Clostridioides difficile dynamic electronic order panel, an effective automated intervention to reduce inappropriate inpatient ordering

BACKGROUND

Ordering Clostridioides difficile diagnostics without appropriate clinical indications can result in inappropriate antibiotic prescribing and misdiagnosis of hospital onset C. difficile infection. Manual processes such as provider review of order appropriateness may detract from other infection control or antibiotic stewardship activities.

METHODS

We developed an evidence-based clinical algorithm that defined appropriateness criteria for testing for C. difficile infection. We then implemented an electronic medical record-based order-entry tool that utilized discrete branches within the clinical algorithm including history of prior C. difficile test results, laxative or stool-softener administration, and documentation of unformed bowel movements. Testing guidance was then dynamically displayed with supporting patient data. We compared the rate of completed C. difficile tests after implementation of this intervention at 5 hospitals to a historic baseline in which a best-practice advisory was used.

RESULTS

Using mixed-effects Poisson regression, we found that the intervention was associated with a reduction in the incidence rate of both C. difficile ordering (incidence rate ratio [IRR], 0.74; 95% confidence interval [CI], 0.63-0.88; P = .001) and C. difficile-positive tests (IRR, 0.83; 95% CI, 0.76-0.91; P < .001). On segmented regression analysis, we identified a sustained reduction in orders over time among academic hospitals and a new reduction in orders over time among community hospitals.

CONCLUSIONS

An evidence-based dynamic order panel, integrated within the electronic medical record, was associated with a reduction in both C. difficile ordering and positive tests in comparison to a best practice advisory, although the impact varied between academic and community facilities.

Central Line-Associated Bloodstream Infection in Patients With Hematologic Malignancy Receiving Parenteral Nutrition

PURPOSE

Parenteral nutrition (PN) has been shown to be a safe method of feeding in the intensive care unit with modern infection prevention practices, but similar analysis in the hematology-oncology setting is lacking.

METHODS

A retrospective analysis of 1,617 patients with hematologic malignancies admitted and discharged from the Hospital of the University of Pennsylvania during 3,629 encounters from 2017 to 2019 was undertaken to evaluate the association of PN administration with risk of central line-associated bloodstream infection (CLABSI). Proportions of mucosal barrier injury (MBI)-CLABSI and non-MBI-CLABSI were also compared between groups.

RESULTS

Risk of CLABSI was associated with cancer type and duration of neutropenia but not with PN administration (odds ratio, 1.015; 95% CI, 0.986 to 1.045; P = .305) in a multivariable analysis. MBI-CLABSI comprised 73% of CLABSI in patients exposed to and 70% in patients not exposed to PN, and there was no significant difference between groups (χ2 = 0.06, P = .800).

CONCLUSION

PN was not associated with increased risk of CLABSI in a sample of patients with hematologic malignancy with central venous catheters when adjusting for cancer type, duration of neutropenia, and catheter days. The high proportion of MBI-CLABSI highlights the effect of gut permeability within this population.

Spatial and temporal effects on severe acute respiratory coronavirus virus 2 (SARS-CoV-2) contamination of the healthcare environment

BACKGROUND

The spatial and temporal extent of severe acute respiratory coronavirus virus 2 (SARS-CoV-2) environmental contamination has not been precisely defined. We sought to elucidate contamination of different surface types and how contamination changes over time.

METHODS

We sampled surfaces longitudinally within COVID-19 patient rooms, performed quantitative RT-PCR for the detection of SARS-CoV-2 RNA, and modeled distance, time, and severity of illness on the probability of detecting SARS-CoV-2 using a mixed-effects binomial model.

RESULTS

The probability of detecting SARS-CoV-2 RNA in a patient room did not vary with distance. However, we found that surface type predicted probability of detection, with floors and high-touch surfaces having the highest probability of detection: floors (odds ratio [OR], 67.8; 95% credible interval [CrI], 36.3-131) and high-touch elevated surfaces (OR, 7.39; 95% CrI, 4.31-13.1). Increased surface contamination was observed in room where patients required high-flow oxygen, positive airway pressure, or mechanical ventilation (OR, 1.6; 95% CrI, 1.03-2.53). The probability of elevated surface contamination decayed with prolonged hospitalization, but the probability of floor detection increased with the duration of the local pandemic wave.

