The cost-effectiveness of procalcitonin for guiding antibiotic prescribing in individuals hospitalized with COVID-19: part of the PEACH study.

BACKGROUND: Many hospitals introduced procalcitonin (PCT) testing to help diagnose bacterial coinfection in individuals with COVID-19, and guide antibiotic decision-making during the COVID-19 pandemic in the UK. OBJECTIVES: Evaluating cost-effectiveness of using PCT to guide antibiotic decisions in individuals hospitalized with COVID-19, as part of a wider research programme. METHODS: Retrospective individual-level data on patients hospitalized with COVID-19 were collected from 11 NHS acute hospital Trusts and Health Boards from England and Wales, which varied in their use of baseline PCT testing during the first COVID-19 pandemic wave. A matched analysis (part of a wider analysis reported elsewhere) created groups of patients whose PCT was/was not tested at baseline. A model was created with combined decision tree/Markov phases, parameterized with quality-of-life/unit cost estimates from the literature, and used to estimate costs and quality-adjusted life years (QALYs). Cost-effectiveness was judged at a £20 000/QALY threshold. Uncertainty was characterized using bootstrapping. RESULTS: People who had baseline PCT testing had shorter general ward/ICU stays and spent less time on antibiotics, though with overlap between the groups' 95% CIs. Those with baseline PCT testing accrued more QALYs (8.76 versus 8.62) and lower costs (£9830 versus £10 700). The point estimate was baseline PCT testing being dominant over no baseline testing, though with uncertainty: the probability of cost-effectiveness was 0.579 with a 1 year horizon and 0.872 with a lifetime horizon. CONCLUSIONS: Using PCT to guide antibiotic therapy in individuals hospitalized with COVID-19 is more likely to be cost-effective than not, albeit with uncertainty.

A retrospective propensity-score-matched cohort study of the impact of procalcitonin testing on antibiotic use in hospitalized patients during the first wave of COVID-19.

BACKGROUND: Procalcitonin (PCT) is a blood marker used to help diagnose bacterial infections and guide antibiotic treatment. PCT testing was widely used/adopted during the COVID-19 pandemic in the UK. OBJECTIVES: Primary: to measure the difference in length of early (during first 7 days) antibiotic prescribing between patients with COVID-19 who did/did not have baseline PCT testing during the first wave of the pandemic. Secondary: to measure differences in length of hospital/ICU stay, mortality, total days of antibiotic prescribing and resistant bacterial infections between these groups. METHODS: Multi-centre, retrospective, observational, cohort study using patient-level clinical data from acute hospital Trusts/Health Boards in England/Wales. Inclusion: patients ≥16 years, admitted to participating Trusts/Health Boards and with a confirmed positive COVID-19 test between 1 February 2020 and 30 June 2020. RESULTS: Data from 5960 patients were analysed: 1548 (26.0%) had a baseline PCT test and 4412 (74.0%) did not. Using propensity-score matching, baseline PCT testing was associated with an average reduction in early antibiotic prescribing of 0.43 days [95% confidence interval (CI): 0.22-0.64 days, P < 0.001) and of 0.72 days (95% CI: 0.06-1.38 days, P = 0.03] in total antibiotic prescribing. Baseline PCT testing was not associated with increased mortality or hospital/ICU length of stay or with the rate of antimicrobial-resistant secondary bacterial infections. CONCLUSIONS: Baseline PCT testing appears to have been an effective antimicrobial stewardship tool early in the pandemic: it reduced antibiotic prescribing without evidence of harm. Our study highlights the need for embedded, rapid evaluations of infection diagnostics in the National Health Service so that even in challenging circumstances, introduction into clinical practice is supported by evidence for clinical utility. STUDY REGISTRATION NUMBER: ISRCTN66682918.

Diagnostic MALDI-TOF MS can differentiate between high and low toxic Staphylococcus aureus bacteraemia isolates as a predictor of patient outcome.

