Rapid phenotypic stress-based microfluidic antibiotic susceptibility testing of Gram-negative clinical isolates

Antimicrobial resistance in plant endophytes associated with poultry-manure application revealed by selective culture and whole genome sequencing

Poultry manure is widely used as organic fertilizer in agriculture during the cultivation of crops, but the persistent high-level use of antibiotics in poultry production has raised concerns about the selection for reservoirs of antimicrobial resistance genes (ARGs). Previous studies have shown that the addition of poultry manure can increase the abundance of genes associated with resistance to tetracyclines, aminoglycosides, fluoroquinolones, sulfonamides, bacitracin, chloramphenicol, and macrolide-lincosamide-streptogramin in soil and plants. Understanding the microbial populations that harbor these ARGs is important to identify microorganisms that could enter the human food chain. Here, we test the hypothesis that environmental exposure to poultry manure increases the occurrence of antimicrobial resistance (AMR) in plant endophytes using selective culture, phenotypic Antibiotic Susceptibility Testing (AST), phylogenetic analysis, and whole genome sequencing (WGS). Endophytes from poultry manure treated Sorghum bicolor (L.) Moench plant root and stem samples showed increased phenotypic and genotypic resistance against multiple antibiotics compared to untreated controls. Comparison of AMR phenotype-to-genotype relationships highlighted the detection of multi-drug resistant (MDR) plant endophytes, demonstrating the value of genomic surveillance for emerging drug-resistant pathogens. The increased occurrence of ARGs in poultry manure-exposed endophytes highlights the need for responsible antibiotic use in poultry and animal farming to reduce contamination of ecological niches and transgression into endophytic plant microbiome compartments. It also emphasizes the requirement for proper manure management practices and vigilance in monitoring and surveillance efforts to tackle the growing problem of antibiotic resistance and preserve the efficacy of antibiotics for human and veterinary medicine.

Investigation of cefiderocol resistance prevalence and resistance mechanisms in carbapenem-resistant Pseudomonas aeruginosa, Germany 2019-21

Background

Cefiderocol, a novel siderophore cephalosporin, is a promising therapeutic option for infections caused by multidrug-resistant Pseudomonas aeruginosa. We evaluated the activity of cefiderocol against carbapenem-resistant P. aeruginosa (Cr-Pa) isolates and investigated the potential mechanisms involved in resistance.

Methods

108 CR-Pa isolates collected from patients without prior exposure to the substance were studied. MICs of cefiderocol were determined by broth microdilution using iron-depleted cation-adjusted Mueller-Hinton broth. Whole genome sequencing was performed to investigate the potential resistance mechanisms by comparing resistant and susceptible P. aeruginosa isolates and identifying unique mutations in the resistant group.

Results

Of the 108 isolates, nine were resistant to cefiderocol with MIC values ranging from 4 to 32 mg/L. The genetic analysis revealed a broad spectrum of mutations in the resistant isolates associated with iron uptake systems, efflux pumps, AmpC β-lactamase and penicillin-binding proteins. The most frequently observed mutations among the resistant isolates were located in fptA, fpvB and chtA. Notably, the presence of carbapenemases did not correlate with cefiderocol resistance.

Conclusions

Our findings show the low prevalence of cefiderocol resistance among CR-Pa isolates, showing its potential as an effective treatment option. However, the complex genetic landscape of resistance mechanisms, particularly mutations affecting iron transport and other TonB-dependent receptors, requires continuous monitoring and functional analyses to identify and manage potential resistance mechanisms. This study provides a foundation for future research to improve antimicrobial resistance prediction and develop targeted therapies against CR-Pa.

Microfluidic technologies for advanced antimicrobial susceptibility testing

Antimicrobial resistance is getting serious and becoming a threat to public health worldwide. The improper and excessive use of antibiotics is responsible for this situation. The standard methods used in clinical laboratories, to diagnose bacterial infections, identify pathogens, and determine susceptibility profiles, are time-consuming and labor-intensive, leaving the empirical antimicrobial therapy as the only option for the first treatment. To prevent the situation from getting worse, evidence-based therapy should be given. The choosing of effective drugs requires powerful diagnostic tools to provide comprehensive information on infections. Recent progress in microfluidics is pushing infection diagnosis and antimicrobial susceptibility testing (AST) to be faster and easier. This review summarizes the recent development in microfluidic assays for rapid identification and AST in bacterial infections. Finally, we discuss the perspective of microfluidic-AST to develop the next-generation infection diagnosis technologies.