CONCLUSIONS

Distance from a patient's bed did not predict SARS-CoV-2 RNA deposition in patient rooms, but surface type, severity of illness, and time from local pandemic wave predicted surface deposition.

SARS-CoV-2 RNA persists on surfaces following terminal disinfection of COVID-19 hospital isolation rooms

We evaluated the effect of terminal cleaning on SARS-CoV-2 RNA contamination of COVID-19 isolation rooms in an acute care hospital. SARS-CoV-2 RNA was detected on 32.1% of room surfaces after cleaning; the odds of contamination increased with month. The prevalence of elevated high-touch surface contamination was lower in terminally cleaned rooms than patient-occupied rooms.

Coronavirus disease 2019 (COVID-19) research agenda for healthcare epidemiology

This SHEA white paper identifies knowledge gaps and challenges in healthcare epidemiology research related to coronavirus disease 2019 (COVID-19) with a focus on core principles of healthcare epidemiology. These gaps, revealed during the worst phases of the COVID-19 pandemic, are described in 10 sections: epidemiology, outbreak investigation, surveillance, isolation precaution practices, personal protective equipment (PPE), environmental contamination and disinfection, drug and supply shortages, antimicrobial stewardship, healthcare personnel (HCP) occupational safety, and return to work policies. Each section highlights three critical healthcare epidemiology research questions with detailed description provided in supplementary materials. This research agenda calls for translational studies from laboratory-based basic science research to well-designed, large-scale studies and health outcomes research. Research gaps and challenges related to nursing homes and social disparities are included. Collaborations across various disciplines, expertise and across diverse geographic locations will be critical.

Stopping Hospital Infections With Environmental Services (SHINE): A Cluster-randomized Trial of Intensive Monitoring Methods for Terminal Room Cleaning on Rates of Multidrug-resistant Organisms in the Intensive Care Unit

BACKGROUND

Multidrug-resistant organisms (MDROs) frequently contaminate hospital environments. We performed a multicenter, cluster-randomized, crossover trial of 2 methods for monitoring of terminal cleaning effectiveness.

METHODS

Six intensive care units (ICUs) at 3 medical centers received both interventions sequentially, in randomized order. Ten surfaces were surveyed each in 5 rooms weekly, after terminal cleaning, with adenosine triphosphate (ATP) monitoring or an ultraviolet fluorescent marker (UV/F). Results were delivered to environmental services staff in real time with failing surfaces recleaned. We measured monthly rates of MDRO infection or colonization, including methicillin-resistant Staphylococcus aureus, Clostridioides difficile, vancomycin-resistant Enterococcus, and MDR gram-negative bacilli (MDR-GNB) during a 12-month baseline period and sequential 6-month intervention periods, separated by a 2-month washout. Primary analysis compared only the randomized intervention periods, whereas secondary analysis included the baseline.

RESULTS

The ATP method was associated with a reduction in incidence rate of MDRO infection or colonization compared with the UV/F period (incidence rate ratio [IRR] 0.876; 95% confidence interval [CI], 0.807-0.951; P = .002). Including the baseline period, the ATP method was associated with reduced infection with MDROs (IRR 0.924; 95% CI, 0.855-0.998; P = .04), and MDR-GNB infection or colonization (IRR 0.856; 95% CI, 0.825-0.887; P < .001). The UV/F intervention was not associated with a statistically significant impact on these outcomes. Room turnaround time increased by a median of 1 minute with the ATP intervention and 4.5 minutes with UV/F compared with baseline.

CONCLUSIONS

Intensive monitoring of ICU terminal room cleaning with an ATP modality is associated with a reduction of MDRO infection and colonization.

