Detection and Characterization of Salmonella spp. and Correlation with Microbial Indicators in North Carolina Tomato Production Environments

At $1 billion in 2012, fresh-market tomatoes (Solanum lycopersicum) rank as one of the highest value vegetables in the U.S.  With improved varieties, high nutritional value, and documented health benefits, demand for tomatoes in the American diet is increasing.  However, multiple epidemiologically confirmed salmonellosis outbreaks have been associated with contaminated tomatoes, many of which were produced in eastern United States.  Because of this, tomatoes remain one of the leading causes of produce-related foodborne outbreaks.  Limited data exists on the prevalence of Salmonella or the relationship between the presence of Salmonella and the traditional microbial fecal contamination indicator generic Escherichia coli (E. coli) in these farm production systems.  The primary purpose of this project is to identify endemic environmental niches for Salmonella, to characterize the isolates, and to correlate Salmonella presence with generic E. coli concentrations in tomato production systems.  Environmental samples were collected from three agriculturally diverse farms in North Carolina during the 2012 and 2013 production seasons.  Samples of tomato fruit, blossom, leaf, weeds, soil, and water/sediment samples (n=1010) were analyzed for the presence of Salmonella using a modified FDA Bacteriological Analytical Manual (BAM) method as well as by real-time PCR.  Isolates were genotyped using pulse-field gel electrophoresis.  Generic E. coli was enumerated in monthly water samples (n=48) collected at the source and throughout the field irrigation system using the IDEXX Colilert and Quanti-Tray 2000 system (theoretical detection limit of 1 MPN/100ml). Salmonella was isolated in June (10 isolates for 2013 only), July (28 isolates), August (35 isolates) and September (42 isolates) with 59% (68/115) of isolates from water, 33% (38/115) from sediment, 4% (5/115) from tomato fruit, and 3% (4/115) from soil.  61% (70/115) of the isolates were identified to seven Salmonella serotypes, including Agona, Berta, Hartford, Montevideo, Newport, Paratyphi B and Typhimurium, each having potential clinical significance.  Using a positive threshold value of 235 MPN/100 ml, as proposed in the pending Food Safety Modernization Act, generic E. coli was predictive of Salmonella spp. 81% of the time (False Positives = 17% (8/48); False Negatives = 2% (1/48)).  On these farms, the major environmental niche (92%) for Salmonella appears to be water sources and sediment, which remain prominent and seasonally consistent across both production years.  A better understanding of environmental sources, persistence, and routes of contamination for salmonellae in tomato production environments will assist with identification and implementation of on-farm remediation and better management for potential microbial contaminants.