Newer developments in the treatment of Gram-positive infections

Oritavancin in vitro activity against Gram-positive organisms from European medical centers: a 10-year longitudinal overview from the SENTRY Antimicrobial Surveillance Program (2010-2019)

To assess oritavancin in vitro activity against clinically relevant Gram-positive pathogens in European (EU) hospitals, a total of 51,531 consecutive and unique clinical isolates collected in 2010-2019 were evaluated. All isolates were tested by CLSI broth microdilution methods. The key resistance phenotypes differed considerably between Eastern Europe (E-EU) and Western Europe (W-EU), respectively: methicillin-resistant (MR) Staphylococcus aureus 27.7%/22.9%; multidrug resistant (MDR) S. aureus, 19.7%/15.2%; MR coagulase-negative staphylococci, 77.3%/61.9%; vancomycin-resistant enterococci (E. faecium), 44.2%/20.9%; and MDR E. faecium, 63.8%/55.4%. There were no substantive differences in oritavancin minimum inhibitory concentration (MIC) values for the different species/organism groups over time or by EU region. Oritavancin inhibited 99.9% and 99.1% of all S. aureus and coagulase-negative staphylococci at 0.12 mg/L, respectively, and all isolates of E. faecalis and E. faecium at ≤0.5 mg/L. Oritavancin susceptibility rates against β-hemolytic and Viridans group streptococci isolates were 98.1% and 99.4%, respectively. Oritavancin had potent activity in vitro against this contemporary collection of European Gram-positive isolates from 2010 to 2019.

Library Screening for Synergistic Combinations of FDA-Approved Drugs and Metabolites with Vancomycin against VanA-Type Vancomycin-Resistant Enterococcus faecium

Antimicrobial resistance is a major public health threat, and there is an urgent need for new strategies to address this issue. In a recent study, a library screening strategy was developed in which an FDA-approved drug library was screened against methicillin-resistant Staphylococcus aureus (MRSA) in both its original (unmetabolized [UM]) and its human liver microsome metabolized (postmetabolized [PM]) forms and in the absence and presence of a resistant-to antibiotic. This allows the identification of agents with active metabolites and agents that can act synergistically with the resistant-to antibiotic. In this study, this strategy is applied to VanA-type vancomycin-resistant Enterococcus faecium (VREfm) in the absence and presence of vancomycin. Thirteen drugs with minimum MICs that were ≤12.5 μM under any tested condition (UM/PM vs. -/+vancomycin) were identified. Seven of these appeared to act synergistically with vancomycin, and follow-up checkerboard analyses confirmed synergy (∑FICmin ≤0.5) for six of these. Ultimately four rifamycins, two pleuromutilins, mupirocin, and linezolid were confirmed as synergistic. The most synergistic agent was rifabutin (∑FICmin = 0.19). Linezolid, a protein biosynthesis inhibitor, demonstrated relatively weak synergy (∑FICmin = 0.5). Only mupirocin showed significantly improved activity after microsomal metabolism, indicative of a more active metabolite, but efforts to identify an active metabolite were unsuccessful. Spectra of activity of several hits and related agents were also determined. Gemcitabine showed activity against a number vancomycin-resistant E. faecium and E. faecalis strains, but this activity was substantially weaker than previously observed in MRSA. IMPORTANCE Resistance to currently used antibiotics poses a serious threat to public health. This study reports a complete screen of 1,000 FDA-approved drugs and their metabolites against vancomycin-resistant Enterococcus faecium (VREfm) in both the absence and presence of vancomycin. This identified potentially synergistic combinations of FDA-approved drugs with vancomycin, and a number of these were confirmed in follow-up checkerboard assays. Among intrinsically active FDA-approved drugs, gemcitabine was identified as having activity against a panel of VRE strains.