Staphylococcus aureus bacteraemia (SAB) is a major cause of blood-stream infection (BSI) in both healthcare and community settings. While the underlying comorbidities of a patient significantly contributes to their susceptibility to and outcome following SAB, recent studies show the importance of the level of cytolytic toxin production by the infecting bacterium. In this study we demonstrate that this cytotoxicity can be determined directly from the diagnostic MALDI-TOF mass spectrum generated in a routine diagnostic laboratory. With further development this information could be used to guide the management and improve the outcomes for SAB patients.

Trade-Offs between Antibacterial Resistance and Fitness Cost in the Production of Metallo-ß-Lactamases by Enteric Bacteria Manifest as Sporadic Emergence of Carbapenem Resistance in a Clinical Setting.

Meropenem is a clinically important antibacterial reserved for treatment of multiresistant infections. In meropenem-resistant bacteria of the family Enterobacterales, NDM-1 is considerably more common than IMP-1, despite both metallo-ß-lactamases (MBLs) hydrolyzing meropenem with almost identical kinetics. We show that blaNDM-1 consistently confers meropenem resistance in wild-type Enterobacterales, but blaIMP-1 does not. The reason is higher blaNDM-1 expression because of its stronger promoter. However, the cost of meropenem resistance is reduced fitness of blaNDM-1-positive Enterobacterales. In parallel, from a clinical case, we identified multiple Enterobacter spp. isolates carrying a plasmid-encoded blaNDM-1 having a modified promoter region. This modification lowered MBL production to a level associated with zero fitness cost, but, consequently, the isolates were not meropenem resistant. However, we identified a Klebsiella pneumoniae isolate from this same clinical case carrying the same blaNDM-1 plasmid. This isolate was meropenem resistant despite low-level NDM-1 production because of a ramR mutation reducing envelope permeability. Overall, therefore, we show how the resistance/fitness trade-off for MBL carriage can be resolved. The result is sporadic emergence of meropenem resistance in a clinical setting.

Using artificial intelligence to reduce diagnostic workload without compromising detection of urinary tract infections.

BACKGROUND: A substantial proportion of microbiological screening in diagnostic laboratories is due to suspected urinary tract infections (UTIs), yet approximately two thirds of urine samples typically yield negative culture results. By reducing the number of query samples to be cultured and enabling diagnostic services to concentrate on those in which there are true microbial infections, a significant improvement in efficiency of the service is possible. METHODOLOGY: Screening process for urine samples prior to culture was modelled in a single clinical microbiology laboratory covering three hospitals and community services across Bristol and Bath, UK. Retrospective analysis of all urine microscopy, culture, and sensitivity reports over one year was used to compare two methods of classification: a heuristic model using a combination of white blood cell count and bacterial count, and a machine learning approach testing three algorithms (Random Forest, Neural Network, Extreme Gradient Boosting) whilst factoring in independent variables including demographics, historical urine culture results, and clinical details provided with the specimen. RESULTS: A total of 212,554 urine reports were analysed. Initial findings demonstrated the potential for using machine learning algorithms, which outperformed the heuristic model in terms of relative workload reduction achieved at a classification sensitivity > 95%. Upon further analysis of classification sensitivity of subpopulations, we concluded that samples from pregnant patients and children (age 11 or younger) require independent evaluation. First the removal of pregnant patients and children from the classification process was investigated but this diminished the workload reduction achieved. The optimal solution was found to be three Extreme Gradient Boosting algorithms, trained independently for the classification of pregnant patients, children, and then all other patients. When combined, this system granted a relative workload reduction of 41% and a sensitivity of 95% for each of the stratified patient groups. CONCLUSION: Based on the considerable time and cost savings achieved, without compromising the diagnostic performance, the heuristic model was successfully implemented in routine clinical practice in the diagnostic laboratory at Severn Pathology, Bristol. Our work shows the potential application of supervised machine learning models in improving service efficiency at a time when demand often surpasses resources of public healthcare providers.

