Prevalence and microbial ecology of Enterobacteriaceae on Texas produce and the survival of Salmonella on parsley as affected by processing and storage

To assess the risk factors involved in the contamination of fresh produce with human pathogenic organisms, a total of 1,257 samples were collected from cantaloupe, oranges, and parsley in the field and after processing, and the environment. Samples were collected twice in a season from two farms with operating packing sheds per commodity and analyzed for the presence of Salmonella. Sixteen, 6, and 3 isolates were obtained from irrigation water, packing shed equipment, and washed cantaloupe, respectively. Salmonella was not detected on oranges or parsley. Serotyping, pulsed-field gel electrophoresis, and repetitive sequence-based polymerase chain reaction assays were applied to Salmonella isolates to evaluate their genetic diversity and to determine if there are relationships between sources of contamination. Using PFGE, all Salmonella isolates obtained from irrigation water and equipment were determined to be different from cantaloupe isolates. Only one equipment isolate was related to isolates from irrigation water. Rep-PCR demonstrated some similarity between equipment and cantaloupe isolates, but this technique is less discriminatory. DNA fingerprinting did not conclusively determine relationships between sources of contamination. Isolates were also subjected to antimicrobial susceptibility testing using the disk diffusion method. Five out of 25 of the isolates demonstrated intermediate sensitivity to streptomycin and one isolate was resistant to streptomycin.
Green fluorescent protein was an effective marker system when monitoring the survival of Salmonella on parsley as affected by processing. Dip temperature had little effect on the attachment and survival of Salmonella on parsley. Regardless of the temperature or duration of dip, Salmonella were internalized. Immersion for longer times resulted in higher numbers of attached and internalized cells. Microscopic observations agreed with these results and showed Salmonella near the stomata and within cuticle cracks. Salmonella increased over 7 storage days at 25??C and decreased at 4??C. After 7 days at 4??C, no internalized Salmonella were detected. Examination of the native microflora of parsley showed that bacterial populations were similar for parsley collected in the field and packing shed. Higher bacterial populations and fungi were observed at retail with Pseudomonas the predominant organism. Parsley supports the growth of a diverse group of microorganisms.