Bacterial enzymatic resistance: beta-lactamases and aminoglycoside-modifying enzymes

Transition metal-based nanoparticles as potential antimicrobial agents: recent advancements, mechanistic, challenges, and future prospects

Bacterial transmission is considered one of the potential risks for communicable diseases, requiring promising antibiotics. Traditional drugs possess a limited spectrum of effectiveness, and their frequent administration reduces effectiveness and develops resistivity. In such a situation, we are left with the option of developing novel antibiotics with higher efficiency. In this regard, nanoparticles (NPs) may play a pivotal role in managing such medical situations due to their distinct physiochemical characteristics and impressive biocompatibility. Metallic NPs are found to possess extraordinary antibacterial effects that are useful in vitro as well as in vivo as self-modified therapeutic agents. Due to their wide range of antibacterial efficacy, they have potential therapeutic applications via diverse antibacterial routes. NPs not only restrict the development of bacterial resistance, but they also broaden the scope of antibacterial action without binding the bacterial cell directly to a particular receptor with promising effectiveness against both Gram-positive and Gram-negative microbes. This review aimed at exploring the most relevant types of metal NPs employed as antimicrobial agents, particularly those based on Mn, Fe, Co, Cu, and Zn metals, and their antimicrobial mechanisms. Further, the challenges and future prospects of NPs in biological applications are also discussed.

Efflux-Mediated bile Resistance in Gram-Positive Pathogens

Gram-positive pathogens are causing many serious infections that affect humans and result in mild to severe diseases worldwide. In order to survive and initiate infection, enteric pathogens must resist the physiochemical defence factors in the human intestinal tract. One of these defence factors is bile, a potent antibacterial like compound in the intestine. Efflux pumps are the important mechanism by which bacteria resist antibacterial agents such as bile. Efflux of antimicrobial substances outside the bacterial cell is considered as a key factor for intestinal colonization and virulence of enteric pathogens. This paper will review the research conducted on efflux–mediated bile resistance in Staphylococcus aureus, Listeria monocytogenes, Enterococcus faecalis and Clostridium perfringens. These bacteria colonize in the human & animal gastrointestinal tract and they have a multiple mechanism to resist the innate defences in the gut and antibacterial activity of bile. However, bile resistance in these bacteria is not fully understood. The evidence from this review suggests that Gram-positive pathogens have the ability to active transport of bile. Further research is needed to know how these pathogens sense bile and how bile regulates its virulence factor. In general, therefore, it seems that understanding the specific mechanism of bile resistance in enteric bacteria including gram-positive pathogens may involve in the development of novel strategies to control and treatment of gastrointestinal infections.

Antibacterial and Antibiotic-Modifying Activity of Methanol Extracts from Six Cameroonian Food Plants against Multidrug-Resistant Enteric Bacteria

The present work was designed to investigate the antibacterial activities of methanol extracts from six Cameroonian edible plants and their synergistic effects with some commonly used antibiotics against multidrug-resistant (MDR) Gram-negative bacteria expressing active efflux pumps. The extracts were subjected to qualitative phytochemical screening and the microdilution broth method was used for antibacterial assays. The results of phytochemical tests indicate that all tested crude extracts contained polyphenols, flavonoids, triterpenes, and steroids. Extracts displayed selective antibacterial activities with the minimal inhibitory concentration (MIC) values ranging from 32 to 1024 μg/mL. The lowest MIC value (32 μg/mL) was recorded with Coula edulis extract against E. coli AG102 and K. pneumoniae K2 and with Mangifera indica bark extract against P. aeruginosa PA01 and Citrus sinensis extract against E. coli W3110 which also displayed the best MBC (256 μg/mL) value against E. coli ATCC8739. In combination with antibiotics, extracts from M. indica leaves showed synergistic effects with 75% (6/8) of the tested antibiotics against more than 80% of the tested bacteria. The findings of the present work indicate that the tested plants may be used alone or in combination in the treatment of bacterial infections including the multidrug-resistant bacteria.

Searching for Novel Inhibitors of the S. aureus NorA Efflux Pump: Synthesis and Biological Evaluation of the 3-Phenyl-1,4-benzothiazine Analogues

Bacterial resistance to antimicrobial agents has become an increasingly serious health problem in recent years. Among the strategies by which resistance can be achieved, overexpression of efflux pumps such as NorA of Staphylococcus aureus leads to a sub-lethal concentration of the antibacterial agent at the active site that in turn may predispose the organism to the development of high-level target-based resistance. With an aim to improve both the chemical stability and potency of our previously reported 3-phenyl-1,4-benzothiazine NorA inhibitors, we replaced the benzothiazine core with different nuclei. None of the new synthesized compounds showed any appreciable intrinsic antibacterial activity, and, in particular, 2-(3,4-dimethoxyphenyl)quinoline (6 c) was able to decrease, in a concentration-dependent manner, the ciprofloxacin MIC against the norA-overexpressing strains S. aureus SA-K2378 (norA++) and SA-1199B (norA+/A116E GrlA).