Monitoring intraventricular vancomycin for ventriculostomy access device infection in preterm infants.

PURPOSE: Ventriculitis is a known complication during external CSF drainage in preterm infants with posthaemorrhagic ventricular dilatation. Staphylococci are most frequently isolated in device-associated ventriculitis, and hence, intraventricular vancomycin is a commonly used therapy. Our aim was to study the CSF vancomycin level pattern and drug safety in ventriculostomy access device infection in preterm infants less than 28 weeks gestation. METHODS: This single-centre, retrospective case series included seven infants with a median gestational age of 25 + 4 weeks (range 23 + 6 to 27 + 5 weeks). Ventriculitis was defined as elevated CSF white cell count of > 20/mm3 or positive CSF culture. The CSF vancomycin concentrations following intraventricular vancomycin administration were studied. RESULTS: Forty treatment episodes of intraventricular vancomycin administration were studied in seven preterm infants. Maximum CSF vancomycin concentrations were 24.9 mg/L (3 mg, n = 8, observed concentration-time (OCT), hours (h) = 19), 96.3 mg/L (5 mg, n = 17, OCT(h) = 14), 94 mg/L (10 mg, n = 14, OCT(h) = 24), and 230.7 mg/L (15 mg, n = 1, OCT(h) = 24). The threshold for re-dosage is set at CSF vancomycin level of < 10 mg/L. In all patients, ventriculitis resolution (defined as sterile CSF and CSF WCC of < 20/mm3) was achieved in a median of 5.5 days (range 2-31 days). Individual microbiology data is provided in the online resource. CONCLUSION: Intraventricular vancomycin is an effective treatment for ventriculostomy access device infection in preterm infants. In doses ranging from 3 to 15 mg, sufficient CSF vancomycin level is generated to achieve microbiological cure without any reported adverse effects. Daily CSF drug monitoring is recommended to define dosage interval to maintain drug concentration above breakpoint of minimum inhibitory concentration.

Early warning score: a dynamic marker of severity and prognosis in patients with Gram-negative bacteraemia and sepsis.

BACKGROUND: Early Warning Score (EWS) is a physiological composite score of six bedside vital parameters, routinely used in UK hospitals. We evaluated the prognostic ability of EWS in Gram-negative bacteraemia causing sepsis. METHODS: We prospectively evaluated EWS as a marker of severity and prognosis in adult patients with Gram-negative bacteraemia. All adult patients with Gram-negative bacteraemia admitted to our tertiary Teaching hospital of the National Health Service in England were enrolled over 1 year period. The highest daily EWS score was recorded from 7 days before to 14 days after the date of onset of bacteraemia. The primary outcome was 28-day mortality. MAIN RESULTS: A total of 245 consecutive adult patients with Gram-negative bacteraemia with sepsis were enrolled. On multivariate analysis, following variables were associated with death for every single unit change (odds ratio in the brackets): higher age (1.05), lower mean arterial pressure (1.03), lower serum bicarbonate (1.08), higher EWS (1.27), higher SOFA score (1.36), hospital-onset of infection (5.43) and need for vasopressor agents (16.4). EWS on day 0, 1, 2, and average 14-day score were significantly higher in patients who died by 28 days from the onset of bacteraemia [95 % CI 0.4-0.6] p < 0.001. A stepwise rise in EWS and failure of improvement in EWS by 2 points 48 h after the onset of bacteraemia were associated with poor outcome. CONCLUSION: EWS is a simple and cost-effective bedside tool for the assessment of severity and prognosis of sepsis caused by Gram-negative bacteraemia.

Low circulating B cells in immunocompromised individuals are linked to poorer antibody responses to vaccines and a predisposition to viral infections.

