Antibiotic Resistance to Molecules Commonly Prescribed for the Treatment of Antibiotic-Resistant Gram-Positive Pathogens: What Is Relevant for the Clinician?

Antibiotic resistance in Gram-positive pathogens is a relevant concern, particularly in the hospital setting. Several antibiotics are now available to treat these drug-resistant pathogens, such as daptomycin, dalbavancin, linezolid, tedizolid, ceftaroline, ceftobiprole, and fosfomycin. However, antibiotic resistance can also affect these newer molecules. Overall, this is not a frequent phenomenon, but it is a growing concern in some settings and can compromise the effectiveness of these molecules, leaving few therapeutic options. We reviewed the available evidence about the epidemiology of antibiotic resistance to these antibiotics and the main molecular mechanisms of resistance, particularly methicillin-resistant Sthaphylococcus aureus, methicillin-resistant coagulase-negative staphylococci, vancomycin-resistant Enterococcus faecium, and penicillin-resistant Streptococcus pneumoniae. We discussed the interpretation of susceptibility tests when minimum inhibitory concentrations are not available. We focused on the risk of the emergence of resistance during treatment, particularly for daptomycin and fosfomycin, and we discussed the strategies that can be implemented to reduce this phenomenon, which can lead to clinical failure despite appropriate antibiotic treatment. The judicious use of antibiotics, epidemiological surveillance, and infection control measures is essential to preserving the efficacy of these drugs.

A Review on Five and Six-Membered Heterocyclic Compounds Targeting the Penicillin-Binding Protein 2 (PBP2A) of Methicillin-Resistant Staphylococcus aureus (MRSA)

Staphylococcus aureus is a common human pathogen. Methicillin-resistant Staphylococcus aureus (MRSA) infections pose significant and challenging therapeutic difficulties. MRSA often acquires the non-native gene PBP2a, which results in reduced susceptibility to β-lactam antibiotics, thus conferring resistance. PBP2a has a lower affinity for methicillin, allowing bacteria to maintain peptidoglycan biosynthesis, a core component of the bacterial cell wall. Consequently, even in the presence of methicillin or other antibiotics, bacteria can develop resistance. Due to genes responsible for resistance, S. aureus becomes MRSA. The fundamental premise of this resistance mechanism is well-understood. Given the therapeutic concerns posed by resistant microorganisms, there is a legitimate demand for novel antibiotics. This review primarily focuses on PBP2a scaffolds and the various screening approaches used to identify PBP2a inhibitors. The following classes of compounds and their biological activities are discussed: Penicillin, Cephalosporins, Pyrazole-Benzimidazole-based derivatives, Oxadiazole-containing derivatives, non-β-lactam allosteric inhibitors, 4-(3H)-Quinazolinones, Pyrrolylated chalcone, Bis-2-Oxoazetidinyl macrocycles (β-lactam antibiotics with 1,3-Bridges), Macrocycle-embedded β-lactams as novel inhibitors, Pyridine-Coupled Pyrimidinones, novel Naphthalimide corbelled aminothiazoximes, non-covalent inhibitors, Investigational-β-lactam antibiotics, Carbapenem, novel Benzoxazole derivatives, Pyrazolylpyridine analogues, and other miscellaneous classes of scaffolds for PBP2a. Additionally, we discuss the penicillin-binding protein, a crucial target in the MRSA cell wall. Various aspects of PBP2a, bacterial cell walls, peptidoglycans, different crystal structures of PBP2a, synthetic routes for PBP2a inhibitors, and future perspectives on MRSA inhibitors are also explored.

Penicillin binding protein 2a: An overview and a medicinal chemistry perspective

Antimicrobial resistance is an imminent threat worldwide. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the "superbug" family, manifesting resistance through the production of a penicillin binding protein, PBP2a, an enzyme that provides its transpeptidase activity to allow cell wall biosynthesis. PBP2a's low affinity to most β-lactams, confers resistance to MRSA against numerous members of this class of antibiotics. An Achilles' heel of MRSA, PBP2a represents a substantial target to design novel antibiotics to tackle MRSA threat via inhibition of the bacterial cell wall biosynthesis. In this review we bring into focus the PBP2a enzyme and examine the various aspects related to its role in conferring resistance to MRSA strains. Moreover, we discuss several antibiotics and antimicrobial agents designed to target PBP2a and their therapeutic potential to meet such a grave threat. In conclusion, we consider future perspectives for targeting MRSA infections.

The QM/MM-QTAIM approach reveals the nature of the different reactivity of cephalosporins in the active site of L1 metallo-β-lactamase

We combine the QM/MM and the QTAIM approaches to predict the reactivity of cephalosporins in the active site of L1 metallo-β-lactamase.

