Efflux pumps and nosocomial antibiotic resistance: a primer for hospital epidemiologists

Antibiotic Resistance in Bacteria—A Review

Background: A global problem of multi-drug resistance (MDR) among bacteria is the cause of hundreds of thousands of deaths every year. In response to the significant increase of MDR bacteria, legislative measures have widely been taken to limit or eliminate the use of antibiotics, including in the form of feed additives for livestock, but also in metaphylaxis and its treatment, which was the subject of EU Regulation in 2019/6. Numerous studies have documented that bacteria use both phenotypis and gentic strategies enabling a natural defence against antibiotics and the induction of mechanisms in increasing resistance to the used antibacterial chemicals. The mechanisms presented in this review developed by the bacteria have a significant impact on reducing the ability to combat bacterial infections in humans and animals. Moreover, the high prevalence of multi-resistant strains in the environment and the ease of transmission of drug-resistance genes between the different bacterial species including commensal flora and pathogenic like foodborne pathogens (E. coli, Campylobacter spp., Enterococcus spp., Salmonella spp., Listeria spp., Staphylococcus spp.) favor the rapid spread of multi-resistance among bacteria in humans and animals. Given the global threat posed by the widespread phenomenon of multi-drug resistance among bacteria which are dangerous for humans and animals, the subject of this study is the presentation of the mechanisms of resistance in most frequent bacteria called as “foodborne pathoges” isolated from human and animals. In order to present the significance of the global problem related to multi-drug resistance among selected pathogens, especially those danger to humans, the publication also presents statistical data on the percentage range of occurrence of drug resistance among selected bacteria in various regions of the world. In addition to the phenotypic characteristics of pathogen resistance, this review also presents detailed information on the detection of drug resistance genes for specific groups of antibiotics. It should be emphasized that the manuscript also presents the results of own research i.e., Campylobacter spp., E. coli or Enetrococcus spp. This subject and the presentation of data on the risks of drug resistance among bacteria will contribute to initiating research in implementing the prevention of drug resistance and the development of alternatives for antimicrobials methods of controlling bacteria.

Detection of OqxAB Efflux Pumps, a Multidrug-Resistant Agent in Bacterial Infection in Patients Referring to Teaching Hospitals in Ahvaz, Southwest of Iran

Antibiotic resistance mechanisms in Enterobacteriaceae are causative agents of global health problems. Bacterial infections due to multidrug resistance (MDR) may be mediated by the overexpression of efflux pumps. In this study, we investigated the prevalence of oqxA and oqxB genes as two encoding agents of efflux pumps and the determination of antibiotic resistance rate in clinical isolates of Enterobacteriaceae. In this study, 100 Enterobacteriaceae isolates collected from different clinical specimens of infectious patients, such as wounds, urine, blood, discharge, and abscesses except stool, were examined. Identification of the isolates was performed using standard biochemical tests such as TSI, citrate, urea, lysine, SIM, MR-VP, and gas production. The antimicrobial susceptibility test was carried out by the Kirby-Bauer disk diffusion method according to CLSI guidelines, and finally, the oqxA and oqxB genes were detected by the PCR method. Among 100 Enterobacteriaceae isolates, Escherichia coli and Enterobacter gergoviae were the most common isolates with 71% and 20%, respectively. Also, the lowest isolates belonged to Enterobacter cloacae (3%) and Klebsiella pneumoniae (1%). Out of 100 Enterobacteriaceae isolates, 37 isolates (37%) were positive for at least one of oqxA or oqxB genes, while both of these genes were detected among 12% of them. oqxAB genes were detected in 8 cases of 20 (40%) Enterobacter gergoviae and 4 cases of 71 (5.7%) E. coli isolates. The antimicrobial susceptibility test showed that all isolates (100%) were susceptible to imipenem, while the maximum resistance to piperacillin, ceftriaxone, and cefotaxime were 69%, 55%, and 55%, respectively. Also, the results of this study showed that antibiotic resistance in Enterobacteriaceae isolates caused by oqxAB genes is increasing among patients in Iran. Therefore, identification of resistant isolates and antibiotic monitoring programs are essential to prevent the spread of MDR isolates.

Jatrorrhizine: a review of its pharmacological effects

OBJECTIVES

Jatrorrhizine is an isoquinoline alkaloid found in medicinal plants. It is the main bioactive compound of the Chinese herbs, Coptis chinensis, Rhizoma coptidis, and Phellodendron chinense Schneid, plants that are predominantly used in traditional Chinese medicine (TCM) for the treatment of metabolic disorders, gastritis, stomachache among a host of others. This manuscript aims to provide a comprehensive review of the pharmacological effects of jatrorrhizine, proffer suggestions on research areas that need redress and potentially serve as a reference for future studies.

KEY FINDINGS

Published scientific literature was therefore retrieved from all credible sources including Pubmed, Elsevier, Research Gate, Web of Science, Google Scholar, Science Direct, Europe PMC and Wiley Online library using key words such as 'jatrorrhizine', 'botanical sources', 'pharmacology', 'toxicology', 'pharmacokinetics' or their combinations. A cursory examination of relevant scientific literature using the aforementioned key words produced more than 400 publications.

CONCLUSIONS

Using an inclusion/exclusion criteria the subject matter of this review was adequately addressed. It is our hope that this review will provide a good platform for further research on fully harnessing the potential of this bioactive compound.