Certain Rooms in Intensive Care Units May Harbor Risk for Clostridioides difficile Infection

Abstract Background: Clostridioides difficile infection (CDI) is a common and sometimes life-threatening illness. Patient-, care-, and room hygiene-specific factors are known to impact CDI genesis, but care provider training and room topography have not been explored. We sought to determine if care in specific intensive care unit (ICU) rooms asymmetrically harbored CDI cases. Patients and Methods: Surgical intensive care unit (SICU) patients developing CDI (July 2009 to June 2018) were identified and separated by service (green/gold). Each service cared for their respective 12 rooms, otherwise differing only in resident team composition (July 2009 to August 2017: green, anesthesia; gold, surgery; August 2017 to June 2018: mixed for both). Fixed/mobile room features and provider traffic in three room zones (far/middle/near in relation to the toilet) were compared between high-/low-incidence rooms using observation via telecritical care video cameras. Results: Seventy-four new CDI cases occurred in 7,834 consecutive SICU admissions. In period one, green CDI cases were almost double gold cases (39 vs. 21; p = 0.02) but were similar in period two in which trainee service allocation intermixed. High-incidence rooms had closer toilet-to-intravenous pole proximity than low-incidence rooms (7.7 + 1.8 feet vs. 3.9 + 1.5 feet; p = 0.02). High-incidence rooms consistently housed mobile objects (patient bed, table-on-wheels) farther away from the toilet. Although physician time spent in each zone was similar, nurses spending more than 15 minutes in-room more frequently stayed in the far/middle zones in high-incidence rooms. Conclusions: Distinct SICU room features relative to toilet location and bedside clinician behaviors interact to alter patient CDI acquisition risk. This suggests that CDI risk occurs as a structural aspect of ICU care, offering the potential to reduce patient risk through deliberate room redesign.

3. Stopping Hospital Infections with Environmental Services (SHINE): A Cluster-Randomized Trial of Intensive Monitoring Methods for Terminal Room Cleaning on Rates of Multidrug-Resistant Organisms (MDROs) in the Intensive Care Unit (ICU)

Background

MDROs frequently contaminate hospital environments. We performed a multicenter cluster-randomized, crossover trial of two methods for intensive monitoring of terminal cleaning effectiveness at reducing infection and colonization with MDROs within ICUs.

Methods

Six medical and surgical ICUs at three medical centers received both intensive monitoring interventions sequentially, in a randomized order. The intervention included surveying a minimum of 10 surfaces each in 5 rooms weekly, after terminal cleaning, with adenosine triphosphate (ATP) monitoring or an ultraviolet fluorescent marker (UV/F). Results were delivered to environmental services (EVS) staff in real-time, with failing surfaces recleaned. The primary study outcome was the monthly rate of infection or colonization with MDROs, including methicillin-resistant Staphylococcus aureus, Clostridioides difficile, vancomycin-resistant Enterococcus, and multidrug-resistant gram-negative bacilli (MDR-GNB), assessed during a 12-month baseline comparison period and sequential 6-month intervention periods, separated by a 2-month washout. Outcomes during each intervention period were compared to the combined baseline period plus the alternative intervention period using mixed-effects Poisson regression, with study hospital as a random effect.

Results

The primary outcome rate varied by hospital and ICU (Figure 1). The ATP method was associated with a relative reduction in the incidence rate of infection or colonization with MDROs (incidence rate ratio (IRR) 0.887, 95% confidence-interval (CI) 0.811–0.969, P=0.008) (Table 1), infection with MDROs (IRR 0.924, 95% CI 0.855–0.998, P=0.04), and infection or colonization limited to multidrug-resistant MDR-GNB (IRR 0.856, 95% CI 0.825–0.887, P< 0.001). The UV/F intervention was not associated with a statistically significant impact on these outcomes. Room turn-around time was increased by a median of one minute with the ATP intervention and 4.5 minutes with the UV/F intervention compared to baseline.

Conclusion

Intensive monitoring of ICU terminal room cleaning with an ATP modality is associated with a relative reduction of infection and colonization with MDROs with a negligible impact on TAT.

Disclosures

Hilary Babcock, MD, MPH, FIDSA, FSHEA (nothing to disclose), David K. Warren, MD, MPH, Homburg & Partner (consultant), Ebbing Lautenbach, MD, MPH, MSCE (nothing to disclose), Jennifer Han, MD, MSCE, GlaxoSmithKline (employee, shareholder).

419. SARS-CoV-2 Environmental Surface Contamination of Healthcare Staff Common Areas

Background

There are limited data regarding SARS-CoV-2 (SC2) environmental contamination in staff areas of healthcare settings. We performed environmental sampling of staff areas in wards where coronavirus disease 19 (COVID-19) patients received care and compared findings to surfaces within COVID-19 patient rooms.