Disruption of allosteric response as an unprecedented mechanism of resistance to antibiotics

Ceftaroline, a recently approved β-lactam antibiotic for treatment of infections by methicillin-resistant Staphylococcus aureus (MRSA), is able to inhibit penicillin-binding protein 2a (PBP2a) by triggering an allosteric conformational change that leads to the opening of the active site. The opened active site is now vulnerable to inhibition by a second molecule of ceftaroline, an event that impairs cell-wall biosynthesis and leads to bacterial death. The triggering of the allosteric effect takes place by binding of the first antibiotic molecule 60 Å away from the active site of PBP2a within the core of the allosteric site. We document, by kinetic studies and by determination of three X-ray structures of the mutant variants of PBP2a that result in resistance to ceftaroline, that the effect of these clinical mutants is the disruption of the allosteric trigger in this important protein in MRSA. This is an unprecedented mechanism for antibiotic resistance.

Development of new drugs for an old target: the penicillin binding proteins

The widespread use of β-lactam antibiotics has led to the worldwide appearance of drug-resistant strains. Bacteria have developed resistance to β-lactams by two main mechanisms: the production of β-lactamases, sometimes accompanied by a decrease of outer membrane permeability, and the production of low-affinity, drug resistant Penicillin Binding Proteins (PBPs). PBPs remain attractive targets for developing new antibiotic agents because they catalyse the last steps of the biosynthesis of peptidoglycan, which is unique to bacteria, and lies outside the cytoplasmic membrane. Here we summarize the “current state of the art” of non-β-lactam inhibitors of PBPs, which have being developed in an attempt to counter the emergence of β-lactam resistance. These molecules are not susceptible to hydrolysis by β-lactamases and thus present a real alternative to β-lactams. We present transition state analogs such as boronic acids, which can covalently bind to the active serine residue in the catalytic site. Molecules containing ring structures different from the β-lactam-ring like lactivicin are able to acylate the active serine residue. High throughput screening methods, in combination with virtual screening methods and structure based design, have allowed the development of new molecules. Some of these novel inhibitors are active against major pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and thus open avenues new for the discovery of novel antibiotics.

Microtiter plate-based assay for inhibitors of penicillin-binding protein 2a from methicillin-resistant Staphylococcus aureus

Penicillin-binding protein 2a (PBP2a), the molecular determinant for high-level β-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA), is intrinsically resistant to most β-lactam antibiotics. The development and characterization of new inhibitors targeting PBP2a would benefit from an effective and convenient assay for inhibitor binding. This study was directed toward the development of a fluorescently detected β-lactam binding assay for PBP2a from MRSA. Biotinylated ampicillin and biotinylated cephalexin were tested as tagging reagents for fluorescence detection by using a streptavidin-horseradish peroxidase conjugate. Both bound surprisingly well to PBP2a, with binding constants of 1.6 ± 0.4 μM and 13.6 ± 0.8 μM, respectively. Two forms of the assay were developed, a one-step direct competition form of the assay and a two-step indirect competition form of the assay, and both forms of the assay gave comparable results. This assay was then used to characterize PBP2a binding to ceftobiprole, which gave results consistent with previous studies of ceftobiprole-PBP2a binding. This assay was also demonstrated for screening for PBP2a inhibitors by screening a set of 13 randomly selected β-lactams for PBP2a inhibition at 750 μM. Meropenem was observed to give substantial inhibition in this screen, and a follow-up titration experiment determined its apparent K(i) to be 480 ± 70 μM. The availability of convenient and sensitive microtiter-plate based assays for the screening and characterization of PBP2a inhibitors is expected to facilitate the discovery and development of new PBP2a inhibitors for use in combating the serious public health problem posed by MRSA.

Recombinant antimicrobial peptide hPAB-β expressed in Pichia pastoris, a potential agent active against methicillin-resistant Staphylococcus aureus

As a potential therapeutic agent, antimicrobial peptide has received increased attention in recent years. However, high-level expression of a small peptide with antimicrobial activity is still a challenging task. In this study, the coding sequence of antimicrobial peptide hPAB-β, a variant derived from human beta-defensin 2, was cloned into pPIC9K vector and transformed into Pichia pastoris. P. pastoris transformants harbored with multi-copy plasmids were screened by G418 selection. When the transformed cells were induced by methanol, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blot, and matrix-assisted laser desorption ionization-time of flight mass spectrometry revealed recombinant hPAB-β products consisting of three protein species of 4,680.4, 4,485.3, and 4,881.9 Da at proportions of 58%, 36%, and 6%, respectively, which may be due to the incomplete processing of the fusion signal peptide of α-factor by the STE13 protease. Expressed hPAB-β was secreted into the culture medium at a level of 241.2 ± 29.5 mg/L. Purified hPAB-β with 95% homogeneity was obtained by 10 kDa membrane filtration followed by cation ion-exchange chromatography with a SP-Sepharose XL column. The two major protein species separated through a SOURCE 30RPC reverse phase chromatography column showed definite antimicrobial activities against Staphylococcus aureus. All 22 methicillin-resistant S. aureus (MRSA) isolates with multidrug resistance phenotype were sensitive to the recombinant hPAB-β with minimal inhibitory concentrations of 8-64 μg/ml. Our results show that the methylotrophic yeast-inducible system is suitable for high-level expression of active hPAB-β, and that expressed hPAB-β in P. pastoris may be a potential antimicrobial agent against MRSA infection.