Overexpression of Efflux Pumps Mediate Pan Resistance of Klebsiella pneumoniae Sequence Type 11

A clinically isolated pan-resistant Klebsiella pneumoniae strain (ST11), KPN142 was subjected to whole-genome sequencing. Genomic sequence of KPN142 showed that limited antibiotic resistances (β-lactams [bla_shv-11], sulfonamides [sul1 and dfrA22], bacitracin [bacA], tetracycline [tet34], aminoglycosides [ksgA, kdpE, aph(3)Ia, aac(3)III, and ant(3)Ia], and chloramphenicol [catA1]) were mediated by enzymes, and efflux pumps contributed most to pan resistance. Five types of multidrug resistance efflux pump families were identified, including the resistance nodulation division superfamily (AcrAB-TolC, AcrD, MdtABC, and KexD), the ATP-binding cascade superfamily (MacAB), the small multidrug resistance family (KpnEF), the multidrug and toxic compound extrusion family (KdeA), and the major facilitator superfamily (EmrAB). There was an AcrAB-TolC efflux pump system, and inhibitory regulatory gene acrR and ramR of system carried deletion mutation, which lead to overexpression of AcrAB-TolC efflux pump, and in turn plays key role in the pan resistance of KPN142. Moreover, we did not find mgrb, a suppressor in the expression of phoPQ, overexpression of which may confer the resistance of KPN142 to colistin B. In addition, K. pneumoniae KPN142 carries IS1, IS3, and IntI1, which means that KPN142 is able to transfer drug-resistance genes. Of note, we detected the overexpression of acrB, ramA, phoP, and phoQ by real-time quantitative reverse transcription-polymerase chain reaction, and carbonyl cyanide chlorophenylhydrazone was able to reverse the resistance patterns of K. pneumoniae KPN142. In conclusion, we consider that the overexpression of AcrAB-TolC efflux pump mediates the resistance to most common clinical antimicrobial agents, and the overexpression of phoPQ mediates the resistance to colistin B in K. pneumoniae KPN142.

Research Progress on Antibacterial Activities and Mechanisms of Natural Alkaloids: A Review

Alkaloids are nitrogen-containing heterocyclic compounds typically isolated from plants. They represent one of the most important types of natural products because of their large number and structural diversity and complexity. Based on their chemical core structures, alkaloids are classified as isoquinolines, quinolines, indoles, piperidine alkaloids, etc. In-depth analyses of alkaloids have revealed their antibacterial activities. To date, due to the widespread use of antibiotics, the problem of drug-resistant bacterial infections has been gradually increasing, which severely affects the clinical efficacy of antibacterial therapies and patient safety. Therefore, significant research efforts are focused on alkaloids because they represent a potentially new type of natural antibiotic with a wide antibacterial spectrum, rare adverse reactions, and a low tendency to produce drug resistance. Their main antibacterial mechanisms include inhibition of bacterial cell wall synthesis, change in cell membrane permeability, inhibition of bacterial metabolism, and inhibition of nucleic acid and protein synthesis. This article reviews recent reports about the chemical structures and the antibacterial activities and mechanisms of alkaloids. The purpose is to solve the problem of bacterial resistance and to provide a certain theoretical basis and research ideas for the development of new antibacterial drugs.

Jatrorrhizine suppresses the antimicrobial resistance of methicillin-resistant Staphylococcus aureus

Bacterial resistance to antimicrobial agents, including multidrug resistance, is an increasing problem in the treatment of infectious diseases. The development of resistance-modifying agents represents a potential strategy to alleviate the spread of bacterial resistance to antibiotics. A checkerboard microdilution assay was used to determine the synergy of jatrorrhizine and the antibiotic, norfloxacin (NFX). A bacterial ethidium bromide efflux assay, reverse transcription semi-quantitative polymerase chain reaction analysis and molecular docking study were performed. The three-dimensional structure of NorA multidrug efflux pump (NorA) was generated using a multiple threading approach. A murine thigh infection model was used to evaluate the in vivo synergistic effect. As a natural product, jatrorrhizine exhibited little antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) SA1199B with a minimum inhibitory concentration (MIC) of 64 mg/l. According to the investigations of the mechanism, jatrorrhizine significantly inhibited bacterial drug efflux and the expression of NorA in the mRNA level as it can bind to NorA by hydrogen-bonds, hydrophobic and electrostatic interactions. The in vivo synergistical bactericidal activity of jatrorrhizine and NFX against MRSA was confirmed in a murine thigh infection model. As a novel resistance-modifying agent, jatrorrhizine exhibited in vitro and in vivo synergistic activities against MRSA, and inhibited bacterial drug efflux. The effects were mediated by the suppression of NorA mRNA expression and/or interactions with NorA efflux pump. These data support the hypothesis that jatrorrhizine is a potential agent for therapeutic use in infections caused by MRSA.

Terminalia chebula Retz. Fruit Extracts Inhibit Bacterial Triggers of Some Autoimmune Diseases and Potentiate the Activity of Tetracycline

Terminalia chebula Retz. is a northern Indian plant species known for its anti-inflammatory and antimicrobial properties. T. chebula fruit powder was extracted with solvents of varying polarity and screened for bacterial growth inhibition by disc diffusion assay. The minimum inhibitory concentration (MIC) was quantified by both liquid dilution and disc diffusion techniques. To screen for combinatorial effects, the T. chebula fruit extracts were combined with a range of conventional antibiotics and tested against each bacteria using a liquid dilution assay. Where synergy was detected, the optimal ratios were determined using isobologram analysis. Toxicity was examined using Artemia nauplii and HDF bioassays. T. chebula fruit methanolic, aqueous and ethyl acetate extracts displayed strong antimicrobial activity against the bacterial triggers of all autoimmune inflammatory diseases except K. pneumoniae, for which only moderate inhibition was observed. Indeed, MIC values as low as 195 μg/mL were measured for the aqueous extract against a resistant strain of P. aeruginosa. Of further note, both the aqueous and ethyl acetate extracts interacted synergistically in combination with tetracycline against K. pneumoniae (Σ FIC 0.38 and 0.25 respectively). All extracts were nontoxic in the Artemia and HDF toxicity assays, further indicating their potential for medicinal use.

