A new study led by University of Georgia researchers and published in Applied and Environmental Microbiology investigated the prevalence and characteristics of Salmonella in U.S. watersheds, with important implications for environmental surveillance and agricultural food safety risk management.

What the researchers uncovered were critical gaps in in understanding environmental Salmonella transmission, reinforcing the importance of a One Health surveillance approach to protect food safety and public health. They emphasize the need to develop a robust surveillance platform for source attribution and, ultimately, the prevention of future outbreaks.

Salmonella in Watersheds, the Environment, and Foodborne Illness Outbreaks

In recent and significant Salmonella outbreaks linked to foods like cucumbers and peppers, investigations have found Salmonella-positive agricultural water and environmental samples, suggesting the role of agricultural water in facilitating produce-related foodborne illness outbreaks. With 198 billion gallons of surface water used daily in the U.S., and 61 billion used for irrigation alone, the researchers underline that untreated water remains a critical food safety vulnerability.

In this context, the researchers sought to assess the prevalence, population complexity, and antimicrobial resistance (AMR) of non-typhoidal Salmonella within freshwater environments.

Importantly, the findings confirmed that surface water harbors complex, clinically relevant Salmonella populations that differ from food animal datasets.

Over two years, researchers sampled 456 water samples from 19 sites across four Southeastern U.S. watersheds, representing animal agriculture, suburban, and forested land uses. Salmonella was detected in 69 percent of samples.

Whole genome sequencing (WGS) identified 37 distinct serovars in Salmonella-positive samples, with 89 percent of positive samples harboring multiple serovars each, averaging 3.7 serovars per sample.

Seasonal Variability an Important Consideration for Agricultural Water Management

Modeling revealed that spring weather conditions significantly increased Salmonella detection and serovar complexity, while summer months showed the lowest prevalence. During 2023, Salmonella incidence shifted from 100 percent of sites to 0 percent, then back to 100 percent within three months. The researchers hypothesize that rain and increased temperature during the summer months could contribute to reduced nutrient availability and Salmonella influx to the environment.

This seasonal variability complicates predictive modeling and reinforces the Agricultural Water Rule requirements under the U.S. Food and Drug Administration’s (FDA’s) Food Safety Modernization Act (FSMA) Produce Safety Rule, which emphasizes systems-based risk assessments that account for watershed dynamics and seasonal factors.

Antimicrobial Resistance Found in 11 Percent of Isolates

The study also detected antimicrobial resistance (AMR) in 11 percent of isolates, with 21 percent classified as multidrug resistant. Even samples from a national forest watershed where human impact is minimal contained AMR strains, highlighting wildlife as a potential reservoir for resistance genes.

Environmental Isolates do not Match Food Animal Data

When comparing their findings against U.S. Department of Agriculture Food Safety and Inspection Service (USDA-FSIS) meat and poultry surveillance data, the researchers found little overlap between Salmonella serovars detected in their study and Salmonella serovars detected in animal agriculture. For example, S. Rubislaw dominated water samples but was absent from USDA-FSIS datasets.

Conversely, S. Infantis and S. Typhimurium were the only serovars collected in the study that were also routinely identified in primary domestic food animals, but aquatic isolates were genetically distinct and more closely related to human clinical cases than to food animal isolates.