Genetic evidence that InhA of Mycobacterium smegmatis is a target for triclosan

Label-Free Flow Cytometry: A Powerful Tool to Rapidly and Accurately Assess the Efficacy of Chemical Disinfectants

A rapid and accurate evaluation of a chemical disinfectant's bactericidal efficacy is crucial for ensuring effective infection control, preventing the spread of pathogens, and supporting the development of new disinfectant formulations. In this study, we report a rapid, label-free flow cytometry (FCM) protocol for evaluating the bactericidal efficacy of disinfectants. Five commercial disinfectants (alcohols, oxidizing agents, and alkylating agents) were evaluated against type strains recommended by EN 13727+A2 and ten clinical strains. The label-free FCM method allowed the determination of disinfectant efficacy through assessment of scatter light profiles (FSC-H/SSC-H) and count differences. The label-free FCM provided the results in approximately 4 h and showed strong correlation with standard tests (91.4%, sensitivity 0.94 and specificity 0.98) that can take up to 48 h. Our results represent a proof-of-principle that label-free FCM can reliably assess the efficacy of chemical disinfectants, the same day, and substantially faster than the current growth-based methods. Additionally, the study highlights the potential of the FCM method for detecting the occurrence of viable but non-culturable bacteria following treatment with chlorine-based disinfectants. With its speed, accuracy, and capability to identify bacterial injuries at a single-cell level, the FCM method is a powerful tool for assessing the efficacy of new disinfectant formulations.

Disinfectants and antiseptics: mechanisms of action and resistance

Chemical biocides are used for the prevention and control of infection in health care, targeted home hygiene or controlling microbial contamination for various industrial processes including but not limited to food, water and petroleum. However, their use has substantially increased since the implementation of programmes to control outbreaks of methicillin-resistant Staphylococcus aureus, Clostridioides difficile and severe acute respiratory syndrome coronavirus 2. Biocides interact with multiple targets on the bacterial cells. The number of targets affected and the severity of damage will result in an irreversible bactericidal effect or a reversible bacteriostatic one. Most biocides primarily target the cytoplasmic membrane and enzymes, although the specific bactericidal mechanisms vary among different biocide chemistries. Inappropriate usage or low concentrations of a biocide may act as a stressor while not killing bacterial pathogens, potentially leading to antimicrobial resistance. Biocides can also promote the transfer of antimicrobial resistance genes. In this Review, we explore our current understanding of the mechanisms of action of biocides, the bacterial resistance mechanisms encompassing both intrinsic and acquired resistance and the influence of bacterial biofilms on resistance. We also consider the impact of bacteria that survive biocide exposure in environmental and clinical contexts.

Biocides in the Hospital Environment: Application and Tolerance Development

Hospital-acquired infections are a rising problem with consequences for patients, hospitals, and health care workers. Biocides can be employed to prevent these infections, contributing to eliminate or reduce microorganisms' concentrations at the hospital environment. These antimicrobials belong to several groups, each with distinct characteristics that need to be taken into account in their selection for specific applications. Moreover, their activity is influenced by many factors, such as compound concentration and the presence of organic matter. This article aims to review some of the chemical biocides available for hospital infection control, as well as the main factors that influence their efficacy and promote susceptibility decreases, with the purpose to contribute for reducing misusage and consequently for preventing the development of resistance to these antimicrobials.

A review on triclosan in wastewater: Mechanism of action, resistance phenomenon, environmental risks, and sustainable removal techniques

Triclosan, belonging to the bisphenols, is a known antiseptic broad-spectrum biocide. It has a very wide range of applications, both in health care and in the household. Triclosan enters the environment, both water bodies and soil, because of its high prevalence and the ability to accumulation. Excessive use of antimicrobial formulations may cause the generation of resistance among microorganisms. Reduced susceptibility to triclosan is observed more frequently and in an expanded group of microorganisms and is conditioned by a number of different mechanisms occurring on the molecular level. Conventional wastewater treatment processes are not always able to provide a reliable barrier to triclosan. Therefore, additional advanced treatment technologies are being considered in areas, where a triclosan contamination problem has been identified. Removal of triclosan from wastewater is carried out using different biological and chemical techniques; however, it should be pointed out that physico-chemical methods often generate toxic by-products. Toxicity of triclosan and its degradation products, bacterial resistance to this compound, and evident problems with triclosan elimination from wastewater are currently the main problems faced by companies creating products containing triclosan. PRACTITIONER POINTS: Triclosan is an emerging pollutant in the environment because of its ability to accumulation and high prevalence. Reduced susceptibility to triclosan is being observed more frequently. Conventional wastewater treatment processes are not always able to provide a reliable barrier to triclosan. Additional advanced treatment technologies should be implemented to remove triclosan from wastewater.

New therapeutic strategies for Mycobacterium abscessus pulmonary diseases - untapping the mycolic acid pathway

INTRODUCTION

Treatment options against Mycobacterium abscessus infections are very limited. New compounds are needed to cure M. abscessus pulmonary diseases. While the mycolic acid biosynthetic pathway has been largely exploited for the treatment of tuberculosis, this metabolic process has been overlooked in M. abscessus, although it offers many potential drug targets for the treatment of this opportunistic pathogen.

AREAS COVERED

Herein, the authors review the role of the MmpL3 membrane protein and the enoyl-ACP reductase InhA involved in the transport and synthesis of mycolic acids, respectively. They discuss their importance as two major vulnerable drug targets in M. abscessus and report the activity of MmpL3 and InhA inhibitors. In particular, they focus on NITD-916, a direct InhA inhibitor against M. abscessus, particularly warranted in the context of multidrug resistance.

EXPERT OPINION

There is an increasing body of evidence validating the mycolic acid pathway as an attractive drug target to be further exploited for M. abscessus lung disease treatments. The NITD-916 studies provide a proof-of-concept that direct inhibitors of InhA are efficient in vitro, in macrophages and in zebrafish. Future work is now required to improve the activity and pharmacological properties of these inhibitors and their evaluation in pre-clinical models.

Antimicrobial Resistance and Its Drivers-A Review

Antimicrobial resistance (AMR) is a critical issue in health care in terms of mortality, quality of services, and financial damage. In the battle against AMR, it is crucial to recognize the impacts of all four domains, namely, mankind, livestock, agriculture, and the ecosystem. Many sociocultural and financial practices that are widespread in the world have made resistance management extremely complicated. Several pathways, including hospital effluent, agricultural waste, and wastewater treatment facilities, have been identified as potential routes for the spread of resistant bacteria and their resistance genes in soil and surrounding ecosystems. The overuse of uncontrolled antibiotics and improper treatment and recycled wastewater are among the contributors to AMR. Health-care organizations have begun to address AMR, although they are currently in the early stages. In this review, we provide a brief overview of AMR development processes, the worldwide burden and drivers of AMR, current knowledge gaps, monitoring methodologies, and global mitigation measures in the development and spread of AMR in the environment.

