The distribution of conjugative-plasmid-mediated 16S rRNA methylase genes among amikacin-resistant Enterobacteriaceae collected between 1995 and 1998 and between 2001 and 2006 at a university hospital in South Korea was examined, and conjugative plasmids carrying the 16S rRNA methylase genes were characterized by PCR-based replicon typing and by determination of their antimicrobial resistance pattern. Among the 7,127 isolates, 463 isolates showed a high level of resistance to amikacin, and 218 of the 463 isolates transferred amikacin resistance by conjugation. Among the 218 isolates, armA was detected in 153 isolates (88 Klebsiella pneumoniae, 28 Escherichia coli, 19 Enterobacter cloacae, and 6 Serratia marcescens isolates and 12 isolates of other organisms), and rmtB was detected in 51 isolates (32 K. pneumoniae isolates, 18 E. coli isolates, and 1 Citrobacter freundii isolate). The first appearance of armA was in 1997. The armA gene was carried by conjugative plasmids of replicon groups IncL/M, IncFIIAs, IncF, IncA/C, IncHI2, and Inc(unidentified) in 38, 20, 7, 9, 4, and 75 strains, respectively. The rmtB gene was carried by conjugative plasmids of groups IncA/C, IncF, and IncI1-I␥ in 43 strains, 7 strains, and 1 strain, respectively. Transconjugants that received the IncL/M plasmid carrying armA or the IncA/C plasmid carrying rmtB showed an additional resistance to cefotaxime. Transconjugants that received the IncFIIA plasmid or Inc(unidentified) plasmid carrying the armA gene showed an additional resistance to cefoxitin and a high MIC 50 (0.25 mg/liter) of ciprofloxacin. In conclusion, this study demonstrated that the dissemination of 16S rRNA methylase genes among the Enterobacteriaceae is mediated by conjugative plasmids of various incompatibility groups that confer resistance to multiple drugs, including aminoglycosides, extended-spectrum ␤-lactams, and/or quinolones.Aminoglycosides have a high affinity for the 16S rRNA of the bacterial 30S ribosome, and they block protein synthesis (14,20). Over the past few decades, there have been many studies conducted regarding the mechanisms of resistance to aminoglycosides. One of the most common mechanisms of resistance to aminoglycosides is the production of aminoglycoside-modifying enzymes, such as 1-N-aminoglycoside acetyltransferase (AAC), adenyltransferase, and phosphotransferase (14,20). Amikacin was developed to suppress a variety of aminoglycoside-modifying enzymes from their accessing target sites (12), and therefore rare, amikacin-resistant bacteria could be expected. Recently, a series of special methylases that protect microbial 16S rRNA, however, were identified in several nosocomial pathogens, and these enzymes are capable of conferring extraordinarily high levels of resistance (MIC Ͼ 512 mg/liter) to most clinically important aminoglycosides, including amikacin, isepamicin, arbekacin, kanamycin, tobramycin, and gentamicin (5)(6)(7)(8)(21)(22)(23)(24). Since the first identification of a gene encoding 16S rRNA methylase, rmtA, from a Pseudomonas aeru...

Ischemic pre-conditioning confers renal resistance to I/R-induced apoptosis via activation of the Akt signal pathway.

The characteristics of the guided electromagnetic wave propagation through a subwavelength hole surrounded by a doubly dispersive metamaterial are investigated. Characteristic equations are derived for the surface polariton modes related to the subwavelength hole and mode classifications established. The surface polariton modes for two different hole-radii are numerically obtained and their electromagnetic dispersion curves and power flux characteristics analyzed and compared with each other. In particular, it was found that the border of the counter-propagation between the forward and backward Poynting vectors was located within the metamaterial, rather than at the interface between the metamaterial and the free space.

Light transmission along dispersive plasmonic gap with varied gap widths and its subwavelength guidance characteristics are numerically investigated over a wide frequency range. Mode numbers for each guided modes of the dispersive plasmonic gaps are properly assigned in order to be in consistency with the parallel plate waveguide composed of the perfect electric conductor. Overall and salient features of the role of the gap widths on the guided propagation characteristics are clearly understood by investigating several dispersion curves of varied gap widths. Cutoff frequency downshifts of the dispersive plasmonic gap compared with the perfect electric conductor based parallel plate waveguides are also observed. Finally, surface plasmon polariton modes having subwavelength guidance capability are described in more detail, which are directly governed by the plasmonic property of the metals. The results are expected to be utilized in designing various potential subwavelength nanophotonic devices.

The resonant microwave transmission characteristics of several coupled subwavelength ridged circular aperture arrangements in a thin metallic film are investigated using the three-dimensional finite-difference time-domain (3D FDTD) method. Simple equivalent circuits represented by the self and mutual conductances that have been quantitatively extracted for each resonance condition, including the mutual coupling effects, are also devised with the help of virtual magnetic current elements. Furthermore, a duality is identified between the ridged circular apertures and conventional half-wavelength dipole arrays based on comparing the respective resonance conditions.