Therapeutic options for infections caused by gram-negative organisms expressing plasmid-mediated AmpC ␤-lactamases are limited because these organisms are usually resistant to all the ␤-lactam antibiotics, except for cefepime, cefpirome, and the carbapenems. These organisms are a major concern in nosocomial infections and should therefore be monitored in surveillance studies. Six families of plasmid-mediated AmpC ␤-lactamases have been identified, but no phenotypic test can differentiate among them, a fact which creates problems for surveillance and epidemiology studies. This report describes the development of a multiplex PCR for the purpose of identifying family-specific AmpC ␤-lactamase genes within gram-negative pathogens. The PCR uses six sets of ampC-specific primers resulting in amplicons that range from 190 bp to 520 bp and that are easily distinguished by gel electrophoresis. ampC multiplex PCR differentiated the six plasmid-mediated ampCspecific families in organisms such as Klebsiella pneumoniae, Escherichia coli, Proteus mirabilis, and Salmonella enterica serovar Typhimurium. Family-specific primers did not amplify genes from the other families of ampC genes. Furthermore, this PCR-based assay differentiated multiple genes within one reaction. In addition, WAVE technology, a high-pressure liquid chromatography-based separation system, was used as a way of decreasing analysis time and increasing the sensitivity of multiple-gene assays. In conclusion, a multiplex PCR technique was developed for identifying family-specific ampC genes responsible for AmpC ␤-lactamase expression in organisms with or without a chromosomal AmpC ␤-lactamase gene.

The dielectric description of the dynamical potential induced by swift protons in solids and the related stopping power is analyzed, using a combination of Mermin-type dielectric functions, which are fitted to available experimental data, to describe the optical properties of various materials. We apply this method to represent the energy loss functions of aluminum, silicon, amorphous carbon, and copper on a wide range of energy and momentum transfers. Using these functions we calculate the shape of the wake potential induced by swift protons; significant differences are obtained in the cases of carbon and copper, with respect to the results derived from simplified dielectric models. The energy loss functions are also applied to calculate the proton stopping power of each element, which are compared with experimental values. ͓S1050-2947͑98͒03007-8͔

Symmetrical tilt and twist grain boundary structures have been simulated in bcc iron using a many-body potential of the Finnis-Sinclair form. Initial structures were relaxed to the local minimum energy configuration using molecular dynamics. The width and relative energies of the resulting grain boundaries have been calculated. Collision cascades have been initiated in the structure by imparting initial energy to a single Fe atom and the interaction of the cascade with the grain boundary has been studied again using molecular dynamics simulations. The cascades were chosen where the primary knock-on atom (PKA) had initial energy of 1 keV and the orientation and distance of the PKA were changed in order to generate some statistical information concerning the radiation damage near the interface. The results show an increased radiation damage in the grain boundary region compared to the bulk material. The interstitials that form in the boundary region seem to be stable and do not move away from the boundary during the recrystallisation phase of the collision cascade. Clusters of interstitials are easily produced at the boundary in either structure but the defects induced near the twist boundary are more extensive that those near the tilt boundary.

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