Bacterial Enoyl-Reductases: The Ever-Growing List of Fabs, Their Mechanisms and Inhibition

Enoyl-ACP reductases (ENRs) are enzymes that catalyze the last step of the elongation cycle during fatty acid synthesis. In recent years, new bacterial ENR types were discovered, some of them with structures and mechanisms that differ from the canonical bacterial FabI enzymes. Here, we briefly review the diversity of structural and catalytic properties of the canonical FabI and the new FabK, FabV, FabL, and novel ENRs identified in a soil metagenome study. We also highlight recent efforts to use the newly discovered Fabs as targets for drug development and consider the complex evolutionary history of this diverse set of bacterial ENRs.

Emerging impact of triazoles as anti-tubercular agent

Tuberculosis, a disease of poverty is a communicable infection with a reasonably high mortality rate worldwide. 10 Million new cases of TB were reported with approx 1.4 million deaths in the year 2019. Due to the growing number of drug-sensitive and drug-resistant tuberculosis cases, there is a vital need to develop new and effective candidates useful to combat this deadly disease. Despite tremendous efforts to identify a mechanism-based novel antitubercular agent, only a few have entered into clinical trials in the last six decades. In recent years, triazoles have been well explored as the most valuable scaffolds in drug discovery and development. Triazole framework possesses favorable properties like hydrogen bonding, moderate dipole moment, enhanced water solubility, and also the ability to bind effectively with biomolecular targets of M. tuberculosis and therefore this scaffold displayed excellent potency against TB. This review is an endeavor to summarize an up-to-date innovation of triazole-appended hybrids during the last 10 years having potential in vitro and in vivo antitubercular activity with structure activity relationship analysis. This review may help medicinal chemists to explore the triazole scaffolds for the rational design of potent drug candidates having better efficacy, improved selectivity and minimal toxicity so that these hybrid NCEs can effectively be explored as potential lead to fight against M. tuberculosis.

Biomarkers-based assessment of triclosan toxicity in aquatic environment: A mechanistic review

Triclosan (TCS), an emergent pollutant, is raising a global concern due to its toxic effects on organisms and aquatic ecosystems. The non-availability of proven treatment technologies for TCS remediation is the central issue stressing thorough research on understanding the underlying mechanisms of toxicity and assessing vital biomarkers in the aquatic organism for practical monitoring purposes. Given the unprecedented circumstances during COVID 19 pandemic, a several-fold higher discharge of TCS in the aquatic ecosystems cannot be considered a remote possibility. Therefore, identifying potential biomarkers for assessing chronic effects of TCS are prerequisites for addressing the issues related to its ecological impact and its monitoring in the future. It is the first holistic review on highlighting the biomarkers of TCS toxicity based on a comprehensive review of available literature about the biomarkers related to cytotoxicity, genotoxicity, hematological, alterations of gene expression, and metabolic profiling. This review establishes that biomarkers at the subcellular level such as oxidative stress, lipid peroxidation, neurotoxicity, and metabolic enzymes can be used to evaluate the cytotoxic effect of TCS in future investigations. Micronuclei frequency and % DNA damage proved to be reliable biomarkers for genotoxic effects of TCS in fishes and other aquatic organisms. Alteration of gene expression and metabolic profiling in different organs provides a better insight into mechanisms underlying the biocide's toxicity. In the concluding part of the review, the present status of knowledge about mechanisms of antimicrobial resistance of TCS and its relevance in understanding the toxicity is also discussed referring to the relevant reports on microorganisms.

Environmental Spread of Antibiotic Resistance

Antibiotic resistance represents a global health concern. Soil, water, livestock and plant foods are directly or indirectly exposed to antibiotics due to their agricultural use or contamination. This selective pressure has acted synergistically to bacterial competition in nature to breed antibiotic-resistant (AR) bacteria. Research over the past few decades has focused on the emergence of AR pathogens in food products that can cause disease outbreaks and the spread of antibiotic resistance genes (ARGs), but One Health approaches have lately expanded the focus to include commensal bacteria as ARG donors. Despite the attempts of national and international authorities of developed and developing countries to reduce the over-prescription of antibiotics to humans and the use of antibiotics as livestock growth promoters, the selective flow of antibiotic resistance transmission from the environment to the clinic (and vice-versa) is increasing. This review focuses on the mechanisms of ARG transmission and the hotspots of antibiotic contamination resulting in the subsequent emergence of ARGs. It follows the transmission of ARGs from farm to plant and animal food products and provides examples of the impact of ARG flow to clinical settings. Understudied and emerging antibiotic resistance selection determinants, such as heavy metal and biocide contamination, are also discussed here.

Distinct expression profiles of regulatory RNAs in the response to biocides in Staphylococcus aureus and Enterococcus faecium

The aim of the study was to characterize the antimicrobial activity of clinically-relevant biocides (chlorhexidine digluconate, benzalkonium chloride, PVP-iodine and triclosan) and to determine the sRNA expression profiles under biocide exposure in two major bacterial opportunistic pathogens, Enterococcus faecium and Staphylococcus aureus. In vitro activities were evaluated against S. aureus HG003 and E. faecium Aus0004. We determined MIC, MBC, sub-inhibitory concentrations (SIC) and growth curves under SIC conditions. sRNA expression study under SIC exposure of biocides was performed by RT-qPCR on 3 sRNAs expressed in S. aureus (RNAIII, SprD and SprX) and the first 9 sRNAs identified as expressed in E. faecium. MICs were higher against E. faecium than for S. aureus. Growth curves under increasing biocide concentrations highlighted two types of bactericidal activity: "on/off" effect for chlorhexidine, benzalkonium chloride, PVP-iodine and a "concentration-dependent" activity for triclosan. Exposure to biocide SICs led to an alteration of several sRNA expression profiles, mostly repressed. The distinct biocide activity profiles must be evaluated with other compounds and bacterial species to enrich the prediction of resistance risks associated with biocide usage. Biocide exposure induces various sRNA-mediated responses in both S. aureus and E. faecium, and further investigations are needed to decipher sRNA-driven regulatory networks.

The food-grade antimicrobial xanthorrhizol targets the enoyl-ACP reductase (FabI) in Escherichia coli

Xanthorrhizol, isolated from the Indonesian Java turmeric Curcuma xanthorrhiza, displays broad-spectrum antibacterial activity. We report herein the evidence that mechanism of action of xanthorrhizol may involve FabI, an enoyl-(ACP) reductase, inhibition. The predicted Y156V substitution in the FabI enzyme promoted xanthorrhizol resistance, while the G93V mutation originally known for triclosan resistance was not effective against xanthorrhizol. Two other mutations, F203L and F203V, conferred FabI enzyme resistance to both xanthorrhizol and triclosan. These results showed that xanthorrhizol is a food-grade antimicrobial compound targeting FabI but with a different mode of binding from triclosan.