Identification of a Novel Polyamine Scaffold With Potent Efflux Pump Inhibition Activity Toward Multi-Drug Resistant Bacterial Pathogens

We have previously reported the use of combinatorial chemistry to identify broad-spectrum antibacterial agents. Herein, we extend our analysis of this technology toward the discovery of anti-resistance molecules, focusing on efflux pump inhibitors. Using high-throughput screening against multi-drug resistant Pseudomonas aeruginosa, we identified a polyamine scaffold that demonstrated strong efflux pump inhibition without possessing antibacterial effects. We determined that these molecules were most effective with an amine functionality at R1 and benzene functionalities at R2 and R3. From a library of 188 compounds, we studied the properties of 5 lead agents in detail, observing a fivefold to eightfold decrease in the 90% effective concentration of tetracycline, chloramphenicol, and aztreonam toward P. aeruginosa isolates. Additionally, we determined that our molecules were not only active toward P. aeruginosa, but toward Acinetobacter baumannii and Staphylococcus aureus as well. The specificity of our molecules to efflux pump inhibition was confirmed using ethidium bromide accumulation assays, and in studies with strains that displayed varying abilities in their efflux potential. When assessing off target effects we observed no disruption of bacterial membrane polarity, no general toxicity toward mammalian cells, and no inhibition of calcium channel activity in human kidney cells. Finally, combination treatment with our lead agents engendered a marked increase in the bactericidal capacity of tetracycline, and significantly decreased viability within P. aeruginosa biofilms. As such, we report a unique polyamine scaffold that has strong potential for the future development of novel and broadly active efflux pump inhibitors targeting multi-drug resistant bacterial infections.

Boeravinone B, A Novel Dual Inhibitor of NorA Bacterial Efflux Pump of Staphylococcus aureus and Human P-Glycoprotein, Reduces the Biofilm Formation and Intracellular Invasion of Bacteria

This study elucidated the role of boeravinone B, a NorA multidrug efflux pump inhibitor, in biofilm inhibition. The effects of boeravinone B plus ciprofloxacin, a NorA substrate, were evaluated in NorA-overexpressing, wild-type, and knocked-out Staphylococcus aureus (SA-1199B, SA-1199, and SA-K1758, respectively). The mechanism of action was confirmed using the ethidium bromide accumulation and efflux assay. The role of boeravinone B as a human P-glycoprotein (P-gp) inhibitor was examined in the LS-180 (colon cancer) cell line. Moreover, its role in the inhibition of biofilm formation and intracellular invasion of S. aureus in macrophages was studied. Boeravinone B reduced the minimum inhibitory concentration (MIC) of ciprofloxacin against S. aureus and its methicillin-resistant strains; the effect was stronger in SA-1199B. Furthermore, time–kill kinetics revealed that boeravinone B plus ciprofloxacin, at subinhibitory concentration (0.25 × MIC), is as equipotent as that at the MIC level. This combination also had a reduced mutation prevention concentration. Boeravinone B reduced the efflux of ethidium bromide and increased the accumulation, thus strengthening the role as a NorA inhibitor. Biofilm formation was reduced by four–eightfold of the minimal biofilm inhibitory concentration of ciprofloxacin, effectively preventing bacterial entry into macrophages. Boeravinone B effectively inhibited P-gp with half maximal inhibitory concentration (IC₅₀) of 64.85 μM. The study concluded that boeravinone B not only inhibits the NorA-mediated efflux of fluoroquinolones but also considerably inhibits the biofilm formation of S. aureus. Its P-gp inhibition activity demonstrates its potential as a bioavailability and bioefficacy enhancer.

Computational Study Reveals the Molecular Mechanism of the Interaction between the Efflux Inhibitor PAβN and the AdeB Transporter from Acinetobacter baumannii

Phenylalanine-arginine β-naphthylamide (PAβN) is a broad-spectrum efflux pump inhibitor that has shown to potentiate the activity of antibiotics in Gram-negative bacteria. AdeB is a part of the AdeABC tripartite pump that plays a pivotal role in conferring efflux-mediated resistance in Acinetobacter baumannii. To understand the molecular mechanism of efflux pump inhibition by PAβN, we investigated the interaction of PAβN with AdeB using different computational methods. We observed that PAβN does not have specific binding interactions with the proximal binding site and interacts strongly with the distal binding pocket. The Phe loop located between the proximal and distal binding pockets plays a key role in the PAβN-mediated inhibition and acts as a gate between the binding pockets. Molecular dynamics simulations suggested that PAβN behaved like a climber as we observed switching of the interaction energies between the ligand and the key Phe residues of the binding site during the course of the simulation. PAβN uses the hydrophobic microenvironment formed by Phe residues in the distal binding pocket to keep the binding monomer in the binding conformation. The simulation data suggests that this binding event should result in the inhibition of the peristaltic mechanism and prevent the exporter from extruding any other substrates leading to the inhibition of the tripartite pump.

Contribution of EmrAB efflux pumps to colistin resistance in Acinetobacter baumannii

Efflux pumps play an important role in antimicrobial resistance for Acinetobacter baumannii. However, the function of the Emr pump system and the relationship between Emr and drug resistance has not been characterized in A. baumannii. In this study, four possible groups of emr-like genes were found by searching a genome database. Among them, A1S_1772 (emrB) and A1S_1773 (emrA) were demonstrated to be co-transcribed as a single operon. Moreover, during osmotic stress, A1S_1772 showed the largest change in gene expression compared to the other emrB-like genes, and deletion of A1S_1772 (AB ΔemrB) significantly slowed cell growth in 20% sucrose. Using a phenotypic microarray analysis, the AB ΔemrB mutant was more susceptible to colistin and nafcillin, paromomycin, spiramycin, and D,L-serine hydroxmate than the wild type. The spot assay, time kill assay and minimal inhibition concentration determination also indicated that the wild type could tolerate colistin better than the AB ΔemrB mutant. Finally, the increased expression levels of all emrB-like genes, including A1S_0775, A1S_0909, A1S_1772, and A1S_1799, in colistin resistance-induced A. baumannii further supported the possible involvement of the emrB genes in A. baumannii colistin resistance. Together, the Emr pump systems in A. baumannii contribute to adaptation to osmotic stress and resistance to colistin.