Methods

The study was conducted at the Hospital of the University of Pennsylvania (Philadelphia, PA) from 9/15/20-1/26/21. Sampling of 20cm² surfaces in staff common areas (breakroom high-touch surfaces comprising tables and microwave/refrigerator handles; bathroom surfaces comprising toilet, sink, and doorknob; and floors), nurse workstations (computer mice and floors), and COVID-19 patient rooms (high-touch surfaces comprising bedrail, computer mice/keyboards, and doorknobs; bathroom surfaces; and floors) was performed using flocked swabs one or more times per week. Specimens underwent RNA extraction and quantitative real-time polymerase chain reaction to detect the SC2 N1 region. Median comparisons were performed using Wilcoxon rank sum test. Trends in odds were evaluated using Score test.

Results

Proportions of surface specimens with detectable SC2 RNA are summarized in Table 1. Median copy numbers were lower among staff toilets compared to COVID-19 patient toilets (135.6 vs. 503.8 copies/specimen, p=0.02), lower among staff breakroom compared to patient room high-touch surfaces (104.3 vs. 220.3 copies/specimen, p=0.007), and similar between staff and patient room samples from sinks and floors. At nurse workstations, SC2 RNA was detected among 22/177 (12.4%) computer mouse and 147/178 (82.6%) floor samples. Odds of SC2 detection increased by study week among common area (p< 0.001) and nurse workstation samples (p< 0.001) (Figures 1 and 2).

Table 1. SARS-CoV-2 (SC2) RNA detection on staff common area and coronavirus disease 19 (COVID-19) patient room surfaces at the Hospital of the University of Pennsylvania, 9/15/20-1/26/21.

Figure 1. Proportion of environmental surface specimens with detectable SARS-CoV-2 RNA from a) staff common areas and b) nurse workstations of inpatient wards where coronavirus disease-19 patients received care at the Hospital of the University of Pennsylvania, 9/15/20-1/26/21.

Figure 2. Proportion of environmental surface specimens with detectable SARS-CoV-2 RNA in staff common areas of inpatient wards where coronavirus disease-19 patients received care at the Hospital of the University of Pennsylvania, 9/15/20-1/26/21, by surface type: a) staff breakroom surfaces, b) staff bathroom surfaces, c) staff common area floors.

Conclusion

A low prevalence of detectable SC2 RNA was observed among staff area high-touch surfaces; however, the likelihood of detection increased over time. Environmental SC2 RNA detection may reflect primary contamination from infected healthcare workers or secondary contamination from contact with infected patients, though a direct relationship between surface SC2 RNA viral detection and transmission risk has not been established.

Disclosures

Michael Z. David, MD PhD, GSK (Board Member) Ebbing Lautenbach, MD, MPH, MSCE, Merck (Other Financial or Material Support, Member of Data and Safety Monitoring Board (DSMB))

275. Clinical and Laboratory Predictors of Stroke Associated with COVID-19 Disease

Background

Although SARS-CoV-2 predominantly targets the respiratory system, it has also been associated with vascular complications including stroke. Identifying COVID-19 patients at elevated risk for stroke can help inform target anticoagulation strategies. We sought to understand how symptoms and laboratory markers at presentation with COVID-19 relate to stroke risk.

Methods

We enrolled a cohort of 1324 subjects who were hospitalized with COVID-19 across six PennMedicine hospitals between April and August 2020 and performed retrospective, manual chart review to measure exposures including presenting symptoms and admission inflammatory markers. Data were organized with a REDCap database, and analyses were performed using R statistical software, with Bayesian binomial regression models fit using Stan Hamiltonian Monte Carlo via the “brms” package.

Results

Among 1324 subjects, 19 stroke events were observed within 30 days of COVID-19 diagnosis. Admission inflammatory markers, including C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), ferritin, and D-dimer, were poor predictors of stroke risk. Among presenting symptoms, including respiratory, gastrointestinal, dermatologic, and neurologic features of COVID-19 disease, only altered mental status documented on presentation (in 529 subjects) was significantly associated with stroke risk (odds ratio 6.06, 95% credible interval 2.16 - 18.7).