Role of membrane proteins in bacterial resistance to antimicrobial peptides

Several natural antimicrobial peptides (AMPs), including the novel semisynthetic lipoglycopeptide antibiotics telavancin, dalbavancin, and oritavancin, have been approved for clinical use to address the growing problem of multiple antibiotic-resistant Gram-positive bacterial infections. Nevertheless, the efficacy of these antibiotics has already been compromised. The SARS-CoV-2 pandemic led to the increased clinical use of all antibiotics, further promoting the development of bacterial resistance. Therefore, it is critical to gain a deeper understanding of the role of resistance mechanisms to minimize the consequential risks of long-term antibiotic use and misuse. Here, we summarize for the first time the current knowledge of resistance mechanisms that have been shown to cause resistance to clinically used AMPs, with particular focus on membrane proteins that have been reported to interfere with the activity of AMPs by affecting the binding of AMPs to bacteria.

Effect of Vancomycin on Cytoplasmic Peptidoglycan Intermediates and van Operon mRNA Levels in VanA-Type Vancomycin-Resistant Enterococcus faecium

Resistance in VanA-type vancomycin-resistant Enterococcus faecium (VREfm) is due to an inducible gene cassette encoding seven proteins (vanRSHAXYZ). This provides for an alternative peptidoglycan (PG) biosynthesis pathway whereby D-Ala-D-Ala is replaced by D-Ala-d-lactate (Lac), to which vancomycin cannot bind effectively. This study aimed to quantify cytoplasmic levels of normal and alternative pathway PG intermediates in VanA-type VREfm by liquid chromatography-tandem mass spectrometry before and after vancomycin exposure and to correlate these changes with changes in vanA operon mRNA levels measured by real-time quantitative PCR (RT-qPCR). Normal pathway intermediates predominated in the absence of vancomycin, with low levels of alternative pathway intermediates. Extended (18-h) vancomycin exposure resulted in a mixture of the terminal normal (UDP-N-acetylmuramic acid [NAM]-l-Ala-D-Glu-l-Lys-D-Ala-D-Ala [UDP-Penta]) and alternative (UDP-NAM-l-Ala-γ-D-Glu-l-Lys-D-Ala-D-Lac [UDP-Pentadepsi]) pathway intermediates (2:3 ratio). Time course analyses revealed normal pathway intermediates responding rapidly (peaking in 3 to 10 min) and alternative pathway intermediates responding more slowly (peaking in 15 to 45 min). RT-qPCR demonstrated that vanA operon mRNA transcript levels increased rapidly after exposure, reaching maximal levels in 15 min. To resolve the effect of increased van operon protein expression on PG metabolite levels, linezolid was used to block protein biosynthesis. Surprisingly, linezolid dramatically reduced PG intermediate levels when used alone. When used in combination with vancomycin, linezolid only modestly reduced alternative UDP-linked PG intermediate levels, indicating substantial alternative pathway presence before vancomycin exposure. Comparison of PG intermediate levels between VREfm, vancomycin-sensitive Enterococcus faecium, and methicillin-resistant Staphylococcus aureus after vancomycin exposure demonstrated substantial differences between S. aureus and E. faecium PG biosynthesis pathways. IMPORTANCE VREfm is highly resistant to vancomycin due to the presence of a vancomycin resistance gene cassette. Exposure to vancomycin induces the expression of genes in this cassette, which encode enzymes that provide for an alternative PG biosynthesis pathway. In VanA-type resistance, these alternative pathway enzymes replace the D-Ala-D-Ala terminus of normal PG intermediates with D-Ala-D-Lac terminated intermediates, to which vancomycin cannot bind. While the general features of this resistance mechanism are well known, the details of the choreography between vancomycin exposure, vanA gene induction, and changes in the normal and alternative pathway intermediate levels have not been described previously. This study comprehensively explores how VREfm responds to vancomycin exposure at the mRNA and PG intermediate levels.

