Ecological prevalence, genetic diversity, and epidemiological aspects of Salmonella isolated from tomato agricultural regions of the Virginia Eastern Shore.

Virginia is the third largest producer of fresh-market tomatoes in the United States. Tomatoes grown along the eastern shore of Virginia are implicated almost yearly in Salmonella illnesses. Traceback implicates contamination occurring in the pre-harvest environment. To get a better understanding of the ecological niches of Salmonella in the tomato agricultural environment, a 2-year study was undertaken at a regional agricultural research farm in Virginia. Environmental samples, including tomato (fruit, blossoms, and leaves), irrigation water, surface water and sediment, were collected over the growing season. These samples were analyzed for the presence of Salmonella using modified FDA-BAM methods. Molecular assays were used to screen the samples. Over 1500 samples were tested. Seventy-five samples tested positive for Salmonella yielding over 230 isolates. The most commonly isolated serovars were S. Newport and S. Javiana with pulsed-field gel electrophoresis yielding 39 different patterns. Genetic diversity was further underscored among many other serotypes, which showed multiple PFGE subtypes. Whole genome sequencing (WGS) of several S. Newport isolates collected in 2010 compared to clinical isolates associated with tomato consumption showed very few single nucleotide differences between environmental isolates and clinical isolates suggesting a source link to Salmonella contaminated tomatoes. Nearly all isolates collected during two growing seasons of surveillance were obtained from surface water and sediment sources pointing to these sites as long-term reservoirs for persistent and endemic contamination of this environment.

Rapid identification and differentiation of non-O157 Shiga toxin-producing Escherichia coli using polymerase chain reaction coupled to electrospray ionization mass spectrometry.

A polymerase chain reaction (PCR)-mass spectroscopy assay was developed to identify non-O157 Shiga toxin-producing Escherichia coli (STEC) with Plex-ID biosensor system, a platform identifying short PCR amplicons by specific base compositions. This assay simultaneously amplifies five fragments of two housekeeping genes, two subunits of stx2 gene, and four other virulence genes of STEC. A total of 164 well-characterized STEC isolates were examined with the assay to build a DNA base composition database. Another panel of 108 diverse STEC isolates was tested with the established database to evaluate the assay's identification capability. Among the 108 isolates, the assay specificity was 100% for three (stx1, eae, and aggA) out of five tested virulence genes, but 99% for stx2 and 96% for hlyA, respectively. Main stx1/stx2 subtypes and multiple alleles of stx1/stx2 could be differentiated. The assay successfully identified several clinically significant serotypes, including O91:H14, O103:H25, O145:H28/NM, O113:H21, and O104:H4. Meanwhile, it was able to group isolates with different levels of pathogenic potential. The results suggest that this high-throughput method may be useful in clinical and regulatory laboratories for STEC identification, particularly strains with increased pathogenic potential.