Background: B cells play an important role in protection against viral infections, not only through the production of antibodies but also through their ability to act as antigen-presenting cells and produce cytokines. Objectives: To assess whether there is a link between low circulating B-cell counts and a predisposition to viral infections in immunocompromised individuals, we performed a retrospective cohort analysis at 2 National Health Service Clinical Immunology sites in England. Methods: Eligible patients were adults who were either diagnosed with or under investigation for an immunodeficiency and had recorded circulating B-cell counts. Information on viral infections was collected by using the departmental, hospital, and laboratory electronic information systems. A generalized linear model was used to analyze the relationship between B-cell counts and relevant indices of viral infection while controlling for patient age, diagnosis group, and T-cell and natural killer cell counts. Results: A total of 376 eligible patients were identified, 134 of whom had B-cell counts that were below the laboratory-defined refence range (<0.11 ×109/L). Patients with low numbers of circulating B cells had lower pretreatment immunoglobulin levels and poorer antibody responses to vaccines (Streptococcus pneumonia, Clostridium tetani, and Haemophilus influenzae type B). An increased number of chronic or recurrent (P = .001), severe or unusual (P = .001), and PCR-confirmed viral infections (P = .04) were recorded in these patients versus in those with normal numbers of circulating B cells. Conclusion: Overall, there was a statistically significant association between low circulating B-cell counts and the incidence of clinically important viral infections in this patient cohort, even when controlling for relevant covariates. Clinicians caring for patients with immunodeficiency should be vigilant for these types of infections, particularly in patients with low peripheral B-cell counts. A prospective study will be required to confirm these findings.

Developing a model for decision-making around antibiotic prescribing for patients with COVID-19 pneumonia in acute NHS hospitals during the first wave of the COVID-19 pandemic: qualitative results from the Procalcitonin Evaluation of Antibiotic use in COVID-19 Hospitalised patients (PEACH Study).

OBJECTIVE: To explore and model factors affecting antibiotic prescribing decision-making early in the pandemic. DESIGN: Semistructured qualitative interview study. SETTING: National Health Service (NHS) trusts/health boards in England and Wales. PARTICIPANTS: Clinicians from NHS trusts/health boards in England and Wales. METHOD: Individual semistructured interviews were conducted with clinicians in six NHS trusts/health boards in England and Wales as part of the Procalcitonin Evaluation of Antibiotic use in COVID-19 Hospitalised patients study, a wider study that included statistical analysis of procalcitonin (PCT) use in hospitals during the first wave of the pandemic. Thematic analysis was used to identify key factors influencing antibiotic prescribing decisions for patients with COVID-19 pneumonia during the first wave of the pandemic (March to May 2020), including how much influence PCT test results had on these decisions. RESULTS: During the first wave of the pandemic, recommendations to prescribe antibiotics for patients with COVID-19 pneumonia were based on concerns about secondary bacterial infections. However, as clinicians gained more experience with COVID-19, they reported increasing confidence in their ability to distinguish between symptoms and signs caused by SARS-CoV-2 viral infection alone, and secondary bacterial infections. Antibiotic prescribing decisions were influenced by factors such as clinician experience, confidence, senior support, situational factors and organisational influences. A decision-making model was developed. CONCLUSION: This study provides insight into the decision-making process around antibiotic prescribing for patients with COVID-19 pneumonia during the first wave of the pandemic. The importance of clinician experience and of senior review of decisions as factors in optimising antibiotic stewardship is highlighted. In addition, situational and organisational factors were identified that could be optimised. The model presented in the study can be used as a tool to aid understanding of the complexity of the decision-making process around antibiotic prescribing and planning antimicrobial stewardship support in the context of a pandemic. TRIAL REGISTRATION NUMBER: ISRCTN66682918.

Bactericidal activity of multiple combinations of tigecycline and colistin against NDM-1-producing Enterobacteriaceae.