Active efflux in dormant bacterial cells - New insights into antibiotic persistence

Bacterial persisters are phenotypic variants of an isogenic cell population that can survive antibiotic treatment and resume growth after the antibiotics have been removed. Cell dormancy has long been considered the principle mechanism underlying persister formation. However, dormancy alone is insufficient to explain the full range of bacterial persistence. Our recent work revealed that in addition to 'passive defense' via dormancy, persister cells employ 'active defense' via enhanced efflux activity to expel drugs. This finding suggests that persisters combine two seemingly contradictory mechanisms to tolerate antibiotic attack. Here, we review the passive and active aspects of persister formation, discuss new insights into the process, and propose new techniques that can facilitate the study of bacterial persistence.

Genomic characterization and assessment of the virulence and antibiotic resistance of the novel species Paenibacillus sp. strain VT-400, a potentially pathogenic bacterium in the oral cavity of patients with hematological malignancies

BACKGROUND

Paenibacillus sp. strain VT-400, a novel spore-forming bacterium, was isolated from patients with hematological malignancies.

METHODS

Paenibacillus sp. strain VT-400 was isolated from the saliva of four children with acute lymphoblastic leukemia. The genome was annotated using RAST and the NCBI Prokaryotic Genome Annotation Pipeline to characterize features of antibiotic resistance and virulence factors. Susceptibility to antibiotics was determined by the Kirby-Bauer disc diffusion method. We used a mouse model of pneumonia to study virulence in vivo. Mice were challenged with 7.5 log10-9.5 log10 CFU, and survival was monitored over 7 days. Bacterial load was measured in the lungs and spleen of surviving mice 48 h post-infection to reveal bacterial invasion and dissemination.

RESULTS

Whole-genome sequencing revealed a large number of virulence factors such as hemolysin D and CD4+ T cell-stimulating antigen. Furthermore, the strain harbors numerous antibiotic resistance genes, including small multidrug resistance proteins, which have never been previously found in the Paenibacillus genus. We then compared the presence of antibiotic resistance genes against results from antibiotic susceptibility testing. Paenibacillus sp. strain VT-400 was found to be resistant to macrolides such as erythromycin and azithromycin, as well as to chloramphenicol and trimethoprim-sulphamethoxazole. Finally, the isolate caused mortality in mice infected with ≥8.5 log10 CFU.

CONCLUSIONS

Based on our results and on the available literature, there is yet no strong evidence that shows Paenibacillus species as an opportunistic pathogen in immunocompromised patients. However, the presence of spore-forming bacteria with virulence and antibiotic resistance genes in such patients warrants special attention because infections caused by spore-forming bacteria are poorly treatable.

The Role of the Two-Component System BaeSR in Disposing Chemicals through Regulating Transporter Systems in Acinetobacter baumannii

Bacterial two-component regulatory systems (TCSs) facilitate changes in gene expression in response to environmental stimuli. TCS BaeR regulons influence tigecycline susceptibility in Acinetobacter baumannii through positively regulating the pump genes adeA and adeB. In this study, we demonstrate that an additional two transport systems, AdeIJK and MacAB-TolC, are also regulated by BaeSR. In the wild type and clinical tigecycline-resistant A. baumannii strains, gene expression of AdeIJK and MacAB-TolC increased after tigecycline induction, implicating their importance to tigecycline resistance in addition to AdeABC. Phenotypic microarray results showed that A. baumannii is vulnerable to certain chemicals, especially tannic acid, after deleting baeR, which was confirmed using the spot assay. The wild-type strain of A. baumannii also exhibited 1.6-fold and 4.4-fold increase in gene expression of adeJ and macB in the medium with 100 μg/mL tannic acid, but the increase was fully inhibited by baeR deletion. An electrophoretic motility shift assay based on an interaction between His-BaeR and the adeA, adeI and macA promoter regions did not demonstrate direct binding. In conclusion, A. baumannii can use the TCS BaeSR in disposing chemicals, such as tannic acid and tigecycline, through regulating the efflux pumps.

The impact of triclosan on the spread of antibiotic resistance in the environment

Triclosan (TCS) is a commonly used antimicrobial agent that enters wastewater treatment plants (WWTPs) and the environment. An estimated 1.1 × 10(5) to 4.2 × 10(5) kg of TCS are discharged from these WWTPs per year in the United States. The abundance of TCS along with its antimicrobial properties have given rise to concern regarding its impact on antibiotic resistance in the environment. The objective of this review is to assess the state of knowledge regarding the impact of TCS on multidrug resistance in environmental settings, including engineered environments such as anaerobic digesters. Pure culture studies are reviewed in this paper to gain insight into the substantially smaller body of research surrounding the impacts of TCS on environmental microbial communities. Pure culture studies, mainly on pathogenic strains of bacteria, demonstrate that TCS is often associated with multidrug resistance. Research is lacking to quantify the current impacts of TCS discharge to the environment, but it is known that resistance to TCS and multidrug resistance can increase in environmental microbial communities exposed to TCS. Research plans are proposed to quantitatively define the conditions under which TCS selects for multidrug resistance in the environment.