Efficacy and safety of linezolid in methicillin-resistant Staphylococcus aureus (MRSA) complicated skin and soft tissue infection (cSSTI): a meta-analysis

OBJECTIVE

To evaluate the clinical and microbiological outcomes of linezolid versus vancomycin in methicillin-resistant Staphylococcus aureus (MRSA) complicated skin and soft-tissue infection (cSSTI) using a meta-analysis.

RESEARCH DESIGN AND METHODS

Clinical trials were identified using PubMed, the Cochrane Central Register of Controlled Trials, and the International Pharmaceutical Abstracts from inception to March 2009. Primary outcomes evaluated resolution of signs and symptoms of infection in clinically evaluable (CE) patients, and microbiological eradication in both the modified intent-to-treat (MITT) and MRSA evaluable (MRSA ME) patients. MITT patients had a culture-confirmed Gram-positive pathogen (S. aureus) at baseline. Secondary outcomes included mortality, adverse drug reactions (ADR), and discontinuation due to ADRs. The Mantel-Haenszel odds ratios (OR) with 95% confidence intervals (CI) were calculated using the fixed effects model based on the assumption that there was little to no heterogeneity between the studies in the primary analysis. Sensitivity analyses were performed for each outcome by removing the most weighted study.

RESULTS

Five studies with a total of 2652 patients (1361/linezolid; 1291/vancomycin) were identified. Resolution of infection in CE patients (OR = 1.41; 95% CI: 1.03, 1.95) and MITT patients (OR = 1.91; 95% CI: 1.33, 2.76) favored the use of linezolid over vancomycin, but did not remain significant after sensitivity analyses (CE: OR = 1.29; 95% CI: 0.81, 2.05; MITT: OR = 1.73; 95% CI: 0.87, 3.41). Microbiological eradication in MRSA ME patients consistently favored the use of linezolid over vancomycin (OR = 2.90; 95% CI: 1.90, 4.41). No difference in mortality was observed between the two groups (OR = 1.17; 95% CI: 0.85, 1.62). Higher proportions of linezolid patients were found to have diarrhea (119/1361 vs. 52/1291), nausea (102/1361 vs. 46/1291) and thrombocytopenia (54/1121 vs. 8/1071), whereas a higher proportion of vancomycin patients were found to have renal insufficiency compared to linezolid (16/634 vs. 4/703). Inconsistent results were seen with the incidence of anemia and rash between the base-case (anemia: OR = 1.36; 95% CI: 0.90, 2.08; rash: OR = 0.26; 95% CI: 0.10, 0.68) and sensitivity analyses (anemia: OR = 2.33; 95% CI: 1.24, 4.37; rash: OR = 0.57; 95% CI: 0.12, 2.71). Discontinuation due to ADRs was not statistically different between linezolid and vancomycin (OR = 1.06; 95% CI: 0.75, 1.51).

CONCLUSION

Resolution of infection in CE and MITT patients were inconsistent; however, a sub-analysis revealed that linezolid was more likely to consistently achieve microbiologic eradication in MRSA ME patients. Apparent risks of thrombocytopenia, nausea, diarrhea, and possibly anemia may limit linezolid use in treating MRSA cSSTI. This study was limited due to an inability to assess for the effects of hetero-resistance and appropriate vancomycin dosing on outcomes. Moreover, the small number of studies made controlling for heterogeneity challenging.

Bench-to-bedside review: Understanding the impact of resistance and virulence factors on methicillin-resistant Staphylococcus aureus infections in the intensive care unit

Methicillin-resistant Staphylococcus aureus (MRSA) displays a remarkable array of resistance and virulence factors, which have contributed to its prominent role in infections of the critically ill. We are beginning to understand the function and regulation of some of these factors and efforts are ongoing to better characterize the complex interplay between the microorganism and host response. It is important that clinicians recognize the changing resistance patterns and epidemiology of Staphylococcus spp., as these factors may impact patient outcomes. Community-associated MRSA clones have emerged as an increasingly important subset of Staphyloccocus aureus and MRSA can no longer be considered as solely a nosocomial pathogen. When initiating empiric antibiotics, it is of vital importance that this therapy be timely and appropriate, as delays in treatment are associated with adverse outcomes. Although vancomycin has long been considered a first-line therapy for serious MRSA infections, multiple concerns with this agent have opened the door for existing and investigational agents demonstrating efficacy in this role.