The interaction between colistin and tigecycline against eight well-characterized NDM-1-producing Enterobacteriaceae strains was studied. Time-kill methodology was employed using a 4-by-4 exposure matrix with pharmacokinetically achievable free drug peak, trough, and average 24-h serum concentrations. Colistin sulfate and methanesulfonate alone showed good early bactericidal activity, often with subsequent regrowth. Tigecycline alone had poor activity. Addition of tigecycline to colistin does not produce increased bacterial killing; instead, it may cause antagonism at lower concentrations.

Epidemiology, Outcomes and Resource Utilisation in Patients with Carbapenem Non-susceptible Gram-Negative Bacteria in the UK: A Retrospective, Observational Study (CARBAR UK).

INTRODUCTION: Antimicrobial resistance is an urgent medical challenge. In this two-part study, we investigated the epidemiology and management of carbapenem non-susceptible (Carb-NS) Gram-negative bacteria (GNB) in the UK. METHODS: We conducted a retrospective review of data from UK hospitals (ten in part 1, nine in part 2). In part 1, epidemiological data were collected from patients hospitalised between April 2017 and March 2018 with any laboratory detection of Carb-NS GNB, encompassing both colonisation and infection. In part 2, diagnosis and management pathways in a randomly selected population of adults from part 1 with confirmed Carb-NS GNB infection were assessed. Data were obtained from a detailed medical chart review for ≥ 3 months from index (collection date of first positive Carb-NS GNB sample). RESULTS: Of 42,340 GNB isolates from 36,098 patients colonised/infected with GNB in part 1, 7% were Carb-NS. In 157 patients included in part 2, 234 GNB index samples were collected, of which 197 (82%) were Carb-NS (median number of Carb-NS pathogens per patient, 1; range 1-3). The most frequent Carb-NS isolates were Pseudomonas aeruginosa (36%), Stenotrophomonas maltophilia (29%) and Klebsiella pneumoniae (10%). Median length of hospitalisation was 34 days. Median time from index to appropriate therapy was 3 days, with empirical therapy initiated a median of 1 day before index. Carb-NS infection was believed to contribute to 21 (28%) of 76 deaths during the study. CONCLUSIONS: This study highlights the high incidence of Carb-NS GNB colonisation and infection in the UK and the need for improved management of patients with Carb-NS GNB infection.

Procalcitonin Evaluation of Antibiotic Use in COVID-19 Hospitalised Patients (PEACH): Protocol for a Retrospective Observational Study.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel virus responsible for the coronavirus disease 2019 (COVID-19) pandemic. Although COVID-19 is a viral illness, many patients admitted to hospital are prescribed antibiotics, based on concerns that COVID-19 patients may experience secondary bacterial infections, and the assumption that they may respond well to antibiotic therapy. This has led to an increase in antibiotic use for some hospitalised patients at a time when accumulating antibiotic resistance is a major global threat to health. Procalcitonin (PCT) is an inflammatory marker measured in blood samples and widely recommended to help diagnose bacterial infections and guide antibiotic treatment. The PEACH study will compare patient outcomes from English and Welsh hospitals that used PCT testing during the first wave of the COVID-19 pandemic with those from hospitals not using PCT. It will help to determine whether, and how, PCT testing should be used in the NHS in future waves of COVID-19 to protect patients from antibiotic overuse. PEACH is a retrospective observational cohort study using patient-level clinical data from acute hospital Trusts and Health Boards in England and Wales. The primary objective is to measure the difference in antibiotic use between COVID-19 patients who did or did not have PCT testing at the time of diagnosis. Secondary objectives include measuring differences in length of stay, mortality, intensive care unit admission, and resistant bacterial infections between these groups.

Population Pharmacokinetics of Intraventricular Vancomycin in Neonatal Ventriculitis, A Preterm Pilot Study.