Antimicrobial resistance in Acinetobacter baumannii: From bench to bedside

Acinetobacter baumannii (A. baumannii) is undoubtedly one of the most successful pathogens in the modern healthcare system. With invasive procedures, antibiotic use and immunocompromised hosts increasing in recent years, A. baumannii has become endemic in hospitals due to its versatile genetic machinery, which allows it to quickly evolve resistance factors, and to its remarkable ability to tolerate harsh environments. Infections and outbreaks caused by multidrug-resistant A. baumannii (MDRAB) are prevalent and have been reported worldwide over the past twenty or more years. To address this problem effectively, knowledge of species identification, typing methods, clinical manifestations, risk factors, and virulence factors is essential. The global epidemiology of MDRAB is monitored by persistent surveillance programs. Because few effective antibiotics are available, clinicians often face serious challenges when treating patients with MDRAB. Therefore, a deep understanding of the resistance mechanisms used by MDRAB can shed light on two possible strategies to combat the dissemination of antimicrobial resistance: stringent infection control and antibiotic treatments, of which colistin-based combination therapy is the mainstream strategy. However, due to the current unsatisfying therapeutic outcomes, there is a great need to develop and evaluate the efficacy of new antibiotics and to understand the role of other potential alternatives, such as antimicrobial peptides, in the treatment of MDRAB infections.

Efflux pump regulatory genes mutations in multidrug resistance Pseudomonas aeruginosa isolated from wound infections in Isfahan hospitals

BACKGROUND

Multidrug resistance Pseudomonas aeruginosa (MDR-P. aeruginosa) is a worldwide threat for public health. Hyperexpression of efflux pump systems (MexAB-OprM and MexCD-OprJ), which is a well-known mechanisms for MDR emerging, is controlled by regulatory genes, mexR and nfxB, respectively. The aim of this study was to evaluate point mutations in mexR and nfxB genes in MDR- P. aeruginosa isolated from wound infections.

MATERIALS AND METHODS

A total of 34 P. aeruginosa cultures obtained from wound infections were analyzed. Among them eight isolates identified as MDR-P. aeruginosa and were subjected to determination of mutations in mexR and nfxB genes.

RESULTS

We detected eight-point mutations in mexR and 12-point mutations in nfxB. The most common mutations were common G327-A (eight isolates), G384-A (eight isolates), G411-A (eight isolates). Mutations in A371-C and A372-C were the predominant substitution which was seen in nfxB. Amino acid substitutions were also found at position 124 and 126 for NfxB and MexR, respectively.

CONCLUSIONS

P. aeruginosa isolates with mutation in efflux pump regulatory genes such as mexR and nfxB could be a main factor contributed to antibiotic resistance and must be considered in antibiotic treatment.

Mechanisms of resistance to β-lactams in some common Gram-negative bacteria causing nosocomial infections

Gram-negative bacilli remain major killers of hospitalized patients and continue to evolve new resistance mechanisms. This review describes the mechanisms of resistance to beta-lactam antibiotics from those Gram-negative pathogens most often isolated from nosocomial infections.

The ABC-type efflux pump MacAB protects Salmonella enterica serovar typhimurium from oxidative stress

Multidrug efflux pumps are integral membrane proteins known to actively excrete antibiotics. The macrolide-specific pump MacAB, the only ABC-type drug efflux pump in Salmonella, has previously been linked to virulence in mice. The molecular mechanism of this link between macAB and infection is unclear. We demonstrate that macAB plays a role in the detoxification of reactive oxygen species (ROS), compounds that salmonellae are exposed to at various stages of infection. macAB is induced upon exposure to H₂O₂ and is critical for survival of Salmonella enterica serovar Typhimurium in the presence of peroxide. Furthermore, we determined that macAB is required for intracellular replication inside J774.A1 murine macrophages but is not required for survival in ROS-deficient J774.D9 macrophages. macAB mutants also had reduced survival in the intestine in the mouse colitis model, a model characterized by a strong neutrophilic intestinal infiltrate where bacteria may experience the cytotoxic actions of ROS. Using an Amplex red-coupled assay, macAB mutants appear to be unable to induce protection against exogenous H₂O₂in vitro, in contrast to the isogenic wild type. In mixed cultures, the presence of the wild-type organism, or media preconditioned by the growth of the wild-type organism, was sufficient to rescue the macAB mutant from peroxide-mediated killing. Our data indicate that the MacAB drug efflux pump has functions beyond resistance to antibiotics and plays a role in the protection of Salmonella against oxidative stress. Intriguingly, our data also suggest the presence of a soluble anti-H₂O₂ compound secreted by Salmonella cells through a MacAB-dependent mechanism.

The ABC-type multidrug efflux pump MacAB is known to be required for Salmonella enterica serovar Typhimurium virulence after oral infection in mice, yet the function of this pump during infection is unknown. We show that this pump is necessary for colonization of niches in infected mice where salmonellae encounter oxidative stress during infection. MacAB is required for growth in cultured macrophages that produce reactive oxygen species (ROS) but is not needed in macrophages that do not generate ROS. In addition, we show that MacAB is required to resist peroxide-mediated killing in vitro and for the inactivation of peroxide in the media. Finally, wild-type organisms, or supernatant from wild-type organisms grown in the presence of peroxide, rescue the growth defect of macAB mutants in H₂O₂. MacAB appears to participate in the excretion of a compound that induces protection against ROS-mediated killing, revealing a new role for this multidrug efflux pump.

Tigecycline susceptibility in Klebsiella pneumoniae and Escherichia coli causing neonatal septicaemia (2007-10) and role of an efflux pump in tigecycline non-susceptibility

OBJECTIVES

To investigate the trend of tigecycline susceptibility and mechanisms behind tigecycline non-susceptibility in Klebsiella pneumoniae and Escherichia coli isolates causing neonatal septicaemia (2007-10).

METHODS

MICs of tigecycline for the isolates were determined. The isolates were evaluated for β-lactamases and carbapenemases. Molecular typing of the tigecycline-resistant isolates was performed. Expression of efflux pump genes (acrA, acrB and tolC) and regulators (soxS and ramA) was examined by real-time RT-PCR and western blotting. Sequencing of the ramA and ramR genes was carried out to identify mutations within these genes.