Exploring the chemical space of 1,2,3-triazolyl triclosan analogs for discovery of new antileishmanial chemotherapeutic agents

Triclosan and isoniazid are known antitubercular compounds that have proven to be also active against Leishmania parasites. On these grounds, a collection of 37 diverse 1,2,3-triazoles based on the antitubercular molecules triclosan and 5-octyl-2-phenoxyphenol (8PP) were designed in search of novel structures with leishmanicidal activity and prepared using different alkynes and azides. The 37 compounds were assayed against Leishmania donovani, the etiological agent of leishmaniasis, yielding some analogs with activity at micromolar concentrations and against M. tuberculosis H37Rv resulting in scarce active compounds with an MIC of 20 μM. To study the mechanism of action of these catechols, we analyzed the inhibition activity of the library on the M. tuberculosis enoyl-ACP reductase (ENR) InhA, obtaining poor inhibition of the enzyme. The cytotoxicity against Vero cells was also tested, resulting in none of the compounds being cytotoxic at concentrations of up to 20 μM. Derivative 5f could be considered a valuable starting point for future antileishmanial drug development. The validation of a putative leishmanial InhA orthologue as a therapeutic target needs to be further investigated.

Short- and Long-Term Transcriptomic Responses of Escherichia coli to Biocides: a Systems Analysis

The mechanisms of the bacterial response to biocides are poorly understood, despite their broad application. To identify the genetic basis and pathways implicated in the biocide stress response, we exposed Escherichia coli populations to 10 ubiquitous biocides. By comparing the transcriptional responses between a short-term exposure (30 min) and a long-term exposure (8 to 12 h) to biocide stress, we established the common gene and pathway clusters that are implicated in general and biocide-specific stress responses. Our analysis revealed a temporal choreography, starting from the upregulation of chaperones to the subsequent repression of motility and chemotaxis pathways and the induction of an anaerobic pool of enzymes and biofilm regulators. A systematic analysis of the transcriptional data identified a zur-regulated gene cluster to be highly active in the stress response against sodium hypochlorite and peracetic acid, presenting a link between the biocide stress response and zinc homeostasis. Susceptibility assays with knockout mutants further validated our findings and provide clear targets for downstream investigation of the implicated mechanisms of action.IMPORTANCE Antiseptics and disinfectant products are of great importance to control and eliminate pathogens, especially in settings such as hospitals and the food industry. Such products are widely distributed and frequently poorly regulated. Occasional outbreaks have been associated with microbes resistant to such compounds, and researchers have indicated potential cross-resistance with antibiotics. Despite that, there are many gaps in knowledge about the bacterial stress response and the mechanisms of microbial resistance to antiseptics and disinfectants. We investigated the stress response of the bacterium Escherichia coli to 10 common disinfectant and antiseptic chemicals to shed light on the potential mechanisms of tolerance to such compounds.

Improvement of the novel inhibitor for Mycobacterium enoyl-acyl carrier protein reductase (InhA): a structure-activity relationship study of KES4 assisted by in silico structure-based drug screening

InhA or enoyl-acyl carrier protein reductase of Mycobacterium tuberculosis (mtInhA), which controls mycobacterial cell wall construction, has been targeted in the development of antituberculosis drugs. Previously, our in silico structure-based drug screening study identified a novel class of compounds (designated KES4), which is capable of inhibiting the enzymatic activity of mtInhA, as well as mycobacterial growth. The compounds are composed of four ring structures (A-D), and the MD simulation predicted specific interactions with mtInhA of the D-ring and methylene group between the B-ring and C-ring; however, there is still room for improvement in the A-ring structure. In this study, a structure-activity relationship study of the A-ring was attempted with the assistance of in silico docking simulations. In brief, the virtual chemical library of A-ring-modified KES4 was constructed and subjected to in silico docking simulation against mtInhA using the GOLD program. Among the selected candidates, we achieved synthesis of seven compounds, and the bioactivities (effects on InhA activity and mycobacterial growth and cytotoxicity) of the synthesized molecules were evaluated. Among the compounds tested, two candidates (compounds 3d and 3f) exhibited superior properties as mtInhA-targeted anti-infectives for mycobacteria than the lead compound KES4.

Differential Expression of Resistant and Efflux Pump Genes in MDR-TB Isolates

Background:

Numerous investigations demonstrate efflux as a worldwide bacterial mode of action which contributes to the resistance of drugs. The activity of antibiotics, which subjects to efflux, can be improved by the combined usage of efflux inhibitors. However, the efflux role to the overall levels of antibiotic resistance of clinical M. tuberculosis isolates is inadequately comprehended and is still disregarded by many.

Method:

Here, we assessed the contribution of resistant genes associated with isoniazid (INH) and rifampin (R) resistance to the levels of drug resistance in the (27) clinical isolates of MDR-TB. Additionally, the role of the resistance for six putative drug efflux pump genes to the antibiotics was investigated. The level of katG expression was down-regulated in 24/27 (88.88%) of MDR-TB isolates. Of the 27 MDR-TB isolates, inhA, oxyR-ahpC, and rpoB showed either overexpression or up-regulation in 8 (29.62%), 4 (14.81 %), and 24 (88.88%), respectively. Moreover, the efflux pump genes drrA, drrB, efpA, Rv2459, Rv1634, and Rv1250 were overexpressed under INH/RIF plus fresh pomegranate juice (FPJ) stress signifying the efflux pumps contribution to the overall levels of the resistance of MDR-TB isolates.

Conclusion:

These results displayed that the levels of drug resistance of MDR-TB clinical isolates are due to combination among drug efflux pump and the presence of mutations in target genes, a truth which is often ignored by the specialists of tuberculosis in favour of the almost undoubted significance of drug target- gene mutations for the resistance in M. tuberculosis.

Biochemical and Structural Insights Concerning Triclosan Resistance in a Novel YX7K Type Enoyl-Acyl Carrier Protein Reductase from Soil Metagenome

Enoyl-acyl carrier protein reductase (ENR) catalyzes the last reduction step in the bacterial type II fatty acid biosynthesis cycle. ENRs include FabI, FabL, FabL2, FabK, and FabV. Previously, we reported a unique triclosan (TCL) resistant ENR homolog that was predominant in obligate intracellular pathogenic bacteria and Apicomplexa. Herein, we report the biochemical and structural basis of TCL resistance in this novel ENR. The purified protein revealed NADH-dependent ENR activity and shared similarity to prototypic FabI. Thus, this metagenome-derived ENR was designated FabI2. Unlike other prototypic bacterial ENRs with the YX₆K type catalytic domain, FabI2 possessed a unique YX₇K type catalytic domain. Computational modeling followed by site-directed mutagenesis revealed that mild resistance (20 µg/ml of minimum inhibitory concentration) of FabI2 to TCL was confined to the relatively less bulky side chain of A128. Substitution of A128 in FabI2 with bulky valine (V128) elevated TCL resistance. Phylogenetic analysis further suggested that the novel FabI2 and prototypical FabI evolved from a common short-chain dehydrogenase reductase family. To our best knowledge, FabI2 is the only known ENR shared by intracellular pathogenic prokaryotes, intracellular pathogenic lower eukaryotes, and a few higher eukaryotes. This suggests that the ENRs of prokaryotes and eukaryotes diverged from a common ancestral ENR of FabI2.

