Large multidrug resistance plasmids of the A/C incompatibility complex (IncA/C) have been found in a diverse group of Gram-negative commensal and pathogenic bacteria. We present three completed sequences from IncA/C plasmids that originated from Escherichia coli (cattle) and Salmonella enterica serovar Newport (human) and that carry the cephamycinase gene bla CMY-2 . These large plasmids (148 to 166 kbp) share extensive sequence identity and synteny. The most divergent plasmid, peH4H, has lost several conjugationrelated genes and has gained a kanamycin resistance region. Two of the plasmids (pAM04528 and peH4H) harbor two copies of bla CMY-2 , while the third plasmid (pAR060302) harbors a single copy of the gene. The majority of single-nucleotide polymorphisms comprise nonsynonymous mutations in floR. A comparative analysis of these plasmids with five other published IncA/C plasmids showed that the bla CMY-2 plasmids from E. coli and S. enterica are genetically distinct from those originating from Yersinia pestis and Photobacterium damselae and distal to one originating from Yersinia ruckeri. While the overall similarity of these plasmids supports the likelihood of recent movements among E. coli and S. enterica hosts, their greater divergence from Y. pestis or Y. ruckeri suggests less recent plasmid transfer among these pathogen groups.

A method is presented for application of the perfectly matched layer ͑PML͒ absorbing boundary condition ͑ABC͒ to the P-SV velocity-stress finite-difference method. The PML consists of a nonphysical material, containing both passive loss and dependent sources, that provides ''active'' absorption of fields. It has been used in electromagnetic applications where it has provided excellent results for a wide range of angles and frequencies. In this work, numerical simulations are used to compare the PML and an ''optimal'' second-order elastic ABC ͓Peng and Toksöz, J. Acoust. Soc. Am. 95, 733-745 ͑1994͔͒. Reflection factors are used to compare angular performance for continuous wave illumination; snapshots of potentials are used to compare performance for broadband illumination. These comparisons clearly demonstrate the superiority of the PML formulation. Within the PML there is a 60% increase in the number of unknowns per grid cell relative to the velocity-stress formulation. However, the high quality of the PML ABC allows the use of a smaller grid, which can result in a lower overall computational cost.

The small slope approximation (SSA) of Voronovich [Soy. Phys. JETP 62, 65-70 (1985)] is a promising method for application to scattering from many natural surfaces. The theory gives a systematic expansion that can be interpreted as a series in generalized surface slope. The SSA series for the T matrix satisfies the appropriate reciprocity condition at each order and reduces to the standard perturbation series for small surface roughness. In this paper we examine in detail the derivation of the SSA for surfaces subject to the Dirichlet (zero field) boundary condition. A number of points are discussed, including the requirements for determining the series terms. In addition, questions have been raised recently about the SSA: It has been argued that (1) an assumption in the derivation contradicts the exact formulation of the problem and (2) there is an arbitrariness in determining the series terms. These two points are refuted and the assumptions needed to determine the series terms unambiguously are clarified. The meaning of slope orders in the SSA series expansion are examined and the concept of generalized slope is discussed. A future companion paper (Part II. Numerical studies) will present an investigation of the accuracy of the SSA through comparison with exact results.

A Monte-Carlo finite-difference time-domain (FDTD) technique is developed for wave scattering from randomly rough, one-dimensional surfaces satisfying the Dirichlet boundary condition. Both single-scale Gaussian and multiscale Pierson-Moskowitz surface roughness spectra are considered. Bistatic radar cross sections are calculated as a function of scattering angle for incident angles of 0, 45, 70, and 80 degrees measured from the vertical. The contour path FDTD method is shown to improve accuracy for incident angles greater than 45 degrees. Results compare well with those obtained using a Monte-Carlo integral equation technique.

ABSTRACTSuperinfection occurs when a second, genetically distinct pathogen strain infects a host that has already mounted an immune response to a primary strain. For antigenically variant pathogens, the primary strain itself expresses a broad diversity of variants over time. Thus, successful superinfection would require that the secondary strain express a unique set of variants. We tested this hypothesis under conditions of natural transmission in both temperate and tropical regions where, respectively, single-strain infections and strain superinfections of the tick-borne pathogenAnaplasma marginalepredominate. Our conclusion that strain superinfection is associated with a significant increase in variant diversity is supported by progressive analysis of variant composition: (i) animals with naturally acquired superinfection had a statistically significantly greater number of unique variant sequences than animals either experimentally infected with single strains or infected with a single strain naturally, (ii) the greater number of unique sequences reflected a statistically significant increase in primary structural diversity in the superinfected animals, and (iii) the increase in primary structural diversity reflected increased combinations of the newly identified hypervariable microdomains. The role of population immunity in establishing temporal and spatial patterns of infection and disease has been well established. The results of the present study, which examined strain structure under conditions of natural transmission and population immunity, support that high levels of endemicity also drive pathogen divergence toward greater strain diversity.