-induced cell death. Moreover, environmental factors such as oxygen availability, extracellular metabolites, population heterogeneity and multidrug contexts alter antibiotic efficacy by impacting bacterial metabolism and stress responses. Here we review recent studies on antibiotic efficacy and highlight insights gained on the involvement of cellular respiration, redox stress and altered metabolism in antibiotic lethality. We discuss the complexity found in natural environments and highlight knowledge gaps in antibiotic lethality that may be addressed using systems approaches.

Potentiating antibiotics in drug-resistant clinical isolates via stimuli-activated superoxide generation

The rise of multidrug-resistant (MDR) bacteria is a growing concern to global health and is exacerbated by the lack of new antibiotics. To treat already pervasive MDR infections, new classes of antibiotics or antibiotic adjuvants are needed. Reactive oxygen species (ROS) have been shown to play a role during antibacterial action; however, it is not yet understood whether ROS contribute directly to or are an outcome of bacterial lethality caused by antibiotics. We show that a light-activated nanoparticle, designed to produce tunable flux of specific ROS, superoxide, potentiates the activity of antibiotics in clinical MDR isolates of Escherichia coli, Salmonella enterica, and Klebsiella pneumoniae. Despite the high degree of antibiotic resistance in these isolates, we observed a synergistic interaction between both bactericidal and bacteriostatic antibiotics with varied mechanisms of action and our superoxide-producing nanoparticles in more than 75% of combinations. As a result of this potentiation, the effective antibiotic concentration of the clinical isolates was reduced up to 1000-fold below their respective sensitive/resistant breakpoint. Further, superoxide-generating nanoparticles in combination with ciprofloxacin reduced bacterial load in epithelial cells infected with S. enterica serovar Typhimurium and increased Caenorhabditis elegans survival upon infection with S. enterica serovar Enteriditis, compared to antibiotic alone. This demonstration highlights the ability to engineer superoxide generation to potentiate antibiotic activity and combat highly drug-resistant bacterial pathogens.

Lethality of MalE-LacZ hybrid protein shares mechanistic attributes with oxidative component of antibiotic lethality

Downstream metabolic events can contribute to the lethality of drugs or agents that interact with a primary cellular target. In bacteria, the production of reactive oxygen species (ROS) has been associated with the lethal effects of a variety of stresses including bactericidal antibiotics, but the relative contribution of this oxidative component to cell death depends on a variety of factors. Experimental evidence has suggested that unresolvable DNA problems caused by incorporation of oxidized nucleotides into nascent DNA followed by incomplete base excision repair contribute to the ROS-dependent component of antibiotic lethality. Expression of the chimeric periplasmic-cytoplasmic MalE-LacZ_72-47 protein is an historically important lethal stress originally identified during seminal genetic experiments that defined the SecY-dependent protein translocation system. Multiple, independent lines of evidence presented here indicate that the predominant mechanism for MalE-LacZ lethality shares attributes with the ROS-dependent component of antibiotic lethality. MalE-LacZ lethality requires molecular oxygen, and its expression induces ROS production. The increased susceptibility of mutants sensitive to oxidative stress to MalE-LacZ lethality indicates that ROS contribute causally to cell death rather than simply being produced by dying cells. Observations that support the proposed mechanism of cell death include MalE-LacZ expression being bacteriostatic rather than bactericidal in cells that overexpress MutT, a nucleotide sanitizer that hydrolyzes 8-oxo-dGTP to the monophosphate, or that lack MutM and MutY, DNA glycosylases that process base pairs involving 8-oxo-dGTP. Our studies suggest stress-induced physiological changes that favor this mode of ROS-dependent death.

The Role of Reactive Oxygen Species in Antibiotic-Mediated Killing of Bacteria

Recently, it was proposed that there is a common mechanism behind the activity of bactericidal antibiotics, involving the production of reactive oxygen species (ROS). However, the involvement of ROS in antibiotic-mediated killing has become the subject of much debate. In the present review, we provide an overview of the data supporting the ROS hypothesis; we also present data that explain the contradictory results often obtained when studying antibiotic-induced ROS production. For this latter aspect we will focus on the importance of taking the experimental setup into consideration and on the importance of some technical aspects of the assays typically used. Finally, we discuss the link between ROS production and toxin-antitoxin modules, and present an overview of implications for treatment.

Plasmid-mediated quinolone resistance in Enterobacteriaceae: a systematic review with a focus on Mediterranean countries

Quinolones are a family of synthetic broad-spectrum antimicrobial drugs. These molecules have been widely prescribed to treat various infectious diseases and have been classified into several generations based on their spectrum of activity. Quinolones inhibit bacterial DNA synthesis by interfering with the action of DNA gyrase and topoisomerase IV. Mutations in the genes encoding these targets are the most common mechanisms of high-level fluoroquinolone resistance. Moreover, three mechanisms for plasmid-mediated quinolone resistance (PMQR) have been discovered since 1998 and include Qnr proteins, the aminoglycoside acetyltransferase AAC(6')-Ib-cr, and plasmid-mediated efflux pumps QepA and OqxAB. Plasmids with these mechanisms often encode additional antimicrobial resistance (extended spectrum beta-lactamases [ESBLs] and plasmidic AmpC [pAmpC] ß-lactamases) and can transfer multidrug resistance. The PMQR determinants are disseminated in Mediterranean countries with prevalence relatively high depending on the sources and the regions, highlighting the necessity of long-term surveillance for the future monitoring of trends in the occurrence of PMQR genes.

iTRAQ-Based Proteomics Revealed the Bactericidal Mechanism of Sodium New Houttuyfonate against Streptococcus pneumoniae

Sodium new houttuyfonate (SNH), an addition product of active ingredient houttuynin from the plant Houttuynia cordata Thunb., inhibits a variety of bacteria, yet the mechanism by which it induces cell death has not been fully understood. In the present study, we utilized iTRAQ-based quantitative proteomics to analyze the protein alterations in Streptococcus pneumoniae in response to SNH treatment. Numerous proteins related to the production of reactive oxygen species (ROS) were found to be up-regulated by SNH, suggesting that ROS pathways may be involved as analyzed via bioinformatics. As reported recently, cellular reactions stimulated by ROS including superoxide anion (O2(•-)), hydrogen peroxide (H2O2), and hydroxyl radicals (OH(•)) have been implicated as mechanisms whereby bactericidal antibiotics kill bacteria. We then validated that SNH killed S. pneumoniae in a dose-dependent manner accompanied by the increasing level of H2O2. On the other hand, the addition of catalase, which can neutralize H2O2 in cells, showed a significant recovery in bacterial survival. These results indicate that SNH indeed induced H2O2 formation to contribute to the cell lethality, providing new insights into the bactericidal mechanism of SNH and expanding our understanding of the common mechanism of killing induced by bactericidal agents.