Insights into the Physiology and Metabolism of a Mycobacterial Cell in an Energy-Compromised State

Mycobacterium tuberculosis, a bacterium that causes tuberculosis, poses a serious threat, especially due to the emergence of drug-resistant strains. M. tuberculosis and other mycobacterial species, such as M. smegmatis, are known to generate an inadequate amount of energy by substrate-level phosphorylation and mandatorily require oxidative phosphorylation (OXPHOS) for their growth and metabolism. Hence, antibacterial drugs, such as bedaquiline, targeting the multisubunit ATP synthase complex, which is required for OXPHOS, have been developed with the aim of eliminating pathogenic mycobacteria. Here, we explored the influence of suboptimal OXPHOS on the physiology and metabolism of M. smegmatis M. smegmatis harbors two identical copies of atpD, which codes for the β subunit of ATP synthase. We show that upon deletion of one copy of atpD (M. smegmatis ΔatpD), M. smegmatis synthesizes smaller amounts of ATP and enters into an energy-compromised state. The mutant displays remarkable phenotypic and physiological differences from the wild type, such as respiratory slowdown, reduced biofilm formation, lesser amounts of cell envelope polar lipids, and increased antibiotic sensitivity compared to the wild type. Additionally, M. smegmatis ΔatpD overexpresses genes belonging to the dormancy operon, the β-oxidation pathway, and the glyoxylate shunt, suggesting that the mutant adapts to a low energy state by switching to alternative pathways to produce energy. Interestingly, M. smegmatis ΔatpD shows significant phenotypic, metabolic, and physiological similarities with bedaquiline-treated wild-type M. smegmatis We believe that the identification and characterization of key metabolic pathways functioning during an energy-compromised state will enhance our understanding of bacterial adaptation and survival and will open newer avenues in the form of drug targets that may be used in the treatment of mycobacterial infections.IMPORTANCE M. smegmatis generates an inadequate amount of energy by substrate-level phosphorylation and mandatorily requires oxidative phosphorylation (OXPHOS) for its growth and metabolism. Here, we explored the influence of suboptimal OXPHOS on M. smegmatis physiology and metabolism. M. smegmatis harbors two identical copies of the atpD gene, which codes for the ATP synthase β subunit. Here, we carried out the deletion of only one copy of atpD in M. smegmatis to understand the bacterial survival response in an energy-deprived state. M. smegmatis ΔatpD shows remarkable phenotypic, metabolic, and physiological differences from the wild type. Our study thus establishes M. smegmatis ΔatpD as an energy-compromised mycobacterial strain, highlights the importance of ATP synthase in mycobacterial physiology, and further paves the way for the identification of novel antimycobacterial drug targets.

Biochemical and Structural Basis of Triclosan Resistance in a Novel Enoyl-Acyl Carrier Protein Reductase

Enoyl-acyl carrier protein reductases (ENR), such as FabI, FabL, FabK, and FabV, catalyze the last reduction step in bacterial type II fatty acid biosynthesis. Previously, we reported metagenome-derived ENR homologs resistant to triclosan (TCL) and highly similar to 7-α hydroxysteroid dehydrogenase (7-AHSDH). These homologs are commonly found in Epsilonproteobacteria, a class that contains several human-pathogenic bacteria, including the genera Helicobacter and Campylobacter Here we report the biochemical and predicted structural basis of TCL resistance in a novel 7-AHSDH-like ENR. The purified protein exhibited NADPH-dependent ENR activity but no 7-AHSDH activity, despite its high homology with 7-AHSDH (69% to 96%). Because this ENR was similar to FabL (41%), we propose that this metagenome-derived ENR be referred to as FabL2. Homology modeling, molecular docking, and molecular dynamic simulation analyses revealed the presence of an extrapolated six-amino-acid loop specific to FabL2 ENR, which prevented the entry of TCL into the active site of FabL2 and was likely responsible for TCL resistance. Elimination of this extrapolated loop via site-directed mutagenesis resulted in the complete loss of TCL resistance but not enzyme activity. Phylogenetic analysis suggested that FabL, FabL2, and 7-AHSDH diverged from a common short-chain dehydrogenase reductase family. This study is the first to report the role of the extrapolated loop of FabL2-type ENRs in conferring TCL resistance. Thus, the FabL2 ENR represents a new drug target specific for pathogenic Epsilonproteobacteria.

Resistance of Bacteria to Biocides

Biocides and formulated biocides are used worldwide for an increasing number of applications despite tightening regulations in Europe and in the United States. One concern is that such intense usage of biocides could lead to increased bacterial resistance to a product and cross-resistance to unrelated antimicrobials including chemotherapeutic antibiotics. Evidence to justify such a concern comes mostly from the use of health care-relevant bacterial isolates, although the number of studies of the resistance characteristics of veterinary isolates to biocides have increased the past few years. One problem remains the definition of "resistance" and how to measure resistance to a biocide. This has yet to be addressed globally, although the measurement of resistance is becoming more pressing, with regulators both in Europe and in the United States demanding that manufacturers provide evidence that their biocidal products will not impact on bacterial resistance. Alongside in vitro evidence of potential antimicrobial cross-resistance following biocide exposure, our understanding of the mechanisms of bacterial resistance and, more recently, our understanding of the effect of biocides to induce a mechanism(s) of resistance in bacteria has improved. This article aims to provide an understanding of the development of antimicrobial resistance in bacteria following a biocide exposure. The sections provide evidence of the occurrence of bacterial resistance and its mechanisms of action and debate how to measure bacterial resistance to biocides. Examples pertinent to the veterinary field are used where appropriate.