Conclusion

Inflammatory markers associated with COVID-19 disease severity did not discriminate patients at high versus low risk of stroke in this cohort. Altered mental status documented on presentation was significantly associated with incident stroke during COVID-19 disease.

Disclosures

Ebbing Lautenbach, MD, MPH, MSCE, Merck (Other Financial or Material Support, Member of Data and Safety Monitoring Board (DSMB)) Michael Z. David, MD PhD, GSK (Board Member)

Quantifying the Impact of Nasopharyngeal Specimen Quality on Severe Acute Respiratory Syndrome Coronavirus 2 Test Performance

BACKGROUND

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reverse-transcription polymerase chain reaction (RT-PCR) cycle threshold (Ct) has been used to estimate quantitative viral load, with the goal of targeting isolation precautions for individuals with coronavirus disease 2019 (COVID-19) and guiding public health interventions. However, variability in specimen quality can alter the Ct values obtained from SARS-CoV-2 clinical assays. We sought to define how variable nasopharyngeal (NP) swab quality impacts clinical SARS-CoV-2 test sensitivity.

METHODS

We performed amplification of a human gene target (β-actin) in parallel with a clinical RT-PCR targeting the SARS-CoV-2 ORF1ab gene for 1282 NP specimens collected from patients with clinical concern for COVID-19. We evaluated the relationship between NP specimen quality, characterized by late Ct values for the human gene target β-actin Ct, and the probability of SARS-CoV-2 detection via logistic regression, as well as the linear relationship between SARS-CoV-2 and β-actin Ct.

RESULTS

Low-quality NP swabs are less likely to detect SARS-CoV-2 (odds ratio, 0.607 [95% credible interval {CrI}, .487-.753]). We observed a positive linear relationship between SARS-CoV-2 and β-actin Ct values (slope, 0.181 [95% CrI, .097-.264]), consistent with a reduction in detection of 0.181 cycles for each additional cycle of the β-actin target. COVID-19 disease severity was not associated with β-actin Ct values.

CONCLUSIONS

Variability in NP specimen quality significantly impacts the performance of clinical SARS-CoV-2 assays, and caution should be taken when interpreting quantitative SARS-CoV-2 Ct results. If unrecognized, low-quality NP specimens, which are characterized by a low level of amplifiable human DNA target, may limit the successful application of SARS-CoV-2 Ct values to direct infection control and public health interventions.

157. patient to Environment Transmission of Multidrug-resistant Bacteria Within Intensive Care Units

Background

Identifying risk factors for environmental contamination with multidrug-resistant organisms (MDROs) is essential to prioritize methods for prevention of hospital transmission.

Methods

Patients admitted to an ICU with an MDRO detected on clinical culture in the prior 30 days were enrolled. Patients (4 body sites) and high-touch objects (HTO) (3 composite sites) in ICU rooms were sampled. Environmental transmission was defined by shared MDRO species cultured on patient and HTO cultures obtained on multiple time points during the patient’s stay. Risk factors for environmental transmission were identified with logistic regression.

Results

Forty-five patients were included (median 2 days of longitudinal sampling [IQR 1–4 days]). Enrollment anatomic cultures included extended-spectrum beta-lactamase-producing Enterobacterales (ESBLE) (n=12, 27%), carbapenem-resistant organisms (CRO) (n=4, 9%), methicillin-resistant S.aureus (MRSA) (n=11, 24%), vancomycin-resistant Enterococci (VRE) (n=4, 9%), and C.difficile (CDIFF) (n=14, 31%). Patient colonization during serial sampling was common with CRO (n=21, 47%), ESBLE (n=16, 36%), and VRE (n=16, 36%) and less so with MRSA (n=7, 16%) and CDIFF (n=5, 11%). Detection of MDROs on environmental surfaces was also common with identification of CRO in 47% of patient rooms (n=21) and ESBLE in 29% (n=13); MRSA (n=2, 4%), VRE (n=9, 20%), and CDIFF (n=3, 7%) were rarer. Patient to environment transmission was observed in 40% of rooms (n=18). Thirteen (29%) rooms had foreign MDRO contamination (i.e., one not detected on a body culture), most (n=10) with CRO. Environmental MDROs were most common in bathroom/sinks (n=22), followed by surfaces near the patient (n=10), and least common surfaces often touched by staff within the room (n=6).