AIM: Intraventricular vancomycin is an effective treatment for neonatal ventriculitis, as the cerebrospinal fluid (CSF) vancomycin levels reach adequate concentrations to achieve microbiological cure. There is no robust data on intraventricular vancomycin pharmacokinetics in the preterm population. This pilot population pharmacokinetic modelling study examines the pharmacokinetic behaviour of intraventricular vancomycin in the preterm population of < 28 weeks gestation, to inform the feasibility of future prospective studies. METHODS: The study comprised 8 preterm infants with neonatal ventriculitis (median gestation age 25.3 weeks; range 23.9 - 27.7). Population pharmacokinetics (non-linear mixed effects modelling) were described with one- and two-compartment models to fit plasma concentrations of vancomycin. A CSF compartment was added to the plasma modelling and mass transfer examined. Three covariates (serum creatinine, ventricular index (VI) and CSF protein) were tested on the final model. Area under the curve (AUC) and average CSF concentration (C average) predictions were generated from the final model and compared with time to microbiological cure. RESULTS: A one-compartment model provided the best fit to the data. There was no appreciable transfer between plasma and CSF. None of the covariates provided a significant reduction in the objective function value (OFV). Generally, time to sterilisation with higher CSF AUC (0-24) and C average tends to be shorter, however this should be interpreted with caution as data is erratic. CONCLUSION: This pilot population pharmacokinetic analysis provides important information to warrant changes in the management of intraventricular vancomycin treatment in the preterm population, such as the current use of VI as a dosing parameter. Further study with a larger data pool is necessary to investigate the influence of VI on CSF vancomycin and ascertain dosing strategies.

Long-term nitrofurantoin: an analysis of complication awareness, monitoring, and pulmonary injury cases.

BACKGROUND: Long-term nitrofurantoin (NF) treatment can result in pulmonary and hepatic injury. Current guidelines do not outline the type or frequency of monitoring required for detection of these injuries. AIM: To assess 1) awareness of NF complications among prescribers; 2) monitoring practice; and 3) to describe the pulmonary sequelae of NF-related complications. DESIGN & SETTING: Evaluation of prescribing habits by questionnaires and review of GP databases, and case-note review in secondary care. METHOD: The following study procedures were undertaken: 1) an electronic questionnaire was distributed to prescribers, interrogating prescribing and monitoring practices, and awareness of complications; 2) an analysis was undertaken (June-July 2020) of NF monitoring among GPs in the local clinical commissioning group (CCG); and 3) a case review was carried out of patients diagnosed with NF-induced interstitial lung disease (NFILD) at the interstitial lung disease (ILD) centre (2014-2020). RESULTS: A total of 125 prescribers of long-term NF responded to the questionnaire (82.4% GPs; 12.0% urologists). Many were unaware of the potential for liver (42.4%) and lung (28.0%) complications; 40.8% and 52.8% never monitored for these, respectively. Only 53.3% of urologists believed themselves responsible for arranging monitoring, while nearly all GPs believed this to be the prescriber's responsibility (94.2%). One-third of all responders considered current British National Formulary (BNF) guidelines 'not at all sufficient/clear', with mean clarity scoring of 2.2/5. Among patients with NFILD (n = 46), NF had been prescribed most often (69.6%) for treatment of recurrent UTI and 58.6% (n = 27) were prescribed for >6 months. On withdrawal of the medication 61.4% displayed resolution (completely or minimal fibrosis), while 15.9% of patients had progressive lung fibrosis. CONCLUSION: NF can cause marked or irreversible lung complications and there is currently a shortfall in awareness and monitoring. Existing monitoring guidelines should be augmented.

Consensus-based antimicrobial resistance and stewardship competencies for UK undergraduate medical students.