Effects of Oxytetracycline/Lead Pollution Alone and in the Combined Form on Antibiotic Resistance Genes, Mobile Genetic Elements, and Microbial Communities in the Soil

The application of livestock manure is the leading cause of antibiotic and heavy metal pollution in agricultural soil. However, the effects of oxytetracycline (OTC) and lead (Pb) pollution in the single or combined form on antibiotic resistance genes (ARGs) in the soil need to be further studied. This study was planned to investigate the effects of OTC and Pb application on ARGs, mobile genetic elements (MGEs), and bacterial abundance in the soil. The relative abundance of ARGs and MGEs increased by 0.31-fold and 0.03-fold after the addition of 80 mg kg^-1 Pb to the soil, and by 0.49-fold and 0.03-fold after the addition of 160 mg kg^-1 Pb. In addition, under the premise of the existence of OTC, the inhibitory effect of a low concentration of Pb on ARG is stronger than that of a high concentration of Pb, resulting in a lower abundance of ARGs. The abundance of ARGs and MGEs increased by 0.11-fold and 0.17-fold after the addition of OTC (30 mg kg^-1) to the soil at a Pb concentration of 80 mg kg^-1 and by 0.18-fold and 0.04-fold at a Pb concentration of 160 mg kg^-1. The addition of OTC and Pb in the soil also decreased the many bacterial communities such as Bacteroidetes, Proteobacteria, Acidobacteria, and Firmicutes. Redundancy analysis (RDA) showed that organic matter content and pH were positively correlated with the abundance of ARGs and MGEs. At the same time, electrical conductivity (EC) had a negative correlation with the abundance of ARGs and MGEs in the soil. Intl1 was significantly associated with tetB, sul1, tetQ, sul2, and sul3. Network analysis illustrated that Actinobacteria, Bacteroidetes, and Proteobacteria were the main host bacteria causing changes in the abundance of ARGs and MGEs, and they were also predominant phylum in the culture environment. This conclusion can provide a reference for the related research of ARGs in soil.

Ti/PbO2 Electrode Efficiency in Catalytic Chloramphenicol Degradation and Its Effect on Antibiotic Resistance Genes

Livestock farming has led to the rapid accumulation of antibiotic resistance genes in the environment. Chloramphenicol (CAP) was chosen as a model compound to investigate its degradation during electrochemical treatment. Ti/PbO₂ electrodes were prepared using electrodeposition. The prepared Ti/PbO₂-La electrodes had a denser surface and a more complete PbO₂ crystal structure. Ti/PbO₂-Co electrodes exhibited improved electrochemical catalytic activity and lifetime in practice. The impact of different conditions on the effectiveness of CAP electrochemical degradation was investigated, and the most favorable conditions were identified (current density: I = 15.0 mA/cm, electrolyte concentration: c = 0.125 mol/L, solution pH = 5). Most importantly, we investigated the effects of the different stages of treatment with CAP solutions on the abundance of resistance genes in natural river substrates (intI1, cmlA, cmle3, and cata2). When CAP was completely degraded (100% TOC removal), no effect on resistance gene abundance was observed in the river substrate; incomplete CAP degradation significantly increased the absolute abundance of resistance genes. This suggests that when treating solutions with antibiotics, they must be completely degraded (100% TOC removal) before discharge into the environment to reduce secondary pollution. This study provides insights into the deep treatment of wastewater containing antibiotics and assesses the environmental impact of the resulting treated wastewater.