On multivariable logistic regression, naïve to clustering by patient, recent receipt of a proton pump inhibitor (OR 2.35, 95% CI 1.00 – 5.52, P=0.049) and presence of one or more wounds (OR 2.56, 95% CI 1.05 – 6.26, P=0.038) were significantly associated with environmental transmission (OR 1.56, 95% CI 1.01 – 2.43, P=0.046) (Table 1).

Conclusion

MDRO contamination of patient rooms is common with detection of organisms attributed to, and foreign to, the occupant.

Disclosures

Michael Z. David, MD PhD, GSK (Consultant)

Impact of Nasopharyngeal Specimen Quality on SARS-CoV-2 Test Sensitivity

BACKGROUND

The SARS-CoV-2 reverse-transcription polymerase chain reaction (RT-PCR) cycle of threshold (Ct) has been used to estimate quantitative viral load, with the goal of targeting isolation precautions for individuals with COVID-19 and guiding public health interventions. However, variability in specimen quality can alter the Ct values obtained from SARS-CoV-2 clinical assays. We sought to define how variable nasopharyngeal (NP) swab quality impacts clinical SARS-CoV-2 test sensitivity.

METHODS

We performed amplification of a human gene target (β-actin) in parallel with a clinical RT-PCR targeting the SARS-CoV-2 ORF1ab gene for 1311 NP specimens collected from patients with clinical concern for COVID-19. We evaluated the relationship between NP specimen quality, characterized by high Ct values for the human gene target β-actin Ct, and the probability of SARS-CoV-2 detection via logistic regression, as well as the linear relationship between SARS-CoV-2 and β-actin Ct.

RESULTS

Low quality NP swabs are less likely to detect SARS-CoV-2 (odds ratio 0.654, 95%CI 0.523 to 0.802). We observed a positive linear relationship between SARS-CoV-2 and β-actin Ct values (slope 0.169, 95%CI 0.092 to 0.247). COVID-19 disease severity was not associated with β-actin Ct values.

CONCLUSIONS

Variability in NP specimen quality accounts for significant differences in the sensitivity of clinical SARS-CoV-2 assays. If unrecognized, low quality NP specimens, which are characterized by a low level of amplifiable human DNA target, may limit the application of SARS-CoV-2 Ct values to direct infection control and public health interventions.

Reducing unnecessary culturing: a systems approach to evaluating urine culture ordering and collection practices among nurses in two acute care settings

Background

Inappropriate ordering and acquisition of urine cultures leads to unnecessary treatment of asymptomatic bacteriuria (ASB). Treatment of ASB contributes to antimicrobial resistance particularly among hospital-acquired organisms. Our objective was to investigate urine culture ordering and collection practices among nurses to identify key system-level and human factor barriers and facilitators that affect optimal ordering and collection practices.

Methods

We conducted two focus groups, one with ED nurses and the other with ICU nurses. Questions were developed using the Systems Engineering Initiative for Patient Safety (SEIPS) framework. We used iterative categorization (directed content analysis followed by summative content analysis) to code and analyze the data both deductively (using SEIPS domains) and inductively (emerging themes).

Results

Factors affecting optimal urine ordering and collection included barriers at the person, process, and task levels. For ED nurses, barriers included patient factors, physician communication, reflex culture protocols, the electronic health record, urinary symptoms, and ED throughput. For ICU nurses, barriers included physician notification of urinalysis results, personal protective equipment, collection technique, patient body habitus, and Foley catheter issues.

Conclusions

We identified multiple potential process barriers to nurse adherence with evidence-based recommendations for ordering and collecting urine cultures in the ICU and ED. A systems approach to identifying barriers and facilitators can be useful to design interventions for improving urine ordering and collection practices.

Electroporating fields target oxidatively damaged areas in the cell membrane

Reversible electropermeabilization (electroporation) is widely used to facilitate the introduction of genetic material and pharmaceutical agents into living cells. Although considerable knowledge has been gained from the study of real and simulated model membranes in electric fields, efforts to optimize electroporation protocols are limited by a lack of detailed understanding of the molecular basis for the electropermeabilization of the complex biomolecular assembly that forms the plasma membrane. We show here, with results from both molecular dynamics simulations and experiments with living cells, that the oxidation of membrane components enhances the susceptibility of the membrane to electropermeabilization. Manipulation of the level of oxidative stress in cell suspensions and in tissues may lead to more efficient permeabilization procedures in the laboratory and in clinical applications such as electrochemotherapy and electrotransfection-mediated gene therapy.

