Richard Christie

Modeling the Spread of Methicillin-Resistant Staphylococcus aureus (MRSA) Outbreaks throughout the Hospitals in Orange County, California

BACKGROUND Since hospitals in a region often share patients, an outbreak of methicillin-resistant Staphylococcus aureus (MRSA) infection in one hospital could affect other hospitals. METHODS Using extensive data collected from Orange County (OC), California, we developed a detailed agent-based model to represent patient movement among all OC hospitals. Experiments simulated MRSA outbreaks in various wards, institutions, and regions. Sensitivity analysis varied lengths of stay, intraward transmission coefficients (β), MRSA loss rate, probability of patient transfer or readmission, and time to readmission. RESULTS Each simulated outbreak eventually affected all of the hospitals in the network, with effects depending on the outbreak size and location. Increasing MRSA prevalence at a single hospital (from 5% to 15%) resulted in a 2.9% average increase in relative prevalence at all other hospitals (ranging from no effect to 46.4%). Single-hospital intensive care unit outbreaks (modeled increase from 5% to 15%) caused a 1.4% average relative increase in all other OC hospitals (ranging from no effect to 12.7%). CONCLUSION MRSA outbreaks may rarely be confined to a single hospital but instead may affect all of the hospitals in a region. This suggests that prevention and control strategies and policies should account for the interconnectedness of health care facilities.

Mid-infrared characterization of the NH4+∙(H2O)n clusters in the neighborhood of the n=20 “magic” number

Vibrational predissociation spectra are reported for size-selected NH4+ (H2O)n clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+ (H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+(H2O)n clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.

A Modified MSEVB Force Field for Protonated Water Clusters

A multistate empirical valence bond (MSEVB) model for protonated water clusters, which incorporates the TIP4P water model, is presented. This model which is designated MSEVB4P represents a significant improvement over the original model of Voth et al. (J. Phys. Chem. B 1998, 102, 5547) which was based on the TIP3P water model and a smaller improvement over the recently introduced MSEVB3 model (J. Phys. Chem. B 2008, 112, 467) which is based on the SPC/Fw (J. Chem. Phys. 2006, 124, 024503) water model.

Model systems for exploring electron correlation effects in the buckling of SiSi dimers on the Si(100) surface

Si2H4 and Si7H8 cluster models are constructed for studying the role of high-order electron correlation effects on the buckling of SiSi dimers on the Si(100) surface. CASPT3, multi-reference coupled-pair functional, and the diffusion Monte Carlo methods are used to examine whether correlation effects not recovered in CASPT2 calculations are important for the buckling of the dimers. The calculations show that such high-order correlation effects are indeed important for determining the relative stability of the buckled and unbuckled structures.