RESULTS

Tigecycline susceptibility was evaluated in all K. pneumoniae (n = 57) and E. coli (n = 19) blood isolates. The prevalence of extended-spectrum β-lactamase (ESBL)-producing organisms was high, but tigecycline non-susceptibility remained low in these isolates. Though MIC values of tigecycline remained in the susceptible range, there was a 2-fold increase in the value of MIC90 from 2007 to 2010. Over the 4 year period K. pneumoniae showed higher MIC values of tigecycline in comparison with E. coli. Tigecycline non-susceptibility was not observed among carbapenem-resistant isolates. Only two ESBL-producing clonally distinct K. pneumoniae isolates showed tigecycline resistance with overexpression of ramA and the AcrAB-TolC pump. No mutations were present within the ramA and ramR genes that might enhance the expression of the pump.

CONCLUSIONS

The study showed for the first time the trend of tigecycline susceptibility in E. coli and K. pneumoniae causing neonatal septicaemia. Tigecycline still has potent antimicrobial effects against most ESBL- or carbapenemase-producing K. pneumoniae and E. coli, but the increasing MIC values make it essential to be vigilant.

Global fluoroquinolone resistance epidemiology and implictions for clinical use

This paper on the fluoroquinolone resistance epidemiology stratifies the data according to the different prescription patterns by either primary or tertiary caregivers and by indication. Global surveillance studies demonstrate that fluoroquinolone resistance rates increased in the past years in almost all bacterial species except S. pneumoniae and H. influenzae, causing community-acquired respiratory tract infections. However, 10 to 30% of these isolates harbored first-step mutations conferring low level fluoroquinolone resistance. Fluoroquinolone resistance increased in Enterobacteriaceae causing community acquired or healthcare associated urinary tract infections and intraabdominal infections, exceeding 50% in some parts of the world, particularly in Asia. One to two-thirds of Enterobacteriaceae producing extended spectrum β-lactamases were fluoroquinolone resistant too. Furthermore, fluoroquinolones select for methicillin resistance in Staphylococci. Neisseria gonorrhoeae acquired fluoroquinolone resistance rapidly; actual resistance rates are highly variable and can be as high as almost 100%, particularly in Asia, whereas resistance rates in Europe and North America range from <10% in rural areas to >30% in established sexual networks. In general, the continued increase in fluoroquinolone resistance affects patient management and necessitates changes in some guidelines, for example, treatment of urinary tract, intra-abdominal, skin and skin structure infections, and traveller's diarrhea, or even precludes the use in indications like sexually transmitted diseases and enteric fever.

Understanding efflux in Gram-negative bacteria: opportunities for drug discovery

INTRODUCTION

Bacteria evolved an arsenal of mechanisms to deal with toxic compounds and metabolic stresses, including antimicrobial agents. Efflux pumps are major players in the multidrug resistance of Gram-negative bacteria and pose major hurdles in the drug discovery process. However, recent advances in our understanding of efflux in these bacteria provide opportunities and assets for drug discovery.

AREAS COVERED

This review provides an overview of drug efflux in Gram-negative bacteria and its role in antimicrobial resistance, stress responses and other biological processes such as biofilm formation, and virulence. The discussion includes comments on the significance of synergy between a low-permeability outer membrane and efflux, notably the role of porins and lipopolysaccharide. The author then summarizes efforts aimed at inhibiting efflux pumps as a means to extend the utility of clinically useful antibiotics. This includes highlights of identification and characterization of small molecule efflux pump inhibitors (EPIs) from natural and synthetic sources, as well as novel strategies such as gene silencing and inhibitory antibodies.

EXPERT OPINION

Options for treating infections caused by multidrug-resistant bacteria are limited. Given the attractiveness of the therapeutic potential of efflux pump inhibition, further studies exploring novel strategies to interfere with efflux pump expression and function are warranted. This includes rational EPI design facilitated by pump structure information, exploitation of genetically defined efflux-proficient and efflux-compromised strain panels and non-traditional approaches such as pump inhibition by gene silencing, antibodies and perhaps even phage.

Coping with antibiotic resistance: combining nanoparticles with antibiotics and other antimicrobial agents

The worldwide escalation of bacterial resistance to conventional medical antibiotics is a serious concern for modern medicine. High prevalence of multidrug-resistant bacteria among bacteria-based infections decreases effectiveness of current treatments and causes thousands of deaths. New improvements in present methods and novel strategies are urgently needed to cope with this problem. Owing to their antibacterial activities, metallic nanoparticles represent an effective solution for overcoming bacterial resistance. However, metallic nanoparticles are toxic, which causes restrictions in their use. Recent studies have shown that combining nanoparticles with antibiotics not only reduces the toxicity of both agents towards human cells by decreasing the requirement for high dosages but also enhances their bactericidal properties. Combining antibiotics with nanoparticles also restores their ability to destroy bacteria that have acquired resistance to them. Furthermore, nanoparticles tagged with antibiotics have been shown to increase the concentration of antibiotics at the site of bacterium-antibiotic interaction, and to facilitate binding of antibiotics to bacteria. Likewise, combining nanoparticles with antimicrobial peptides and essential oils generates genuine synergy against bacterial resistance. In this article, we aim to summarize recent studies on interactions between nanoparticles and antibiotics, as well as other antibacterial agents to formulate new prospects for future studies. Based on the promising data that demonstrated the synergistic effects of antimicrobial agents with nanoparticles, we believe that this combination is a potential candidate for more research into treatments for antibiotic-resistant bacteria.