The Spread of Antibiotic Resistance Genes In Vivo Model

Infections caused by antibiotic-resistant bacteria are a major public health threat. The emergence and spread of antibiotic resistance genes (ARGs) in the environment or clinical setting pose a serious threat to human and animal health worldwide. Horizontal gene transfer (HGT) of ARGs is one of the main reasons for the dissemination of antibiotic resistance in vitro and in vivo environments. There is a consensus on the role of mobile genetic elements (MGEs) in the spread of bacterial resistance. Most drug resistance genes are located on plasmids, and the spread of drug resistance genes among microorganisms through plasmid-mediated conjugation transfer is the most common and effective way for the spread of multidrug resistance. Experimental studies of the processes driving the spread of antibiotic resistance have focused on simple in vitro model systems, but the current in vitro protocols might not correctly reflect the HGT of antibiotic resistance genes in realistic conditions. This calls for better models of how resistance genes transfer and disseminate in vivo. The in vivo model can better mimic the situation that occurs in patients, helping study the situation in more detail. This is crucial to develop innovative strategies to curtail the spread of antibiotic resistance genes in the future. This review aims to give an overview of the mechanisms of the spread of antibiotic resistance genes and then demonstrate the spread of antibiotic resistance genes in the in vivo model. Finally, we discuss the challenges in controlling the spread of antibiotic resistance genes and their potential solutions.

Nanotechnology Approaches for Rapid Detection and Theranostics of Antimicrobial Resistant Bacterial Infections

As declared by WHO, antimicrobial resistance (AMR) is a high priority issue with a pressing need to develop impactful technologies to curb it. The rampant and inappropriate use of antibiotics due to the lack of adequate and timely diagnosis is a leading cause behind AMR evolution. Unfortunately, populations with poor economic status and those residing in densely populated areas are the most affected ones, frequently leading to emergence of AMR pathogens. Classical approaches for AMR diagnostics like phenotypic methods, biochemical assays, and molecular techniques are cumbersome and resource-intensive and involve a long turnaround time to yield confirmatory results. In contrast, recent emergence of nanotechnology-assisted approaches helps to overcome challenges in classical approaches and offer simpler, more sensitive, faster, and more affordable solutions for AMR diagnostics. Nanomaterial platforms (metallic, quantum-dot, carbon-based, upconversion, etc.), nanoparticle-based rapid point-of-care platforms, nano-biosensors (optical, mechanical, electrochemical), microfluidic-assisted devices, and importantly, nanotheranostic devices for diagnostics with treatment of AMR infections are examples of rapidly growing nanotechnology approaches used for AMR management. This review comprehensively summarizes the past 10 years of research progress on nanotechnology approaches for AMR diagnostics and for estimating antimicrobial susceptibility against commonly used antibiotics. This review also highlights several bottlenecks in nanotechnology approaches that need to be addressed prior to considering their translation to clinics.

An antibiotic concentration gradient microfluidic device integrating surface-enhanced Raman spectroscopy for multiplex antimicrobial susceptibility testing

Antimicrobial susceptibility testing (AST) is a key measure in clinical microbiology laboratories to enable appropriate antimicrobial administration. During an AST, the determination of the minimum inhibitory concentration (MIC) is an important step in which the bacterial responses to an antibiotic at a series of concentrations obtained in separate bacterial growth chambers or sites are compared. However, the preparation of different antibiotic concentrations is time-consuming and labor-intensive. In this paper, we present a microfluidic device that generates a concentration gradient for antibiotics that is produced by diffusion in the laminar flow regime along a series of lateral microwells to encapsulate bacteria for antibiotic treatment. All the AST preparation steps (including bacterium loading, antibiotic concentration generation, buffer washing, and isolated bacterial growth with an antibiotic) can be performed in a single chip. The viable bacterial cells in each microwell after the antibiotic treatment are then quantified by their surface-enhanced Raman scattering (SERS) signals that are acquired after placing a uniform SERS-active substrate in contact with all the microwells. For proof-of-concept, we demonstrated the AST performance of this system on ampicillin (AMP)-susceptible and -resistant E. coli strains. Compared with the parameters for conventional AST methods, the AST procedure based on this chip requires only 20 μL of bacteria solution and 5 h of operation time. This result indicates that this integrated system can greatly shorten and simplify the tedious and labor-intensive procedures required for current standard AST methods.

Electrocatalytic inactivation of antibiotic resistant bacteria and control of antibiotic resistance dissemination risk

Antibiotic resistance in environmental matrices becomes urgently significant for public health and has been considered as an emerging environmental contaminant. In this work, the ampicillin-resistant Escherichia coli (AR E. coli) and corresponding resistance genes (bla_TEM-1) were effectively eliminated by the electrocatalytic process, and the dissemination risk of antibiotic resistance was also investigated. All the AR E. coli (∼8 log) was inactivated and 8.17 log bla_TEM-1 was degraded by the carbon nanotubes/agarose/titanium (CNTs/AG/Ti) electrode within 30 min. AR E. coli was inactivated mainly attributing to the damage of cell membrane, which was attacked by reactive oxygen species and subsequent leakage of intracellular cytoplasm. The bla_TEM-1 was degraded owing to the strand breaking in the process of electrocatalytic degradation. Furthermore, the dissemination risk of antibiotic resistance was effectively controlled after being electrocatalytic treatment. This study provided an effective electrocatalytic technology for the inactivation of antibiotic resistant bacteria and control of antibiotic resistance dissemination risk in the aqueous environment.