QSAR Studies on Bacterial Efflux Pump Inhibitors

Antimicrobial drug resistance occurs when bacteria undergo certain modifications to eliminate the effectiveness of drugs, chemicals, or other agents designed to cure infections. To date, the burden of resistance has remained one of the major clinical concerns as it renders prolonged and complicated treatments, thereby increasing the medical costs with lengthier hospital stays. Of complex causes for bacterial resistance, there has been increasing evidence that proved the significant role of efflux pumps in antibiotic resistance. Coadministration of Efflux Pump Inhibitors (EPIs) with antibiotics has been considered one of the promising ways not only to improve the efficacy but also to extend the clinical utility of existing antibiotics. This chapter begins with outlining current knowledge about bacterial efflux pumps and drug designs applied in identification of their modulating compounds. Following, the chapter addresses and provides a discussion on Quantitative Structure-Activity Relationship (QSAR) analyses in search of novel and potent efflux pump inhibitors.

Resistance mechanisms

By definition, the terms sepsis and septic shock refer to a potentially fatal infectious state in which the early administration of an effective antibiotic is the most significant determinant of the outcome. Because of the global spread of resistant bacteria, the efficacy of antibiotics has been severely compromised. S. pneumonia, Escherichia coli (E. coli), Klebsiella, Acinetobacter, and Pseudomonas are the predominant pathogens of sepsis and septic shock. It is common for E. coli, Klebsiella, Acinetobacter and Pseudomonas to be resistant to multiple drugs. Multiple drug resistance is caused by the interplay of multiple resistance mechanisms those emerge via the acquisition of extraneous resistance determinants or spontaneous mutations. Extended-spectrum beta-lactamases (ESBLs), carbapenemases, aminoglycoside-modifying enzymes (AMEs) and quinolone resistance determinants are typically external and disseminate on mobile genetic elements, while porin-efflux mechanisms are activated by spontaneous modifications of inherited structures. Porin and efflux mechanisms are frequent companions of multiple drug resistance in Acinetobacter and P. aeruginosa, but only occasionally detected among E. coli and Klebsiella. Antibiotic resistance became a global health threat. This review examines the major resistance mechanisms of the leading microorganisms of sepsis.

QSAR Studies on Bacterial Efflux Pump Inhibitors

Antimicrobial drug resistance occurs when bacteria undergo certain modifications to eliminate the effectiveness of drugs, chemicals, or other agents designed to cure infections. To date, the burden of resistance has remained one of the major clinical concerns as it renders prolonged and complicated treatments, thereby increasing the medical costs with lengthier hospital stays. Of complex causes for bacterial resistance, there has been increasing evidence that proved the significant role of efflux pumps in antibiotic resistance. Coadministration of Efflux Pump Inhibitors (EPIs) with antibiotics has been considered one of the promising ways not only to improve the efficacy but also to extend the clinical utility of existing antibiotics. This chapter begins with outlining current knowledge about bacterial efflux pumps and drug designs applied in identification of their modulating compounds. Following, the chapter addresses and provides a discussion on Quantitative Structure-Activity Relationship (QSAR) analyses in search of novel and potent efflux pump inhibitors.

The saci_2123 gene of the hyperthermoacidophile Sulfolobus acidocaldarius encodes an ATP-binding cassette multidrug transporter

Multidrug resistance (MDR) transporters are capable of secreting structurally and functionally unrelated toxic compounds from the cell. Among this group are ATP-binding cassette (ABC) transporters. These membrane proteins are typically arranged as either hetero- or homo-dimers of ABC half-transporters with each subunit consisting of a membrane domain fused at the C-terminus to an ATP-binding domain, or as full transporters in which the two subunits are fused into a single polypeptide. The saci_2123 gene of the thermoacidophilic archaeon Sulfolobus acidocaldarius is the only gene in the genome that encodes an ATP-binding cassette half-transporter, while a homologous gene is present in the genomes of S. solfataricus, S. tokodaii and S islandicus. Saci_2123 shares homology with well-characterized bacterial and mammalian MDR transporters. The saci_2132 gene is up-regulated when cells are exposed to drugs. A deletion mutant of saci_2132 was found to be more vulnerable to a set of toxic compounds, including detergents, antibiotics and uncouplers as compared to the wild-type strain, while the drug resistance could be restored through the plasmid-based expression of saci_2132. These data demonstrate that Saci_2132 is an archaeal ABC-MDR transporter and therefore it was termed Smr1 (Sulfolobus multidrug resistance transporter 1).

