Fifty-one fluoroquinolone-resistant Escherichia coli isolates recovered from pigs, workers and environmental samples in one pig farm were screened for 16S rRNA methylase genes and qepA, a fluoroquinolone efflux pump gene, by PCR. Clonal relatedness of the E. coli isolates was examined by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing and phylogenetic analysis. Plasmids from the E. coli isolates were characterized by incompatibility group, restriction enzyme digestion and Southern hybridization analysis. The genetic environment of rmtB and qepA was also determined by PCR mapping. Eleven isolates that were highly resistant to amikacin and fluoroquinolones were positive for rmtB and qepA. All of these isolates belonged to phylogenetic group A, but most of them had different PFGE patterns or belonged to different sequence types (STs). Four isolates from different sources (two from pigs, one from a farm worker and one from an environmental sample) belonged to the same ST (ST160). Both rmtB and qepA were located on approximately 75-kb IncFII conjugative plasmids with nearly the same EcoRI digestion pattern. Tn3, IS26 and ISCR3 were found to be associated with rmtB and qepA. This study has found, for the first time, the transmission of rmtB and qepA among E. coli isolates from pigs, farm workers and the environment. Both horizontal transfer of IncFII plasmids and clonal dissemination have occurred and been seen to contribute to the dissemination of these resistance genes in a pig farm.
In this paper, filters with rectangular fibres arranged in a staggered and parallel array and placed transverse to the flow are studied numerically. A twodimensional flow field is obtained by solving NavierStokes equations with the control volume method. Periodic boundary conditions are introduced in the calculation. In order to achieve higher accuracy, a secondorder upwind scheme is adopted and a fine mesh is arranged near the fibre and the symmetrical plane of the flow field where large gradients in velocity are expected. Particle trajectories are calculated by solving the corresponding Lagrangian equation of motion to obtain the collection efficiency of a single rectangular fibre, in which positions of the approaching particles on the inlet plane of the flow field are randomly distributed according to the Monte-Carlo principle. The simulation considers all the important mechanisms of particle capture including interception, inertial impaction and Brownian motion. Effects of fibre aspect ratio, filter packing density, particulate size and Reynolds number on the collection efficiency are numerically determined. The volumetric packing density ranges from 0.4 to 4% and the particle diameter is from 0.01 lm to 2 lm. Reynolds number based on the height of computational domain varies from 20 to 100 and the aspect ratio is from 0.1 to 10. Simulations with and without Brownian motion are carried out for different Reynolds numbers, packing densities and aspect ratios and the results show that Brownian effects are significant for particles smaller than 1 lm.
IntroductionFiltration in a structure of fibrous filters is widely used in many industrial areas, such as bag houses for fly ash collection in coal-fired power plants, fibrous filters in air conditioning systems and fibrous traps for particulate emission control of diesel engines. As particulate emission is a major source to air pollution that directly deteriorates our living environment, particulate filtration technique has received increasing attention.Fibrous filtration is one of the most efficient and cost effective ways to remove suspended particulate matter from the gas stream. There are many factors that determine the performance of a fibrous filter, such as volumetric packing density of fibres, geometric shape and surface properties of fibres, gas flow rate, and dust particle loading. The separation of particulate matter from flow streams by fibrous filters is mainly due to several non-additive mechanisms of interception, inertial impaction and Brownian diffusion (Kuwabara, 1959). Although gravitational sedimentation and electrostatics also affect the collection efficiency of fibrous filtration (Davies, 1973;Brown, 1986), the contributions of interception, inertial impaction and Brownian diffusion are of prime importance. Extensive theoretical studies on particle collection by fibres have been reported. Stechkina (1969) calculated the particle collection efficiency in a system of parallel cylinders due to inertial deposition of particles of finite size. The t...
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