Biochar and Hyperthermophiles as Additives Accelerate the Removal of Antibiotic Resistance Genes and Mobile Genetic Elements during Composting

Sewage treatment plants are known as repositories of antibiotic resistance genes (ARGs). Adding biochar and inoculating with exogenous microbial agents are common ways to improve the quality of compost. However, little is known about the effects of these exogenous additives on the fate of ARGs during composting and the related mechanisms. In this study, municipal sludge was taken as the research object to study the ARG-removal effects of four composting methods: ordinary compost (CT), compost with hyperthermophiles (HT), compost with hyperthermophiles and 2.0% biochar (HT2C) and compost with hyperthermophiles and 5.0% biochar (HT5C). Real-time quantitative PCR (qPCR) and 16S rRNA high-throughput sequencing were conducted to analyze the ARGs, MGEs and bacterial community. After composting, the abundance of ARGs in CT was reduced by 72.7%, while HT, HT2C and HT5C were reduced by 80.7%, 84.3% and 84.8%, respectively. Treatments with different proportions of biochar added (HT2C, HT5C) had no significant effect on the abundance of ARGs. Network analysis showed that Firmicutes and Nitrospirae were positively associated with most ARGs and may be potential hosts for them. In addition, redundancy analysis further showed that the class 1 integrase gene (intI1), pH and organic carbon had a greater effect on ARGs. Our findings suggested that the combination of hyperthermophiles and biochar during the composting process was an effective way to control ARGs and mobile genetic elements (MGEs), thus inhibiting the spread and diffusion of ARGs in the environment and improving the efficiency of treating human and animal diseases.

A microfluidic microwell device operated by the automated microfluidic control system for surface-enhanced Raman scattering-based antimicrobial susceptibility testing

Bloodstream infection (BSI) is a serious public health issue worldwide. Timely and effective antibiotics for controlling infection are crucial towards patient outcomes. However, the current culture-based methods of identifying bacteria and antimicrobial susceptibility testing (AST) remain labor-intensive and time-consuming, and are unable to provide early support to physicians in critical hours. To improve the effectiveness of early antibiotic therapy, Surface-enhanced Raman scattering (SERS) technology, has been used in bacterial detection and AST based on its high specificity and label-free features. To simplify sample preparation steps in SERS-AST, we proposed an automated microfluidic control system to integrate all required procedures into a single device. Our preliminary results demonstrated the system can achieve on-chip reagent replacement, bacteria trapping, and buffer exchange. Finally, in-situ SERS-AST was performed within 3.5 h by loading isolates of ampicilin susceptible and resistant E. coli and clear discrimination of two strains under antibiotic treatment was demonstrated. Overall, our system can standardize and simplify the SERS-AST protocol and implicate parallel bacterial detection. This prototypical integration demonstrates timely microbiological support to optimize early antibiotic therapy for fighting bacteremia.

Nucleic acid amplification-based microfluidic approaches for antimicrobial susceptibility testing

Because of the global spread of antimicrobials, there is an urgent need to develop rapid and effective tools for antimicrobial susceptibility testing to help clinicians prescribe accurate and appropriate antibiotic doses sooner. The conventional methods for antimicrobial susceptibility testing are usually based on bacterial culture methods, which are time-consuming, complicated, and labor-intensive. Therefore, other approaches are needed to address these issues. Recently, microfluidic technology has gained significant attention in infection management due to its advantages including rapid detection, high sensitivity and specificity, highly automated assay, simplicity, low cost, and potential for point-of-care testing in low-resource areas. Microfluidic advances for antimicrobial susceptibility testing can be classified into phenotypic (usually culture-based) and genotypic tests. Genotypic antimicrobial susceptibility testing is the detection of resistant genes in a microorganism using methods such as nucleic acid amplification. This review (with 107 references) surveys the different forms of nucleic acid amplification-based microdevices used for genotypic antimicrobial susceptibility testing. The first section reviews the serious threat of antimicrobial-resistant microorganisms and the urgent need for fast check-ups. Next, several conventional antimicrobial susceptibility testing methods are discussed, and microfluidic technology as a promising candidate for rapid detection of antimicrobial-resistant microorganisms is briefly introduced. The next section highlights several advancements of microdevices, with an emphasis on their working principles and performance. The review concludes with the importance of fully integrated microdevices and a discussion on future perspectives.