Recent Advances in Multi-Drug Resistance (MDR) Efflux Pump Inhibitors of Gram-Positive Bacteria S. aureus

The paper focuses on recent achievements in the search for new chemical compounds able to inhibit multidrug resistance (MDR) mechanisms in Gram-positive pathogens. An analysis of the results of the search for new efflux pump inhibitors (EPIs) for Gram-positive bacteria, which have been performed over the last decade, indicates that almost all efforts are focused on the NorA (MFS) efflux pump in S. aureus. Considering the chemical structures of the NorA EPIs that have been identified, it can be observed that the most active agents belong to the families of compounds possessing conjugated double bonds, e.g., chalcones, piperine-like compounds, N-cinnamoylphenalkylamides or citral amide derivatives. Indole-, dihydronaphthyl-, 2-chloro-5-bromo-phenyl- or piperidine moieties seem to be profitable for the EPI properties, as well. These results, together with an increasing knowledge about a variety of efflux pumps that are involved in MDR of Gram-positive pathogens underline that further search for new EPIs should pay more attention to develop MDR efflux protein targets, including SMR, MATE, ABC or other members of the MFS family.

First report of mefA and msrA/msrB multidrug efflux pumps associated with blaTEM-1 β-lactamase in Enterococcus faecalis

OBJECTIVES

Enterococcus faecalis is thought to possess a great deal of intrinsic resistance to several antimicrobial agents. In this study we identified ampicillin- and erythromycin-resistant clinical isolates of E. faecalis and sought to identify the resistance mechanisms among these isolates.

METHODS

Twelve isolates of E. faecalis were collected from 12 different patients. Identification of the isolates and their susceptibility patterns were determined using the Phoenix automated phenotypic identification criteria. PCR amplification and sequencing were used to detect β-lactamase production. Colony blotting was performed in order to screen for multidrug efflux pump production. Extraction and N-terminal sequencing of the multidrug efflux pumps was carried out.

RESULTS

The E. faecalis isolates showed high resistance to erythromycin and ampicillin, with minimum inhibitory concentrations of >16 μg/ml. PCR amplification and sequencing showed that isolates produced TEM-1 β-lactamase. Colony blotting showed that these isolates harbored multidrug efflux pump genes. Multidrug efflux pump extraction, purification, and sequencing showed the distribution of mefA and msrA/msrB efflux pumps.

CONCLUSION

Two resistance mechanisms among E. faecalis are described - the production of TEM β-lactamase and mefA and msrA/msrB efflux pumps. These results are of great interest because this is the first report of the co-existence of these resistance mechanisms among E. faecalis strains.

Essential oils from Moroccan plants as potential chemosensitisers restoring antibiotic activity in resistant Gram-negative bacteria

Bacterial drug resistance is a worrying public health problem. Antibiotic efflux is a major non-specific resistance mechanism used by bacteria, and efflux pumps are involved in the low-level susceptibility of various important Gram-negative pathogens. Use of molecules that can block bacterial pumps is an attractive strategy, but several studies report only partial efficacy owing to limits of these molecules (stability, selectivity, bioavailability, toxicity, etc.). The objective of this study was to search for natural sources of molecules able to inhibit efflux pump systems of resistant Gram-negative bacteria (Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, Salmonella enterica serotype Typhimurium and Pseudomonas aeruginosa). The results indicate that the studied essential oils exhibit interesting activity against the tested bacteria. This activity was significantly enhanced in the presence of an efflux pump inhibitor such as phenylalanine arginyl β-naphthylamide (PAβN). The role of lipopolysaccharide (LPS) structure in the effect of essential oils was also reported in Salmonella LPS deep-rough mutants. In addition, essential oils of Thymus maroccanus and Thymus broussonetii, used at a low concentration (a fraction of the minimum inhibitory concentration), are able to significantly increase chloramphenicol susceptibility of several resistant isolates. These results demonstrate that these essential oils can alter efflux pump activity and may be attractive candidates to develop new drugs for chemosensitising multidrug-resistant strains to clinically used antibiotics.