BACKGROUND: In the UK there is limited coverage of antimicrobial stewardship across postgraduate curricula and evidence that final year medical students have insufficient and inconsistent antimicrobial stewardship teaching. A national undergraduate curriculum for antimicrobial resistance and stewardship is required to standardize an adequate level of understanding for all future doctors. OBJECTIVES: To provide a UK national consensus on competencies for antimicrobial resistance and stewardship for undergraduate medical education. METHODS: Using the modified Delphi method over two online survey rounds, an expert panel comprising leads for infection teaching from 25 UK medical schools reviewed competency descriptors for antimicrobial resistance and stewardship education. RESULTS: There was a response rate of 100% with all 28 experts who agreed to take part completing both survey rounds. Following the first-round survey, of the initial 55 descriptors, 43 reached consensus (78%). The second-round survey included the 12 descriptors from the first round in which agreement had not been reached, four amended descriptors and 12 new descriptors following qualitative feedback from the panel members. Following the second-round survey, a total of 58 consensus-based competency descriptors within six overarching domains were identified. CONCLUSIONS: The consensus-based competency descriptors defined here can be used to inform standards, design curricula, develop assessment tools and direct UK undergraduate medical education.

Post-acute COVID-19 associated with evidence of bystander T-cell activation and a recurring antibiotic-resistant bacterial pneumonia.

Here, we describe the case of a COVID-19 patient who developed recurring ventilator-associated pneumonia caused by Pseudomonas aeruginosa that acquired increasing levels of antimicrobial resistance (AMR) in response to treatment. Metagenomic analysis revealed the AMR genotype, while immunological analysis revealed massive and escalating levels of T-cell activation. These were both SARS-CoV-2 and P. aeruginosa specific, and bystander activated, which may have contributed to this patient's persistent symptoms and radiological changes.

Successful treatment of multicompartmental cerebral ventriculitis caused by Acinetobacter baumanii.

We present a case report of a 58-year- old woman with subarachnoid haemorrhage complicated by non-communicating hydrocephalus. During the course of her neurosurgical management, she developed external-ventricular drain associated ventriculitis which in turn was complicated by lack of communication between third and fourth ventricles. The causative organism was a fully-sensitive Acinetobacter baumanii, a nosocomial pathogen often associated with complicated treatment regimens and poor outcomes. This patient was successfully managed by a multi-disciplinary team involving neurosurgeons, neuroradiologists and infection specialists. Patient made a full recovery following double CSF diversion and intravenous plus intrathecal antimicrobial therapy.

The combination of colistin and fosfomycin is synergistic against NDM-1-producing Enterobacteriaceae in in vitro pharmacokinetic/pharmacodynamic model experiments.

The chemotherapeutic options against NDM-1-producing Enterobacteriaceae infections are limited and therefore combination therapy is gaining momentum to counter the secondary resistance and potential suboptimal efficacy of monotherapy. Colistin and fosfomycin are two separate classes of antimicrobial agents that act on bacterial cells by different mechanisms. Hence, there is a potential for both synergy and antagonism. In this study, the antibacterial effects (ABEs) of colistin and fosfomycin were systematically investigated by time-kill curve studies over 48 h as well as in an in vitro pharmacokinetic model over 96 h against six well characterised strains of NDM-1-producing Enterobacteriaceae (three isolates resistant and three susceptible to fosfomycin) at a standard inoculum of 10(6)CFU/mL. Clinically achievable free serum concentrations of colistin sulphate and fosfomycin were used. In a single-chamber in vitro model, peak/trough concentrations (C(max)/C(min)) and the half-life (t(1/2)) for fosfomycin (250/40 mg/L and 2.7 h, respectively) and colistin sulphate (3.0/0.75 mg/L and 4 h, respectively) were used, along with a growth control. ABEs were measured by the decrease in viable bacterial counts (log kill), area under the bacterial kill curve (AUBKC) and population analysis profile (PAP). The combination of colistin and fosfomycin compared with either agent alone achieved increased bacterial killing and decreased the chance of emergence of resistance. Also, the ABEs of the combination were sustained for a longer duration and were evident both against fosfomycin-sensitive and -resistant strains. This study provides important information and support for the role of combination therapy against multidrug-resistant Gram-negative bacteria with limited therapeutic options.