Pathogenic Escherichia coli in retail meats

Pathogenic Escherichia coli, including diarrheagenic E. coli (DEC) and extraintestinal pathogenic E. coli (ExPEC), cause numerous human infections annually. Retail meats are generally contaminated with E. coliand may serve as a vehicle transmitting pathogenic E. coli. The objectives of this project were to detect and characterize different pathogenic E. coli from retail meats, and to explore their potential in causing human infections. E. coli isolates recovered from ground beef, ground turkey, chicken breast and pork chop during 2002 to 2007 were screened for Shiga toxin genes and Shiga toxin-producing E. coli (STEC) were characterized phenotypically and genotypically. In addition, E. coli isolates recovered in 2006 were also examined for virulence genes specific for other DEC. The results demonstrated that retail meats, especially ground beef, were contaminated with diverse STEC strains and some strains contained Shiga toxin genes associated with severe human infections. The presence of atypical enteropathogenic E. coli in retail meat is also of concern due to their potential to cause human infections. Meanwhile, E. coli isolates recovered in 2006 were investigated for the presence of five ExPEC-defining virulence genes using multiplex PCR. Identified ExPEC isolates were characterized by serotyping and phylogenetic grouping, and their antimicrobial susceptibility data were analyzed. The findings showed that ExPEC, including antimicrobial resistant strains, were widely distributed in retail meats, especially in chicken and turkey products. This indicates that meat may be a vehicle for dissemination of ExPEC strains. ExPEC strains identified were further examined by multiplex PCR for the presence of 21 uropathogenic E. coli (UPEC) virulence genes. Selected strains were further characterized by multi-locus sequence typing and studied for their interactions with human bladder epithelial cells. Data showed that most UPEC virulence genes selected could be detected in isolates from meat. And some isolates belonged to sequence types associated with clinical UPEC. Meat-source isolates exhibited lower level of adherence and invasion compared to a clinical UPEC strain. These observations suggested that a small proportion of E. coli isolates from retail meats resemble human UPEC in various aspects, but their potential in causing human UTI needs further investigation

Inverse electromagnetic obstacle scattering problems with multi-frequency sparse backscattering far field data

This paper is dedicated to design a direct sampling method of inverse electromagnetic scattering problems, which uses multi-frequency sparse backscattering far field data for reconstructing the boundary of perfectly conducting obstacles. We show that a smallest strip containing the unknown object can be approximately determined by the multi-frequency backscattering far field data at two opposite observation directions. The proof is based on the Kirchhoff approximation and Fourier transform. Such a strip is then reconstructed by an indicator, which is the absolute value of an integral of the product of the data and some properly chosen function over the frequency interval. With the increase of the number of the backscattering data, the location and shape of the underlying object can be reconstructed. Numerical examples are conducted to show the validity and robustness of the proposed sampling method. The numerical examples also show that the concave part of the underlying object can be well reconstructed, and the different connected components of the underlying object can be well separated