Distribution of triclosan-resistant genes in major pathogenic microorganisms revealed by metagenome and genome-wide analysis

The substantial use of triclosan (TCS) has been aimed to kill pathogenic bacteria, but TCS resistance seems to be prevalent in microbial species and limited knowledge exists about TCS resistance determinants in a majority of pathogenic bacteria. We aimed to evaluate the distribution of TCS resistance determinants in major pathogenic bacteria (N = 231) and to assess the enrichment of potentially pathogenic genera in TCS contaminated environments. A TCS-resistant gene (TRG) database was constructed and experimentally validated to predict TCS resistance in major pathogenic bacteria. Genome-wide in silico analysis was performed to define the distribution of TCS-resistant determinants in major pathogens. Microbiome analysis of TCS contaminated soil samples was also performed to investigate the abundance of TCS-resistant pathogens. We experimentally confirmed that TCS resistance could be accurately predicted using genome-wide in silico analysis against TRG database. Predicted TCS resistant phenotypes were observed in all of the tested bacterial strains (N = 17), and heterologous expression of selected TCS resistant genes from those strains conferred expected levels of TCS resistance in an alternative host Escherichia coli. Moreover, genome-wide analysis revealed that potential TCS resistance determinants were abundant among the majority of human-associated pathogens (79%) and soil-borne plant pathogenic bacteria (98%). These included a variety of enoyl-acyl carrier protein reductase (ENRs) homologues, AcrB efflux pumps, and ENR substitutions. FabI ENR, which is the only known effective target for TCS, was either co-localized with other TCS resistance determinants or had TCS resistance-associated substitutions. Furthermore, microbiome analysis revealed that pathogenic genera with intrinsic TCS-resistant determinants exist in TCS contaminated environments. We conclude that TCS may not be as effective against the majority of bacterial pathogens as previously presumed. Further, the excessive use of this biocide in natural environments may selectively enrich for not only TCS-resistant bacterial pathogens, but possibly for additional resistance to multiple antibiotics.

An overview on crystal structures of InhA protein: Apo-form, in complex with its natural ligands and inhibitors

The enoyl-ACP reductase InhA from the mycobacterial fatty acid biosynthesis pathway has become a target of interest for the development of new anti-tubercular drugs. This protein has been identified as essential for the survival of Mycobacterium tuberculosis, the causative agent of tuberculosis, and as the main target of two pro-drugs: isoniazid, the frontline anti-tubercular drug, and ethionamide, a second-line medicine. Since most cases of resistance to isoniazid and ethionamide result from mutations in the mycobacterial activating enzyme (KatG for isoniazid and EthA for ethionamide), research of direct InhA inhibitors, avoiding the activation step, has emerged as a promising strategy for combating tuberculosis. Thereby, InhA is drawing much attention and its three-dimensional structure has been particularly studied. A better understanding of key sites of interactions responsible for InhA inhibition arises thus as an essential tool for the rational design of new potent inhibitors. In this paper, we propose an overview of the 80 available crystal structures of wild-type and mutant InhA, in its apo form, in complex with its cofactor, with an analogue of its natural ligands (C16 fatty acid and/or NADH) or with inhibitors. We will first discuss structural and mechanistic aspects in order to highlight key features of the protein before delivering thorough inventory of structures of InhA in the presence of synthetic ligands to underline the key interactions implicated in high affinity inhibition.

Current and Emerging Topical Antibacterials and Antiseptics: Agents, Action, and Resistance Patterns

Bacterial skin infections represent some of the most common infectious diseases globally. Prevention and treatment of skin infections can involve application of a topical antimicrobial, which may be an antibiotic (such as mupirocin or fusidic acid) or an antiseptic (such as chlorhexidine or alcohol). However, there is limited evidence to support the widespread prophylactic or therapeutic use of topical agents. Challenges involved in the use of topical antimicrobials include increasing rates of bacterial resistance, local hypersensitivity reactions (particularly to older agents, such as bacitracin), and concerns about the indiscriminate use of antiseptics potentially coselecting for antibiotic resistance. We review the evidence for the major clinical uses of topical antibiotics and antiseptics. In addition, we review the mechanisms of action of common topical agents and define the clinical and molecular epidemiology of antimicrobial resistance in these agents. Moreover, we review the potential use of newer and emerging agents, such as retapamulin and ebselen, and discuss the role of antiseptic agents in preventing bacterial skin infections. A comprehensive understanding of the clinical efficacy and drivers of resistance to topical agents will inform the optimal use of these agents to preserve their activity in the future.

Impact of Exposure of Methicillin-Resistant Staphylococcus aureus to Polyhexanide In Vitro and In Vivo

Methicillin-resistant Staphylococcus aureus (MRSA) resistant to decolonization agents such as mupirocin and chlorhexidine increases the need for development of alternative decolonization molecules. The absence of reported severe adverse reactions and bacterial resistance to polyhexanide makes it an excellent choice as a topical antiseptic. In the present study, we evaluated the in vitro and in vivo capacity to generate strains with reduced polyhexanide susceptibility and cross-resistance with chlorhexidine and/or antibiotics currently used in clinic. Here we report the in vitro emergence of reduced susceptibility to polyhexanide by prolonged stepwise exposure to low concentrations in broth culture. Reduced susceptibility to polyhexanide was associated with genomic changes in the mprF and purR genes and with concomitant decreased susceptibility to daptomycin and other cell wall-active antibiotics. However, the in vitro emergence of reduced susceptibility to polyhexanide did not result in cross-resistance to chlorhexidine. During in vivo polyhexanide clinical decolonization treatment, neither reduced polyhexanide susceptibility nor chlorhexidine cross-resistance was observed. Together, these observations suggest that polyhexanide could be used safely for decolonization of carriers of chlorhexidine-resistant S. aureus strains; they also highlight the need for careful use of polyhexanide at low antiseptic concentrations.

Disruption of key NADH-binding pocket residues of the Mycobacterium tuberculosis InhA affects DD-CoA binding ability

Tuberculosis (TB) is a global health problem that affects over 10 million people. There is an urgent need to develop novel antimicrobial therapies to combat TB. To achieve this, a thorough understanding of key validated drug targets is required. The enoyl reductase InhA, responsible for synthesis of essential mycolic acids in the mycobacterial cell wall, is the target for the frontline anti-TB drug isoniazid. To better understand the activity of this protein a series of mutants, targeted to the NADH co-factor binding pocket were created. Residues P193 and W222 comprise a series of hydrophobic residues surrounding the cofactor binding site and mutation of both residues negatively affect InhA function. Construction of an M155A mutant of InhA results in increased affinity for NADH and DD-CoA turnover but with a reduction in V_max for DD-CoA, impairing overall activity. This suggests that NADH-binding geometry of InhA likely permits long-range interactions between residues in the NADH-binding pocket to facilitate substrate turnover in the DD-CoA binding region of the protein. Understanding the precise details of substrate binding and turnover in InhA and how this may affect protein-protein interactions may facilitate the development of improved inhibitors enabling the development of novel anti-TB drugs.

Clinical implication of novel drug resistance-conferring mutations in resistant tuberculosis

Evolving novel and/or unfamiliar mutations are revolutionizing the pathways of antibiotic resistance of clinical tuberculosis. The accumulation and interaction of these poorly characterized mutations augment the complexity of resistant pathogenic strains and raise public health concerns. This article reviews our current understanding of the genetic changes that characterize drug resistance in tuberculosis and highlights the imperative for further investigations focusing on the effects of an individual mutation and interacting mutations with detailed strain epidemiology, particularly as these pertain to technology-limited countries with high tuberculosis incidence rates. Concomitantly, there is a need for the development, testing, and uptake of new tools for studying the effects of these mutations in drug resistance and fitness cost of the pathogen. Such genetic data are critical for effective localized and global tuberculosis control interventions and for accurate epidemiological predictions.