Dynamic Laser Speckle Imaging Meets Machine Learning to Enable Rapid Antibacterial Susceptibility Testing (DyRAST)

Rapid antibacterial susceptibility testing (RAST) methods are of significant importance in healthcare, as they can assist caregivers in timely administration of the correct treatments. Various RAST techniques have been reported for tracking bacterial phenotypes, including size, shape, motion, and redox state. However, they still require bulky and expensive instruments-which hinder their application in resource-limited environments-and/or utilize labeling reagents which can interfere with antibiotics and add to the total cost. Furthermore, the existing RAST methods do not address the potential gradual adaptation of bacteria to antibiotics, which can lead to a false diagnosis. In this work, we present a RAST approach by leveraging machine learning to analyze time-resolved dynamic laser speckle imaging (DLSI) results. DLSI captures the change in bacterial motion in response to antibiotic treatments. Our method accurately predicts the minimum inhibitory concentration (MIC) of ampicillin and gentamicin for a model strain of Escherichia coli (E. coli K-12) in 60 min, compared to 6 h using the currently FDA-approved phenotype-based RAST technique. In addition to ampicillin (a β-lactam) and gentamicin (an aminoglycoside), we studied the effect of ceftriaxone (a third-generation cephalosporin) on E. coli K-12. The machine learning algorithm was trained and validated using the overnight results of a gold standard antibacterial susceptibility testing method enabling prediction of MIC with a similarly high accuracy yet substantially faster.

Bacterial Isolation Microwell-Plug (μWELLplug) for Rapid Antibiotic Susceptibility Testing Using Morphology Analysis

The rapid and accurate diagnosis of infectious diseases with high morbidity rates is crucial because it can minimize the misuse and overuse of antibiotics and increase survival rates in dreadful conditions. The conventional antibiotic susceptibility test (AST) systems used to choose appropriate antibiotics require long wait times to obtain results and cannot prevent the misuse or overuse of antibiotics by clinicians who need to quickly treat patients and cannot wait to identify the underlying cause of their symptoms. Therefore, several rapid AST (rAST) methods have been developed to provide quick test results, but they are complicated to operate, require additional equipment or materials, and give less accurate results than the conventional AST methods. In this study, we propose an rAST method that can obtain precise outcomes from a simple process with a short running time using a bacterial isolation microwell-plug (μWELLplug) in a conventional 96-well plate. The specifically designed hydrogel component of the μWELLplug provides a simple process for cell isolation and the observation of bacterial growth and morphological changes induced by a variety of antibiotic concentrations. The μWELLplug is placed over each well of the 96-well plate, and then bacterial or eukaryotic cells are isolated in the microwells and treated with different antibiotic concentrations to observe their effects. Saccharomyces cerevisiae (yeast, eukaryote), Streptomyces atratus (actinomycetes, prokaryote), Escherichia coli, Staphylococcus aureus, and methicillin-resistant S. aureus were cultivated and tested using the μWELLplug. The minimum inhibitory concentration values from this system were obtained in 3-4 h and correlated well with those from the conventional AST methods whose running time is 18-24 h.

Emerging technologies for antibiotic susceptibility testing

Superbugs such as infectious bacteria pose a great threat to humanity due to an increase in bacterial mortality leading to clinical treatment failure, lengthy hospital stay, intravenous therapy and accretion of bacteraemia. These disease-causing bacteria gain resistance to drugs over time which further complicates the treatment. Monitoring of antibiotic resistance is therefore necessary so that bacterial infectious diseases can be diagnosed rapidly. Antimicrobial susceptibility testing (AST) provides valuable information on the efficacy of antibiotic agents and their dosages for treatment against bacterial infections. In clinical laboratories, most widely used AST methods are disk diffusion, gradient diffusion, broth dilution, or commercially available semi-automated systems. Though these methods are cost-effective and accurate, they are time-consuming, labour-intensive, and require skilled manpower. Recently much attention has been on developing rapid AST techniques to avoid misuse of antibiotics and provide effective treatment. In this review, we have discussed emerging engineering AST techniques with special emphasis on phenotypic AST. These techniques include fluorescence imaging along with computational image processing, surface plasmon resonance, Raman spectra, and laser tweezer as well as micro/nanotechnology-based device such as microfluidics, microdroplets, and microchamber. The mechanical and electrical behaviour of single bacterial cell and bacterial suspension for the study of AST is also discussed.