Antimicrobial resistance in the intensive care unit: mechanisms, epidemiology, and management of specific resistant pathogens

Infections caused by drug-resistant and multidrug-resistant microbial pathogens pose tremendous challenges to health care systems, including challenges related to the diagnosis, treatment, and containment of these infections. These challenges are amplified in the intensive care unit (ICU), where pressures for selection and emergence of resistance and risks of transmission of resistant pathogens are highest, and where the threat of resistance drives selection of empiric antimicrobial regimens. This article reviews basic concepts of resistance to antibacterial agents including mechanisms and modes of transmission, and discusses management issues for the important drug-resistant pathogens found in the ICU.

Distribution of 16S rRNA methylases among different species of Gram-negative bacilli with high-level resistance to aminoglycosides

16S rRNA methylases confer high-level resistance to most aminoglycosides in Gram-negative bacteria. Seven 16S rRNA methylase genes, armA, rmtA, rmtB, rmtC, rmtD, rmtE and npmA, have been identified since 2003. We studied the distribution of methylase genes in more than 200 aminoglycoside-resistant Gram-negative clinical isolates collected in 2007 at our hospital in Shanghai, China. 16S rRNA methylase genes were amplified by polymerase chain reaction (PCR) among 217 consecutive clinical isolates of Gram-negative bacilli resistant to gentamicin and amikacin by a disk diffusion method. 16S rRNA methylase genes were present in 97.5% (193/198) of clinical isolates highly resistant to amikacin (≥512 μg/ml), with armA and rmtB detected in 67.2 and 30.3% of strains, respectively, while no 16S rRNA methylase genes were detected in 19 strains with amikacin minimum inhibitory concentration (MIC) ≤256 μg/ml. armA or rmtB genes were detected in 100% of 104 strains of Enterobacteriaceae, and these two genes were equally represented (49 vs. 55 strains). Genes for armA or rmtB were detected in 94.7% (89/94) of Acinetobacter baumannii and Pseudomonas aeruginosa strains, and armA was predominant (84 vs. 5 strains with rmtB). No rmtA, rmtC, rmtD or npmA genes were found. Enterobacterial repetitive intergenic consensus sequence (ERIC-PCR) indicated that armA and rmtB genes were spread by both horizontal transfer and clonal dissemination.

A review of tigecycline

The first article in this supplement is an overall review of the first glycylcycline, tigecycline, which includes a brief overview of the problem of tetracycline resistance as well as tigecycline's mode of action, antibacterial activity, pharmacokinetics, pharmacodynamics, clinical efficacy, safety and tolerability. The remaining articles in the supplement report the European clinical experience from the pivotal clinical trials in complicated intra-abdominal infections, complicated skin and skin structure infections and community acquired pneumonia.

Synergistic antibacterial effect between silybin and N,N'-dicyclohexylcarbodiimide in clinical Pseudomonas aeruginosa isolates

Silybin is a composition of the silymarin group as a hepatoprotective agent, and it exhibits various biological activities, including an antibacterial activity. In this study, the effects of a combination of silybin with N,N'-dicyclohexylcarbodiimide (DCCD) against clinical isolates of Pseudomonas aeruginosa were investigated. In the results of susceptibility assay, silybin showed more potent antibacterial activity in methicillin-resistant Staphylococcus aureus (MRSA) than in P. aeruginosa, but DCCD significantly increased the antibacterial activity of silybin in P. aeruginosa. The antibacterial activity of silybin was affected by the strong action of multidrug-resistant pumps rather than by a permeable disruption of lipopolysaccharide and silybin showed a remarkable synergistic activity in combination with some antibiotic agents against drug-resistant bacteria. Therefore, silybin has a potential as a combination therapeutic agent for treatment of infectious diseases by multidrug-resistant bacteria.

Clinical implications of multiple drug resistance efflux pumps of pathogenic bacteria

Resistance of microorganisms to many classes of antibiotics and other drugs is a major problem throughout the world. This antimicrobial resistance can be mediated by various mechanisms such as enzymatic inactivation of the drug, alteration of the target and decreased intracellular concentration of the antimicrobial. The latter mechanism is mediated by either decreased influx or increased efflux or a combination of both. Recently, efflux has become increasingly recognized as a major component of resistance. Some efflux pumps selectively extrude specific antibiotics such as macrolides, lincosamides and/or streptogramins and tetracyclines, whereas others, referred to as multiple drug resistance pumps, expel a variety of structurally diverse anti-infectives with different modes of action. This phenomenon, whereby a single transporter is able to recognize and transport multiple antimicrobials with no common structural homology, was first described in the late 1980s in higher eukaryotes where P-glycoprotein was found to play a role in resistance to anti-cancer chemotherapeutic agents. Later, it became apparent that efflux systems were also present in microorganisms. Efflux pump inhibitors offer considerable promise as therapeutic agents, as they should restore the activity of standard antibiotics.

Fluoroquinolone resistance in Haemophilus influenzae is associated with hypermutability

Forty-three percent (12/28) of ciprofloxacin (CIP)-nonsusceptible respiratory isolates of Haemophilus influenzae were hypermutable, compared with 8.5% (3/35) in the CIP-susceptible control group (P=0.002). CIP-nonsusceptible mutants were obtained with hypermutable strains only; these mutants developed three resistance mechanisms in a step-by-step process: target modifications, loss of a porin protein, and increased efflux.

Antimicrobial resistance: mechanisms and strategies

Antimicrobial resistance is a growing threat worldwide. S. aureus strains that combine resistance and virulence genes have become a major treatment problem in Europe and the U.S. Extended-spectrum beta-lactamases are an emerging problem in many settings. Resistance mechanisms have been found for every class of antibiotic agent. Development of new classes of antibiotics or more robust versions of old classes will be essential in the years to come. Judicious use of antimicrobial agents and improved infection control methods must become health care priorities.