Calcium binding and head group dipole angle in phosphatidylserine-phosphatidylcholine bilayers

Manipulating the plasma membrane, the gateway to the cell interior, with chemical and physical agents for genetic and pharmacological therapy, and understanding the interactions of lipid membrane components with proteins and other structural and functional elements of the cell, require a detailed biomolecular membrane model. We report here progress along one path toward such a model: molecular dynamics simulations of mixed, zwitterionic-anionic, asymmetric phospholipid bilayers with monovalent and divalent cations. With phosphatidylcholine/phosphatidylserine systems, we identify temporal and concentration boundaries for equilibration of calcium with the bilayer and saturation of the calcium capacity of the membrane, we demonstrate the electrostatic- and entropic-driven associations of calcium and sodium ions with polar groups in the bilayer interface region, expressed in spatial distribution profiles and in changes in the orientation of the phospholipid head groups, and we describe for the first time simulations of dynamic, calcium-mediated adjustments in the conformation of mixed phospholipid species coresident in the same leaflet of the bilayer. The results are consistent with experimental observations and point the way to further refinement and increased realism of these molecular models.

Nanosecond field alignment of head group and water dipoles in electroporating phospholipid bilayers

To investigate the mechanism of biological cell membrane electroporation at the nanosecond and nanometer scale, we tracked pore-forming lipids and water in molecular dynamics simulations of a palmitoyloleoylphosphatidylcholine bilayer in a minimum porating electric field. Although the field-generated torque tilts the mean head group dipole a few degrees away from its equilibrium, zero-field position relative to the bilayer plane, this change in conformation does not appear to contribute directly to the development of the pore-initiating aggregation of lipid head groups and water that leads to the formation of a membrane-spanning hydrophilic pore. Field-directed rotation of the head group dipoles in the plane of the incipient pore wall, in combination with water dipole and solvation interactions at the aqueous-lipid interface, is one component in the coordinated ensemble of electroporation events.

Nanopore-facilitated, voltage-driven phosphatidylserine translocation in lipid bilayers--in cells and in silico

Nanosecond, megavolt-per-meter pulses--higher power but lower total energy than the electroporative pulses used to introduce normally excluded material into biological cells--produce large intracellular electric fields without destructively charging the plasma membrane. Nanoelectropulse perturbation of mammalian cells causes translocation of phosphatidylserine (PS) to the outer face of the cell, intracellular calcium release, and in some cell types a subsequent progression to apoptosis. Experimental observations and molecular dynamics (MD) simulations of membranes in pulsed electric fields presented here support the hypothesis that nanoelectropulse-induced PS externalization is driven by the electric potential that appears across the lipid bilayer during a pulse and is facilitated by the poration of the membrane that occurs even during pulses as brief as 3 ns. MD simulations of phospholipid bilayers in supraphysiological electric fields show a tight association between PS externalization and membrane pore formation on a nanosecond time scale that is consistent with experimental evidence for electropermeabilization and anode-directed PS translocation after nanosecond electric pulse exposure, suggesting a molecular mechanism for nanoelectroporation and nanosecond PS externalization: electrophoretic migration of the negatively charged PS head group along the surface of nanometer-diameter electropores initiated by field-driven alignment of water dipoles at the membrane interface.

Nanopore Formation and Phosphatidylserine Externalization in a Phospholipid Bilayer at High Transmembrane Potential

Atomic-resolution molecular dynamics simulations of lipid bilayers containing 7% phosphatidylserine (PS) on one leaflet are consistent with experimental observations of membrane poration and PS externalization in living cells exposed to nanosecond, megavolt-per-meter electric pulses. Nanometer-diameter aqueous pores develop within nanoseconds after application of an electric field of 450 mV/nm, and electrophoretic transport of the anionic PS headgroup along the newly constructed hydrophilic pore surface commences even while pore formation is still in progress.