Association of Salmonella with Arachis hypgaea (peanut plants and seedpods)

Abstract

Salmonella is the leading cause of bacterial foodborne illness in the United States and its presence in food results in the recall of many products every year. An increasing concern in the food industry is the contamination of peanuts with Salmonella. The goals of this research were to determine if S. Typhimurium was capable of internalizing in peanut seedpods and plants and identify factors involved in this uptake such as moisture, seedpod status, soil type and the influence of the peanut symbiont, Bradyrhizobium. Intact dry Virginia (DV) seedpods were exposed to S. Typhimurium suspensions and inoculated soil under different conditions. S. Typhimurium suspensions containing 7 Log CFU/mL were examined for different times, and temperatures. DV and green Virginia (GV) seedpods were also exposed to potting media and Hubbard series soil inoculated with 6.5 Log CFU/g. The initial moisture content of each soil was adjusted and the results were compared. Internalization was measured by swabbing and rinsing the interior surface of the seedpods following exposure. S. Typhimurium was quantified using differential tryptic soy agar (dTSA). Internalization of S. Typhimurium from soil into peanut plants was also examined. Soil was inoculated with 6 Log CFU/g. Sterile seeds were sowed and grown for 35 days after inoculation. Following surface sterilization, whole plants were divided into root, stem, and leaf samples. Each plant section was homogenized and quantified using dTSA. For experiments involving Bradyrhizobium, seeds were dip-inoculated in a suspension containing 8 Log CFU/mL of B. NC92 and then tested as described above. Intact DV seedpods were susceptible to S. Typhimurium internalization over all of the temperatures and times tested when immersed in cell suspensions. After exposing seedpods to the suspension for 22 h, S. Typhimurium was recovered internally at a level of 6.4 Log CFU/pod. Internalization also occurred rapidly in as little as 0.5 h. Significant differences were observed between the recovery of S. Typhimurium from the internal surface of DV and GV seedpods when exposed to soil. Overall differences were also identified in the ability of S. Typhimurium to infiltrate seedpods when delivered through potting media and Hubbard series soil at specific soil moisture contents. S. Typhimurium was capable of internalizing in peanut plant tissues and remained present at all testing times present. S. Typhimurium was recovered from stem samples (3.5 Log CFU/g) at greater levels than was observed for root (2.6 Log CFU/g) and leaf (1.7 Log CFU/g) samples. Overall results for stem, root, and leaf samples were recovered at lower levels when B. NC92 was inoculated on seeds before sowing. However, this difference was not significant for any time point, or plant section. Overall, this study’s results suggested the importance of water for S. Typhimurium to internalize within peanut seedpods. Moreover, the initial soil moisture content in relation to the water-holding capacity also impacted the ability of S. Typhimurium to internalize in seedpods. This work also observed that Salmonella was capable of internalization in peanut plants through inoculated soil. Moreover, the results indicated that the nodulating symbiont, B. NC92, did not significantly influence the internalization of S. Typhimurium when seeds are sowed in inoculated soil. This work provides some of the first evidence that peanut seedpods and plants are susceptible to Salmonella internalization, which may represent a potential route of entry of Salmonella into a processing facility from the field.