Sequencing-based methods and resources to study antimicrobial resistance

Antimicrobial resistance extracts high morbidity, mortality and economic costs yearly by rendering bacteria immune to antibiotics. Identifying and understanding antimicrobial resistance are imperative for clinical practice to treat resistant infections and for public health efforts to limit the spread of resistance. Technologies such as next-generation sequencing are expanding our abilities to detect and study antimicrobial resistance. This Review provides a detailed overview of antimicrobial resistance identification and characterization methods, from traditional antimicrobial susceptibility testing to recent deep-learning methods. We focus on sequencing-based resistance discovery and discuss tools and databases used in antimicrobial resistance studies.

Microfluidics-Based Organism Isolation from Whole Blood: An Emerging Tool for Bloodstream Infection Diagnosis

The diagnosis of bloodstream infections presents numerous challenges, in part, due to the low concentration of pathogens present in the peripheral bloodstream. As an alternative to existing time-consuming, culture-based diagnostic methods for organism identification, microfluidic devices have emerged as rapid, high-throughput and integrated platforms for bacterial and fungal enrichment, detection, and characterization. This focused review serves to highlight and compare the emerging microfluidic platforms designed for the isolation of sepsis-causing pathogens from blood and suggest important areas for future research.

Recent Advances in the Race to Design a Rapid Diagnostic Test for Antimicrobial Resistance

Even with advances in antibiotic therapies, bacterial infections persistently plague society and have amounted to one of the most prevalent issues in healthcare today. Moreover, the improper and excessive administration of antibiotics has led to resistance of many pathogens to prescribed therapies, rendering such antibiotics ineffective against infections. While the identification and detection of bacteria in a patient's sample is critical for point-of-care diagnostics and in a clinical setting, the consequent determination of the correct antibiotic for a patient-tailored therapy is equally crucial. As a result, many recent research efforts have been focused on the development of sensors and systems that correctly guide a physician to the best antibiotic to prescribe for an infection, which can in turn, significantly reduce the instances of antibiotic resistance and the evolution of bacteria "superbugs." This review details the advantages and shortcomings of the recent advances (focusing from 2016 and onward) made in the developments of antimicrobial susceptibility testing (AST) measurements. Detection of antibiotic resistance by genomic AST techniques relies on the prediction of antibiotic resistance via extracted bacterial DNA content, while phenotypic determinations typically track physiological changes in cells and/or populations exposed to antibiotics. Regardless of the method used for AST, factors such as cost, scalability, and assay time need to be weighed into their design. With all of the expansive innovation in the field, which technology and sensing systems demonstrate the potential to detect antimicrobial resistance in a clinical setting?

Optimization of Stress-Based Microfluidic Testing for Methicillin Resistance in Staphylococcusaureus Strains

The rapid evolution of antibiotic resistance in bacterial pathogens is driving the development of innovative, rapid antibiotic susceptibility testing (AST) tools as a way to provide more targeted and timely antibiotic treatment. We have previously presented a stress-based microfluidic method for the rapid determination of antibiotic susceptibility in methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA). In this method, stress is used to potentiate the action of antibiotics, and cell death is measured as a proxy for susceptibility. The method allows antibiotic susceptibility to be determined within an hour from the start of the antibiotic introduction. However, the relatively low dynamic range of the signal (2–10% cell response) even with high antibiotic concentrations (10–50 µg/mL) left room for the method’s optimization. We have conducted studies in which we varied the flow patterns, the media composition, and the antibiotic concentration to increase the cell death response and concordantly decrease the required antibiotic concentration down to 1–3 µg/mL, in accordance with the Clinical and Laboratory Standards Institute’s (CLSI) guidelines for AST breakpoint concentrations.