From 6-aminoquinolone antibacterials to 6-amino-7-thiopyranopyridinylquinolone ethyl esters as inhibitors of Staphylococcus aureus multidrug efflux pumps

The thiopyranopyridine moiety was synthesized as a new heterocyclic base to be inserted at the C-7 position of selected quinolone nuclei followed by a determination of antibacterial activity against strains of Staphylococcus aureus. Selected thiopyranopyridinylquinolones showed significant antimicrobial activity, including strains having mutations in gyrA and grlA as well as other strains overexpressing the NorA multidrug (MDR) efflux pump. Most derivatives did not appear to be NorA substrates. The effect of the thiopyranopyridinyl substituent on making these quinolones poor substrates for NorA was investigated further. Several quinolone ester intermediates, devoid of any intrinsic antibacterial activity, were tested for their abilities to inhibit the activities of NorA (MFS family) and MepA (MATE family) S. aureus MDR efflux pumps. Selected quinolone esters were capable of inhibiting both MDR pumps more efficiently than the reference compound reserpine. Moreover, they also were able to restore, and even enhance, the activity of ciprofloxacin toward some genetically modified resistant S. aureus strains.

Synthesis and antimicrobial evaluation of amphiphilic neamine derivatives

The aminoglycoside antibiotics bind to the 16S bacterial rRNA and disturb the protein synthesis. One to four hydroxyl functions of the small aminoglycoside neamine were capped with phenyl, naphthyl, pyridyl, or quinolyl rings. The 3',4'- (6), 3',6- (7a), and the 3',4',6- (10a) 2-naphthylmethylene derivatives appeared to be active against sensitive and resistant Staphylococcus aureus strains. 10a also showed marked antibacterial activities against Gram (-) bacteria, including strains expressing enzymes modifying aminoglycosides, efflux pumps, or rRNA methylases. 7a and 10a revealed a weak and aspecific binding to a model bacterial 16S rRNA. Moreover, as compared to neomycin B, 10a showed a lower ability to decrease (3)H leucine incorporation into proteins in Pseudomonas aeruginosa. All together, our results suggest that the 3',4',6-tri-2-naphthylmethylene neamine derivative 10a should act against Gram (-) bacteria through a mechanism different from inhibition of protein synthesis, probably by membrane destabilization.

Molecular dynamic simulations of the metallo-beta-lactamase from Bacteroides fragilis in the presence and absence of a tight-binding inhibitor

The beta-lactam-based antibiotics are among the most prescribed and effective antibacterial agents. Widespread use of these antibiotics, however, has created tremendous pressure for the emergence of resistance mechanisms in bacteria. The most common cause of antibiotic resistance is bacterial production of actamases that efficiently degrade antibiotics. The metallo-beta-lactamases are of particular clinical concern due to their transference between bacterial strains. We used molecular dynamics (MD) simulations to further study the conformational changes that occur due to binding of an inhibitor to the dicanzinc metallo-beta-lactamase from Bacteroides fragilis. Our studies confirm previous findings that the major flap is a major source of plasticity within the active site, therefore its dynamic response should be considered in drug development. However, our results also suggest the need for care in using MD simulations in evaluating loop mobility, both due to relaxation times and to the need to accurately model the zinc active site. Our study also reveals two new robust responses to ligand binding. First, there are specific localized changes in the zinc active site--a local loop flip--due to ligand intercalation that may be critical to the function of this enzyme. Second, inhibitor binding perturbs the dynamics throughout the protein, without otherwise perturbing the enzyme structure. These dynamic perturbations radiate outward from the active site and their existence suggests that long-range communication and dynamics may be important in the activity of this enzyme.

From phenothiazine to 3-phenyl-1,4-benzothiazine derivatives as inhibitors of the Staphylococcus aureus NorA multidrug efflux pump

Overexpression of efflux pumps is an important mechanism by which bacteria evade effects of substrate antimicrobial agents and inhibition of such pumps is a promising strategy to circumvent this resistance mechanism. NorA is a Staphylococcus aureus multidrug efflux pump, the activity of which confers decreased susceptibility to many structurally unrelated agents, including fluoroquinolones, resulting in a multidrug resistant (MDR) phenotype. In this work, a series of 1,4-benzothiazine derivatives were designed and synthesized as a minimized structural template of phenothiazine MDR efflux pump inhibitors (EPIs) in an effort to identify more potent S. aureus NorA EPIs. Almost all derivatives evaluated showed good activity in combination with ciprofloxacin against S. aureus ATCC 25923; some were capable of completely restoring ciprofloxacin activity in a norA-overexpressing strain (SA-K2378). Compounds 6k and 7j displayed good activity against SA-1199B, a strain that also overexpresses norA, in an ethidium bromide (EtBr) efflux inhibition assay.