Triclosan Resistome from Metagenome Reveals Diverse Enoyl Acyl Carrier Protein Reductases and Selective Enrichment of Triclosan Resistance Genes

Triclosan (TCS) is a widely used antimicrobial agent and TCS resistance is considered to have evolved in diverse organisms with extensive use of TCS, but distribution of TCS resistance has not been well characterized. Functional screening of the soil metagenome in this study has revealed that a variety of target enoyl acyl carrier protein reductases (ENR) homologues are responsible for the majority of TCS resistance. Diverse ENRs similar to 7-α-hydroxysteroid dehydrogenase (7-α-HSDH), FabG, or the unusual YX7K-type ENR conferred extreme tolerance to TCS. The TCS-refractory 7-α HSDH-like ENR and the TCS-resistant YX7K-type ENR seem to be prevalent in human pathogenic bacteria, suggesting that a selective enrichment occurred in pathogenic bacteria in soil. Additionally, resistance to multiple antibiotics was found to be mediated by antibiotic resistance genes that co-localize with TCS resistance determinants. Further comparative analysis of ENRs from 13 different environments has revealed a huge diversity of both prototypic and metagenomic TCS-resistant ENRs, in addition to a selective enrichment of TCS-resistant specific ENRs in presumably TCS-contaminated environments with reduced ENR diversity. Our results suggest that long-term extensive use of TCS can lead to the selective emergence of TCS-resistant bacterial pathogens, possibly with additional resistance to multiple antibiotics, in natural environments.

Triclosan-induced genes Rv1686c-Rv1687c and Rv3161c are not involved in triclosan resistance in Mycobacterium tuberculosis

A key issue towards developing new chemotherapeutic approaches to fight Mycobacterium tuberculosis is to understand the mechanisms underlying drug resistance. Previous studies have shown that genes Rv1686c-Rv1687c and Rv3161c, predicted to encode an ATP-binding cassette transporter and a dioxygenase respectively, are induced in the presence of triclosan and other antimicrobial compounds. Therefore a possible role in drug resistance has been suggested for the products of these genes although no functional studies have been done. The aim of the present study was to clarify the role of Rv1686c-Rv1687c and Rv3161c in M. tuberculosis resistance to triclosan and other drugs. To this end, deficient mutants and overproducing strains for both systems were constructed and their minimal inhibitory concentration (MIC) against over 20 compounds, including triclosan, was evaluated. Unexpectedly, no differences between the MIC of these strains and the wild-type H37Rv were observed for any of the compounds tested. Moreover the MIC of triclosan was not affected by efflux pump inhibitors that inhibit the activity of transporters similar to the one encoded by Rv1686c-Rv1687c. These results suggest that none of the two systems is directly involved in M. tuberculosis resistance to triclosan or to any of the antimicrobials tested.

In vitro Resistance Testing of Porphyromonas gingivalis, Prevotella intermedia, and Tannerella forsythia to Triclosan

AIM

To determine the sensitivity of Porphyromonas gingivalis, Prevotella intermedia, and Tannerella forsythia to triclosan, and determine if these bacteria develop resistance to triclosan upon prolonged exposure.

MATERIALS AND METHODS

Susceptibility to triclosan was tested against three periodontal pathogens P. gingivalis, P. intermedia, and T. forsythia. Escherichia coli strains sensitive and resistant to triclosan were used as biological controls to confirm the efficacy of triclosan in the assays. Agar plates were prepared locally with vitamin K and hemin-supplemented medium.

RESULTS

Porphyromonas gingivalis and P. intermedia did not grow on plates containing ≥ 2 μg/ml triclosan, while T. forsythia did not grow on ≥ 1.66 μg/ml. Colonies of P. intermedia resistant to triclosan developed after prolonged incubation at 2 μg/ml, but this resistance disappeared during subculture in the absence of triclosan.

CONCLUSION

No significant resistance to triclosan was detected for these species.

CLINICAL SIGNIFICANCE

Dental products containing triclosan can be beneficial in controlling periodontal disease.

Efficacy of triclosan as an antimicrobial hand soap and its potential impact on antimicrobial resistance: a focused review

Triclosan is a synthetic biocide found in many household products, including antimicrobial hand soap. Levels of triclosan have been found throughout the environment and in human urine, blood, and even breast milk. Increasing levels of exposure to triclosan have led to concerns over the development of resistance to triclosan and cross-resistance to other antimicrobials. We performed a literature search to assess whether the widespread use of triclosan displays a favorable benefit: risk ratio, defined by evaluation of triclosan's efficacy as an antimicrobial hand soap and its potential effect on the development of antimicrobial resistance. Data from laboratory-based studies regarding the efficacy of triclosan are conflicting, although well-designed studies suggest no significant difference in efficacy over nonantimicrobial soap. In addition, when triclosan was introduced in a community setting, no beneficial effects were observed on the reduction of infections over nonantimicrobial soap. Resistance to triclosan and cross-resistance to antimicrobials have been consistently demonstrated in laboratory settings, although overall resistance rates and cross-resistance rates in the community setting are low. Based on the available evidence, the risk of potential antimicrobial resistance outweighs the benefit of widespread triclosan use in antimicrobial soaps.

An Efficient and Economical Assay to Screen for Triclosan Binding to FabI

Triclosan is an effective inhibitor for enoyl acyl carrier protein reductase (ENR) in fatty acid biosynthesis. Triclosan-resistant mutants of ENR have emerged. Thus, it is important to detect these triclosan-resistant mutations in ENR. Generally, enzyme activity assays on the mutants are used to determine the effect of triclosan on ENR activity. Since the substrates are linked to acyl carrier protein (ACP), the assays are challenging due to the need to prepare the ACP and link it to the substrates. Non-ACP-linked (coenzyme A [CoA]-linked) substrates can be used in some ENR, but not in all. Consequently, screening for triclosan-resistant mutants is also challenging. We have developed a simple thermal shift assay, which does not use ACP-linked substrates, to determine the binding ability of triclosan to the ENR active site, and thus it can be used for screening for triclosan-resistant mutants. Staphylococcus aureus FabI enzyme and its mutants were used to demonstrate the binding ability of triclosan with NADP(+) to FabI. The direct correlation between the binding ability and enzyme activity was demonstrated with Francisella tularensis FabI. This method may also be applied to select effective triclosan analogues that inhibit ENR activity.