Dissemination of 16S rRNA methylase-mediated highly amikacin-resistant isolates of Klebsiella pneumoniae and Acinetobacter baumannii in Korea

Novel 16S rRNA methylase-mediated high-level resistance to amikacin and arbekacin has been reported recently in clinical isolates of Gram-negative bacilli only from several countries. We tested amikacin- or arbekacin-nonsusceptible Gram-negative bacilli isolated in 2003 and 2005 at a tertiary-care hospital in Korea by polymerase chain reaction to detect 16S rRNA methylase genes. armA alleles were detected in 14 isolates of Klebsiella pneumoniae, 10 other species of Enterobacteriaceae, and 16 Acinetobacter baumannii, whereas the rmtB allele was detected in 1 K. pneumoniae isolate. The resistance 1st detected in 2003 persisted in 2005. 16S rRNA methylase-producing isolates were highly resistant to arbekacin and amikacin, and were mostly coresistant to levofloxacin. Most K. pneumoniae isolates also produced extended-spectrum beta-lactamases and plasmid-mediated AmpC beta-lactamases, and most A. baumannii isolates were nonsusceptible to carbapenems.

Multidrug-resistance efflux pumps - not just for resistance

It is well established that multidrug-resistance efflux pumps encoded by bacteria can confer clinically relevant resistance to antibiotics. It is now understood that these efflux pumps also have a physiological role(s). They can confer resistance to natural substances produced by the host, including bile, hormones and host-defence molecules. In addition, some efflux pumps of the resistance nodulation division (RND) family have been shown to have a role in the colonization and the persistence of bacteria in the host. Here, I present the accumulating evidence that multidrug-resistance efflux pumps have roles in bacterial pathogenicity and propose that these pumps therefore have greater clinical relevance than is usually attributed to them.

Clinically relevant chromosomally encoded multidrug resistance efflux pumps in bacteria

Efflux pump genes and proteins are present in both antibiotic-susceptible and antibiotic-resistant bacteria. Pumps may be specific for one substrate or may transport a range of structurally dissimilar compounds (including antibiotics of multiple classes); such pumps can be associated with multiple drug (antibiotic) resistance (MDR). However, the clinical relevance of efflux-mediated resistance is species, drug, and infection dependent. This review focuses on chromosomally encoded pumps in bacteria that cause infections in humans. Recent structural data provide valuable insights into the mechanisms of drug transport. MDR efflux pumps contribute to antibiotic resistance in bacteria in several ways: (i) inherent resistance to an entire class of agents, (ii) inherent resistance to specific agents, and (iii) resistance conferred by overexpression of an efflux pump. Enhanced efflux can be mediated by mutations in (i) the local repressor gene, (ii) a global regulatory gene, (iii) the promoter region of the transporter gene, or (iv) insertion elements upstream of the transporter gene. Some data suggest that resistance nodulation division systems are important in pathogenicity and/or survival in a particular ecological niche. Inhibitors of various efflux pump systems have been described; typically these are plant alkaloids, but as yet no product has been marketed.

Efflux systems in bacterial pathogens: An opportunity for therapeutic intervention? An industry view

The efflux systems of bacteria protect cells from antibiotics and biocides by actively transporting compounds out of the cytoplasm and/or periplasm and thereby limit their steady-state accumulation at their site(s) of action. The impact of efflux systems on the efficacy of antibiotics used in human medicine and animal husbandry is becoming increasingly apparent from the characterization of drug-resistant strains with altered drug efflux properties. In most instances, efflux-mediated antibiotic resistance arises from mutational events that result in their elevated expression and, in the case of efflux pumps with broad substrate specificity, can confer multi-drug resistance (MDR) to structurally unrelated antibiotics. Knowledge of the role of efflux systems in conferring antibiotic resistance has now been successfully exploited in the pharmaceutical industry and contributed, in part, to the development of new members of the macrolide and tetracycline classes of antibiotics that circumvent the efflux-based resistance mechanisms that have limited the clinical utility of their progenitors. The therapeutic utility of compounds that inhibit bacterial drug efflux pumps and therein potentiate the activity of a co-administered antibiotic agent remains to be validated in the clinical setting, but the approach holds promise for the future in improving the efficacy and/or extending the clinical utility of existing antibiotics. This review discusses the potential of further exploiting the knowledge of efflux-mediated antibiotic resistance in bacteria toward the discovery and development of new chemotherapeutic agents.

Resistance to antibiotics: are we in the post-antibiotic era?

Serious infections caused by bacteria that have become resistant to commonly used antibiotics have become a major global healthcare problem in the 21st century. They not only are more severe and require longer and more complex treatments, but they are also significantly more expensive to diagnose and to treat. Antibiotic resistance, initially a problem of the hospital setting associated with an increased number of hospital-acquired infections usually in critically ill and immunosuppressed patients, has now extended into the community causing severe infections difficult to diagnose and treat. The molecular mechanisms by which bacteria have become resistant to antibiotics are diverse and complex. Bacteria have developed resistance to all different classes of antibiotics discovered to date. The most frequent type of resistance is acquired and transmitted horizontally via the conjugation of a plasmid. In recent times new mechanisms of resistance have resulted in the simultaneous development of resistance to several antibiotic classes creating very dangerous multidrug-resistant (MDR) bacterial strains, some also known as "superbugs". The indiscriminate and inappropriate use of antibiotics in outpatient clinics, hospitalized patients and in the food industry is the single largest factor leading to antibiotic resistance. In recent years, the number of new antibiotics licensed for human use in different parts of the world has been lower than in the recent past. In addition, there has been less innovation in the field of antimicrobial discovery research and development. The pharmaceutical industry, large academic institutions or the government are not investing the necessary resources to produce the next generation of newer safe and effective antimicrobial drugs. In many cases, large pharmaceutical companies have terminated their anti-infective research programs altogether due to economic reasons. The potential negative consequences of all these events are relevant because they put society at risk for the spread of potentially serious MDR bacterial infections.