The Genetic and Phenotypic Diversity of Salmonella Strains Associated with the U.S. Pistachio Environment

The genetic diversity of Salmonella isolated associated with U.S. pistachios was determined by using the U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition Single Nucleotide Polymorphism (SNP) Pipeline. Additionally, representative Salmonella isolates obtained from pistachios stored in silos were selected to evaluate phenotypical characteristics that may play a role in persistence in pistachio orchards or during postharvest handling. A total of 11 Salmonella serovars and 15 strains have been isolated from U.S. pistachios from 2008 to 2018. Seven Salmonella strains have persisted in the California pistachio environment for ≥3 years, and some of these strains have been reported exclusively in association with pistachios. The copper homeostasis and silver resistant island sequence was present in three of the persistent strains and was associated with an increase in tolerance to CuSO4 under anaerobic but not aerobic conditions. Growth of all strains except for Salmonella Enteritidis strain A (sporadic) was similar in pistachio hull slurry held at 30°C over 48 h. All Salmonella strains produced weak to strong biofilms after 4 days at 25°C, with seven strains, including two sporadic strains, producing moderate biofilms and one persistent strain (Salmonella Liverpool) producing a strong biofilm. The rdar+ and rdar- morphotypes were observed in both persistent and sporadic Salmonella strains. All Salmonella strains contained nine of the 10 genes previously associated with desiccation tolerance. The fate of Salmonella strains during simulated desiccation and subsequent dry storage was evaluated by inoculating sterile filters with individual Salmonella strains, which were dried overnight, and then stored at 24 ± 1 °C and 35% relative humidity for 50 days. After drying, population reductions of 0.50–1.25 logs were observed for eight of the nine Salmonella strains. The population reductions (3.98–5.12 log) of eight strains were not significantly different at day 50. All but one of the Salmonella strains isolated from California pistachios were able to survive desiccation and storage, irrespective of their multicellular morphology or the presence of genes thought to aid in their survival during these processes. No single characteristic studied here fully explained the association of specific Salmonella strains with pistachios. Instead, it appears that multiple factors, including phenotypic and genotypic features and environmental factors, may be involved in the persistence of this foodborne pathogen in this environment