Predictive Studies Suggest that the Risk for the Selection of Antibiotic Resistance by Biocides Is Likely Low in Stenotrophomonas maltophilia

Biocides are used without restriction for several purposes. As a consequence, large amounts of biocides are released without any control in the environment, a situation that can challenge the microbial population dynamics, including selection of antibiotic resistant bacteria. Previous work has shown that triclosan selects Stenotrophomonas maltophilia antibiotic resistant mutants overexpressing the efflux pump SmeDEF and induces expression of this pump triggering transient low-level resistance. In the present work we analyze if two other common biocides, benzalkonium chloride and hexachlorophene, trigger antibiotic resistance in S. maltophilia. Bioinformatic and biochemical methods showed that benzalkonium chloride and hexachlorophene bind the repressor of smeDEF, SmeT. Only benzalkonium chloride triggers expression of smeD and its effect in transient antibiotic resistance is minor. None of the hexachlorophene-selected mutants was antibiotic resistant. Two benzalkonium chloride resistant mutants presented reduced susceptibility to antibiotics and were impaired in growth. Metabolic profiling showed they were more proficient than their parental strain in the use of some dipeptides. We can then conclude that although bioinformatic predictions and biochemical studies suggest that both hexachlorophene and benzalkonium chloride should induce smeDEF expression leading to transient S. maltophilia resistance to antibiotics, phenotypic assays showed this not to be true. The facts that hexachlorophene resistant mutants are not antibiotic resistant and that the benzalkonium chloride resistant mutants presenting altered susceptibility to antibiotics were impaired in growth suggests that the risk for the selection (and fixation) of S. maltophilia antibiotic resistant mutants by these biocides is likely low, at least in the absence of constant selection pressure.

Mutations upstream of fabI in triclosan resistant Staphylococcus aureus strains are associated with elevated fabI gene expression

Background

The enoyl-acyl carrier protein (ACP) reductase enzyme (FabI) is the target for a series of antimicrobial agents including novel compounds in clinical trial and the biocide triclosan. Mutations in fabI and heterodiploidy for fabI have been shown to confer resistance in S. aureus strains in a previous study. Here we further determined the fabI upstream sequence of a selection of these strains and the gene expression levels in strains with promoter region mutations.

Results

Mutations in the fabI promoter were found in 18% of triclosan resistant clinical isolates, regardless the previously identified molecular mechanism conferring resistance. Although not significant, a higher rate of promoter mutations were found in strains without previously described mechanisms of resistance. Some of the mutations identified in the clinical isolates were also detected in a series of laboratory mutants. Microarray analysis of selected laboratory mutants with fabI promoter region mutations, grown in the absence of triclosan, revealed increased fabI expression in three out of four tested strains. In two of these strains, only few genes other than fabI were upregulated. Consistently with these data, whole genome sequencing of in vitro selected mutants identified only few mutations except the upstream and coding regions of fabI, with the promoter mutation as the most probable cause of fabI overexpression. Importantly the gene expression profiling of clinical isolates containing similar mutations in the fabI promoter also showed, when compared to unrelated non-mutated isolates, a significant up-regulation of fabI.

Conclusions

In conclusion, we have demonstrated the presence of C34T, T109G, and A101C mutations in the fabI promoter region of strains with fabI up-regulation, both in clinical isolates and/or laboratory mutants. These data provide further observations linking mutations upstream fabI with up-regulated expression of the fabI gene.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1544-y) contains supplementary material, which is available to authorized users.

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.

On the need and speed of regulating triclosan and triclocarban in the United States

The polychlorinated aromatic antimicrobials triclosan and triclocarban are in widespread use for killing microorganisms indiscriminately, rapidly, and by nonspecific action. While their utility in healthcare settings is undisputed, benefits to users of antimicrobial personal care products are few to none. Yet, these latter, high-volume uses have caused widespread contamination of the environment, wildlife, and human populations. This feature article presents a timeline of scientific evidence and regulatory actions in the U.S. concerning persistent polychlorinated biocides, showing a potential path forward to judicious and sustainable uses of synthetic antimicrobials, including the design of greener and safer next-generation alternatives.

In vitro and in silico antimalarial activity of 2-(2-hydrazinyl)thiazole derivatives

A series of 2-(2-hydrazinyl)thiazole derivatives with a wide range of substitutions at 2-, 4- and 5-positions were synthesized, characterized and evaluated their inhibitory potentials against plasmodium falciparum, NF54, by in vitro blood stage assay. The compounds, ethyl-4-methyl-2-[(E)-2-[1-(pyridin-2-yl)ethylidene]hydrazin-1-yl]-1,3-thiazole-5-carboxylate, 4d, and 1-{4-methyl-2-[(E)-2-[1-(pyridin-2-yl)ethylidene]hydrazin-1-yl]-1,3-thiazol-5-yl}ethan-1-one, 5d showed significant antimalarial activity with IC50 values of 0.725 μM and 0.648 μM respectively. To understand the mechanism, the binding interactions between 2-(2-hydrazinyl)thiazole derivatives and trans-2-enoyl acyl carrier protein reductase of P. falciparum were studied through docking studies. The half maximal inhibitory concentration (IC50) through docking studies for the compounds, 4d and 5d were found to be 22.88 μM and 631.84 μM respectively.

Design, synthesis, and biological and crystallographic evaluation of novel inhibitors of Plasmodium falciparum enoyl-ACP-reductase (PfFabI)

Malaria, a disease of worldwide significance, is responsible for over one million deaths annually. The liver-stage of Plasmodium's life cycle is the first, obligatory, but clinically silent step in malaria infection. The P. falciparum type II fatty acid biosynthesis pathway (PfFAS-II) has been found to be essential for complete liver-stage development and has been regarded as a potential antimalarial target for the development of drugs for malaria prophylaxis and liver-stage eradication. In this paper, new coumarin-based triclosan analogues are reported and their biological profile is explored in terms of inhibitory potency against enzymes of the PfFAS-II pathway. Among the tested compounds, 7 and 8 showed the highest inhibitory potency against Pf enoyl-ACP-reductase (PfFabI), followed by 15 and 3. Finally, we determined the crystal structures of compounds 7 and 11 in complex with PfFabI to identify their mode of binding and to confirm outcomes of docking simulations.

Biocide tolerance in bacteria

Biocides have been employed for centuries, so today a wide range of compounds showing different levels of antimicrobial activity have become available. At the present time, understanding the mechanisms of action of biocides has also become an important issue with the emergence of bacterial tolerance to biocides and the suggestion that biocide and antibiotic resistance in bacteria might be linked. While most of the mechanisms providing antibiotic resistance are agent specific, providing resistance to a single antimicrobial or class of antimicrobial, there are currently numerous examples of efflux systems that accommodate and, thus, provide tolerance to a broad range of structurally unrelated antimicrobials, both antibiotics and biocides. If biocide tolerance becomes increasingly common and it is linked to antibiotic resistance, not only resistant (even multi-resistant) bacteria could be passed along the food chain, but also there are resistance determinants that can spread and lead to the emergence of new resistant microorganisms, which can only be detected and monitored when the building blocks of resistance traits are understood on the molecular level. This review summarizes the main advances reached in understanding the mechanism of action of biocides, the mechanisms of bacterial resistance to both biocides and antibiotics, and the incidence of biocide tolerance in bacteria of concern to human health and the food industry.