Oritavancin in vitro activity against gram-positive organisms from European and United States medical centers: results from the SENTRY Antimicrobial Surveillance Program for 2010-2014

The in vitro activity of oritavancin was assessed against 44,715 gram-positive pathogens causing infections in European and United States (US) hospitals (2010-2014). There were no substantive differences (>±2-fold dilution) in oritavancin MIC₅₀ or MIC₉₀ values for different species/organism groups over time or by region. Oritavancin (99.9% susceptible) showed modal MIC, MIC₅₀, and MIC₉₀ results of 0.03, 0.03, and 0.06-0.12 mg/L when tested against Staphylococcus aureus, regardless of methicillin susceptibility, year, or region. Coagulase-negative staphylococci from the US and Europe demonstrated equal MIC₅₀ values for oritavancin (MIC₅₀, 0.03 mg/L). Oritavancin inhibited 99.9% of Enterococcus faecalis and all E. faecium at ≤0.5 mg/L, including vancomycin-resistant isolates. Oritavancin exhibited MIC₅₀ results of 0.03 and ≤0.008 mg/L when tested against β-hemolytic and viridans group streptococci isolates, respectively, regardless of geographical region. Oritavancin maintained potent activity in vitro against this contemporary collection of European and US gram-positive isolates over 5 years (2010-2014).

RelA Mutant Enterococcus faecium with Multiantibiotic Tolerance Arising in an Immunocompromised Host

Serious bacterial infections in immunocompromised patients require highly effective antibacterial therapy for cure, and thus, this setting may reveal novel mechanisms by which bacteria circumvent antibiotics in the absence of immune pressure. Here, an infant with leukemia developed vancomycin-resistant Enterococcus faecium (VRE) bacteremia that persisted for 26 days despite appropriate antibiotic therapy. Sequencing of 22 consecutive VRE isolates identified the emergence of a single missense mutation (L152F) in relA, which constitutively activated the stringent response, resulting in elevated baseline levels of the alarmone guanosine tetraphosphate (ppGpp). Although the mutant remained susceptible to both linezolid and daptomycin in clinical MIC testing and during planktonic growth, it demonstrated tolerance to high doses of both antibiotics when growing in a biofilm. This biofilm-specific gain in resistance was reflected in the broad shift in transcript levels caused by the mutation. Only an experimental biofilm-targeting ClpP-activating antibiotic was able to kill the mutant strain in an established biofilm. The relA mutation was associated with a fitness trade-off, forming smaller and less-well-populated biofilms on biological surfaces. We conclude that clinically relevant relA mutations can emerge during prolonged VRE infection, causing baseline activation of the stringent response, subsequent antibiotic tolerance, and delayed eradication in an immunocompromised state.

The increasing prevalence of antibiotic-resistant bacterial pathogens is a major challenge currently facing the medical community. Such pathogens are of particular importance in immunocompromised patients as these individuals may favor emergence of novel resistance determinants due to lack of innate immune defenses and intensive antibiotic exposure. During the course of chemotherapy, a patient developed prolonged bacteremia with vancomycin-resistant Enterococcus faecium that failed to clear despite multiple front-line antibiotics. The consecutive bloodstream isolates were sequenced, and a single missense mutation identified in the relA gene, the mediator of the stringent response. Strains harboring the mutation had elevated baseline levels of the alarmone and displayed heightened resistance to the bactericidal activity of multiple antibiotics, particularly in a biofilm. Using a new class of compounds that modulate ClpP activity, the biofilms were successfully eradicated. These data represent the first clinical emergence of mutations in the stringent response in vancomycin-resistant entereococci.

A liquid chromatography-tandem mass spectrometry assay for d-Ala-d-Lac: a key intermediate for vancomycin resistance in vancomycin-resistant enterococci