Characterization of an integron carrying blaIMP-1 and a new aminoglycoside resistance gene, aac(6')-31, and its dissemination among genetically unrelated clinical isolates in a Brazilian hospital

Seven bla(IMP-1)-harboring Acinetobacter sp. isolates and one Pseudomonas putida clinical isolate were recovered from hospitalized patients. All isolates possessed a class 1 integron, named In86, carrying the same cassette array [bla(IMP1), aac(6')-31, and aadA1], which was plasmid located in five of the isolates. This report describes the ability of nonfermentative nosocomial pathogens to acquire and disseminate antimicrobial resistance determinants.

Bacterial efflux systems and efflux pumps inhibitors

It is now well established that bacterial resistance to antibiotics has become a serious problem of public health that concerns almost all antibacterial agents and that manifests in all fields of their application. Among the three main mechanisms involved in bacterial resistance (target modification, antibiotic inactivation or default of its accumulation within the cell), efflux pumps, responsible for the extrusion of the antibiotic outside the cell, have recently received a particular attention. Actually, these systems, classified into five families, can confer resistance to a specific class of antibiotics or to a large number of drugs, thus conferring a multi-drug resistance (MDR) phenotype to bacteria. To face this issue, it is urgent to find new molecules active against resistant bacteria. Among the strategies employed, the search for inhibitors of resistance mechanisms seems to be attractive because such molecules could restore antibiotic activity. In the case of efflux systems, efflux pump inhibitors (EPIs) are expected to block the pumps and such EPIs, if active against MDR pumps, would be of great interest. This review will focus on the families of bacterial efflux systems conferring drug resistance, and on the EPIs that have been identified to restore antibiotic activity.

Fighting tuberculosis: An old disease with new challenges

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a leading cause of mortality worldwide into 21st century. The mortality and spread of this disease has further been aggravated because of synergy of this disease with HIV. A number of anti-TB drugs are ineffective against this disease because of development of resistance strains. Internationally efforts are being made to develop new anti-tubercular agents. A number of drug targets from cell wall biosynthesis, nucleic acid biosynthesis, and many other biosynthetic pathways are being unraveled throughout the world and are being utilized for drug development. In this review, socioeconomic problems in developing countries, efforts to control this disease in different individuals, the targets (known already and newly discovered), existing anti-tubercular agents including natural products and lead molecules, and the future prospects to develop new anti-TB agents are described.

In vitro and in vivo activities of AM-112, a novel oxapenem

AM-112 [(1'R,5R,6R)-3-(4-amino-1,1-dimethyl-butyl)-6-(1'-hydroxyethyl)oxapenem-3-carboxylate] is a novel oxapenem compound which possesses potent beta-lactamase-inhibitory properties. Fifty-percent inhibitory concentrations (IC(50)s) of AM-112 for class A enzymes were between 0.16 and 2.24 micro M for three enzymes, compared to IC(50)s of 0.008 to 0.12 micro M for clavulanic acid. Against class C and class D enzymes, however, the activity of AM-112 was between 1,000- and 100,000-fold greater than that of clavulanic acid. AM-112 had affinity for the penicillin-binding proteins (PBPs) of Escherichia coli DC0, with PBP2 being inhibited by the lowest concentration of AM-112 tested, 0.1 micro g/ml. Ceftazidime was combined with AM-112 at 1:1 and 2:1 ratios in MIC determination studies against a panel of beta-lactamase-producing organisms. These studies demonstrated that AM-112 was effective at protecting ceftazidime against extended-spectrum beta-lactamase-producing strains and derepressed class C enzyme producers, reducing ceftazidime MICs by 16- and 2,048-fold. Similar results were obtained when AM-112 was combined with ceftriaxone, cefoperazone, or cefepime in a 1:2 ratio. Protection of ceftazidime with AM-112 was maintained against Enterobacter cloacae P99 and Klebsiella pneumoniae SHV-5 in a murine intraperitoneal sepsis model. The 50% effective dose of ceftazidime against E. cloacae P99 and K. pneumoniae SHV-5 was reduced from >100 and 160 mg/kg of body weight to 2 and 33.6 mg/kg, respectively, when it was combined with AM-112 at a 1:1 ratio. AM-112 demonstrates potential as a new beta-lactamase inhibitor.