The Francisella tularensis FabI enoyl-acyl carrier protein reductase gene is essential to bacterial viability and is expressed during infection

Francisella tularensis is classified as a category A priority pathogen and causes fatal disseminated disease in humans upon inhalation of less than 50 bacteria. Although drugs are available for treatment, they are not ideal because of toxicity and route of delivery, and in some cases patients relapse upon withdrawal. We have an ongoing program to develop novel FAS-II FabI enoyl-ACP reductase enzyme inhibitors for Francisella and other select agents. To establish F. tularensis FabI (FtFabI) as a clinically relevant drug target, we demonstrated that fatty acid biosynthesis and FabI activity are essential for growth even in the presence of exogenous long-chain lipids and that FtfabI is not transcriptionally altered in the presence of exogenous long-chain lipids. Inhibition of FtFabI or fatty acid synthesis results in loss of viability that is not rescued by exogenous long-chain lipid supplementation. Importantly, whole-genome transcriptional profiling of F. tularensis with DNA microarrays from infected tissues revealed that FtfabI and de novo fatty acid biosynthetic genes are transcriptionally active during infection. This is the first demonstration that the FabI enoyl-ACP-reductase enzyme encoded by F. tularensis is essential and not bypassed by exogenous fatty acids and that de novo fatty acid biosynthetic components encoded in F. tularensis are transcriptionally active during infection in the mouse model of tularemia.

Global transcriptional responses to triclosan exposure in Pseudomonas aeruginosa

Global gene transcription was assessed by microarray experiments following treatment of a triclosan-susceptible Δ(mexAB-oprM) Pseudomonas aeruginosa strain with subinhibitory concentrations of triclosan. Expression patterns of selected genes were verified by quantitative real-time PCR analysis. The results showed that triclosan exposure had a profound effect on gene expression, affecting 44% of the genes present on the Affymetrix GeneChip(®), with 28% of genes being significantly upregulated and 16% being significantly downregulated in triclosan-treated cells. Genes encoding membrane proteins, transporters of small molecules, aspects of amino acid metabolism, and transcriptional regulators were significantly over-represented among the more strongly upregulated or downregulated genes in triclosan-treated cells. Quorum sensing-regulated genes were among the most strongly downregulated genes, presumably because of decreased acyl-acyl carrier protein pools and the resulting reduced acyl-homoserine lactone molecule synthesis. Surprisingly, iron homeostasis was completed perturbed in triclosan-exposed cells, with iron acquisition systems being strongly downregulated and iron storage systems significantly upregulated, thus mimicking conditions of excess iron. The profound perturbations of cellular metabolism via specific and global mechanisms may explain why triclosan is such a potent antimicrobial in susceptible bacteria.

Evolution of antibiotic resistance at non-lethal drug concentrations

Human use of antimicrobials in the clinic, community and agricultural systems has driven selection for resistance in bacteria. Resistance can be selected at antibiotic concentrations that are either lethal or non-lethal, and here we argue that selection and enrichment for antibiotic resistant bacteria is often a consequence of weak, non-lethal selective pressures - caused by low levels of antibiotics - that operates on small differences in relative bacterial fitness. Such conditions may occur during antibiotic therapy or in anthropogenically drug-polluted natural environments. Non-lethal selection increases rates of mutant appearance and promotes enrichment of highly fit mutants and stable mutators.

Impairment of the bacterial biofilm stability by triclosan

The accumulation of the widely-used antibacterial and antifungal compound triclosan (TCS) in freshwaters raises concerns about the impact of this harmful chemical on the biofilms that are the dominant life style of microorganisms in aquatic systems. However, investigations to-date rarely go beyond effects at the cellular, physiological or morphological level. The present paper focuses on bacterial biofilms addressing the possible chemical impairment of their functionality, while also examining their substratum stabilization potential as one example of an important ecosystem service. The development of a bacterial assemblage of natural composition – isolated from sediments of the Eden Estuary (Scotland, UK) – on non-cohesive glass beads (<63 µm) and exposed to a range of triclosan concentrations (control, 2 – 100 µg L⁻¹) was monitored over time by Magnetic Particle Induction (MagPI). In parallel, bacterial cell numbers, division rate, community composition (DGGE) and EPS (extracellular polymeric substances: carbohydrates and proteins) secretion were determined. While the triclosan exposure did not prevent bacterial settlement, biofilm development was increasingly inhibited by increasing TCS levels. The surface binding capacity (MagPI) of the assemblages was positively correlated to the microbial secreted EPS matrix. The EPS concentrations and composition (quantity and quality) were closely linked to bacterial growth, which was affected by enhanced TCS exposure. Furthermore, TCS induced significant changes in bacterial community composition as well as a significant decrease in bacterial diversity. The impairment of the stabilization potential of bacterial biofilm under even low, environmentally relevant TCS levels is of concern since the resistance of sediments to erosive forces has large implications for the dynamics of sediments and associated pollutant dispersal. In addition, the surface adhesive capacity of the biofilm acts as a sensitive measure of ecosystem effects.

Sublethal triclosan exposure decreases susceptibility to gentamicin and other aminoglycosides in Listeria monocytogenes

The human food-borne pathogen Listeria monocytogenes is capable of persisting in food processing plants despite cleaning and sanitation and is likely exposed to sublethal biocide concentrations. This could potentially affect susceptibility of the bacterium to biocides and other antimicrobial agents. The purpose of the present study was to determine if sublethal biocide concentrations affected antibiotic susceptibility in L. monocytogenes. Exposure of L. monocytogenes strains EGD and N53-1 to sublethal concentrations of Incimaxx DES (containing peroxy acids and hydrogen peroxide) and Triquart Super (containing quaternary ammonium compound) in four consecutive cultures did not alter the frequency of antibiotic-tolerant isolates, as determined by plating on 2× the MIC for a range of antibiotics. Exposure of eight strains of L. monocytogenes to 1 and 4 μg/ml triclosan did not alter triclosan sensitivity. However, all eight strains became resistant to gentamicin (up to 16-fold increase in MIC) after exposure to sublethal triclosan concentrations. Gentamicin-resistant isolates of strains N53-1 and 4446 were also resistant to other aminoglycosides, such as kanamycin, streptomycin, and tobramycin. Gentamicin resistance remained at a high level also after five subcultures without triclosan or gentamicin. Aminoglycoside resistance can be caused by mutations in the target site, the 16S rRNA gene. However, such mutations were not detected in the N53-1-resistant isolates. A combination of gentamicin and ampicillin is commonly used in listeriosis treatment. The triclosan-induced resistance is, hence, of great concern. Further investigations are needed to determine the molecular mechanisms underlying the effect of triclosan.