Vancomycin exerts its antibacterial activity by binding to d-Ala-d-Ala in bacterial cell wall precursors. Vancomycin resistance in vancomycin-resistant enterococci (VRE) is due to an alternative cell wall biosynthesis pathway in which d-Ala-d-Ala is replaced, most commonly by d-Ala-d-Lac. In this study, we extend our recently developed Marfey's derivatization-based liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for l-Ala, d-Ala, and d-Ala-d-Ala to d-Ala-d-Lac and apply it to the quantitation of these metabolites in VRE. The first step in this effort was the development of an effective washing method for removing medium components from VRE cells. Mar-d-Ala-d-Lac was well resolved chromatographically from Mar-d-Ala-d-Ala, a prerequisite for MS/MS quantitation of d-Ala-d-Ala and d-Ala-d-Lac. Mar-d-Ala-d-Lac gave similar detection parameters, sensitivity, and linearity as Mar-d-Ala-d-Ala. l-Ala, d-Ala, d-Ala-d-Ala, and d-Ala-d-Lac levels in VRE were then determined in the presence of variable vancomycin levels. Exposure to vancomycin resulted in a dramatic reduction of d-Ala-d-Ala, with a response midpoint at approximately 0.06μg/ml vancomycin and with a broad response profile up to 128μg/ml vancomycin. In contrast, d-Ala-d-Lac was present in the absence of vancomycin, with its level constant up to 128μg/ml vancomycin. This method will be useful for the discovery, characterization, and refinement of new agents targeting vancomycin resistance in VRE.

Nano-technology for targeted drug delivery to combat antibiotic resistance

Several microbes have evolved clinically significant resistance against almost every available antibiotic. Yet the development of new classes of antibiotics has lagged far behind our growing need. Frequent and suboptimal use of antibiotics particularly in developing countries aggravated the problem by increasing the rate of resistance. Therefore, developing new and multidimensional strategies to combat microbial infections is warranted. These include i) modification of existing antibiotics, ii) searching new and novel antibiotics, iii) development and improvement of antibiotics carrier system to reduce amount and frequency of antibiotic doses, iv) development of targeted antibiotic delivery systems. Here, the authors discuss trends and development of nano-materials and alternative antimicrobials to solve the problem of antibiotic resistance.

The antibiotic R&D pipeline: an update

There is an urgent need for new antibacterials to target emerging multidrug-resistant bacteria. The need for such agents is rising while the efforts in antibacterial research have declined dramatically in the past few decades with the result of only four compounds belonging to new chemical classes being approved for clinical use. The main reasons that led to this critical situation are shortly described. A renewed interest in the research of new effective antimicrobials is nonetheless delivering compounds deriving mainly from modification of existing drugs, yet new chemical classes are appearing. Because many of these activities have started relatively recently, we should expect a long period before new antibiotics are added to the medical armamentarium.

Linezolid, the first oxazolidinone antibacterial agent

Linezolid (Zyvox) is the first member of an entirely new class of antibiotics to reach the market in over 35 years; it was approved for use in 2000. A member of the oxazolidinone class of antibiotics, linezolid is highly effective for the treatment of serious Gram-positive infections and has activity that compares favorably with vancomycin for most clinically relevant pathogens. Zyvox is approved for use against serious Gram-positive infections, including those caused by Streptococcus pneumoniae, and the very challenging methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium organisms. Zyvox inhibits bacterial protein synthesis by binding to 23S rRNA in the catalytic site of the 50S ribosome. It can be administered both orally and intravenously and has good tissue distribution. Recent results have demonstrated that oxazolidinone analogs related to linezolid are effective in treating pulmonary tuberculosis caused by resistant Mycobacterium tuberculosis in animal infection models and suggest additional new therapeutic applications for these antibiotics.

Future challenges and treatment of Staphylococcus aureus bacteremia with emphasis on MRSA

Staphylococcus aureus bacteremia (SAB) is an urgent medical problem due to its growing frequency and its poor associated outcome. As healthcare delivery increasingly involves invasive procedures and implantable devices, the number of patients at risk for SAB and its complications is likely to grow. Compounding this problem is the growing prevalence of methicillin-resistant S. aureus (MRSA) and the dwindling efficacy of vancomycin, long the treatment of choice for this pathogen. Despite the recent availability of several new antibiotics for S. aureus, new strategies for treatment and prevention are required for this serious, common cause of human infection.