Understanding Salmonella 

In our first installment of “Meet the Pathogens” we focused on Listeria species. A genus of bacteria that frequently challenge the food industry with their ubiquitous nature (found almost everywhere) and capacity to grow in wet cold environments common in the food industry. While Listeria is often spoken about in the context of wet processing plants (though, it survives perfectly well in dry warm environments too), Salmonella is conversely spoken about regarding their capacity to survive in dry processing plants and hot conditions. Dry processing plants (plants that don’t use water for processing/cleaning) and warmer, if not hot, temperature conditions, are no deterrent to Salmonella and it can survive in these conditions for years.   

Microorganisms are masters of survival and can/will adapt to meet the challenges of their environments. There are many strategies they can apply to persist in the environment, including comprehensive physiologic structure changes that allow Salmonella to stay functional in extreme conditions. For example, Salmonella can modify their cell membranes to remain more fluid at cold temperatures (increased unsaturated fatty acid composition), change protein structures to function at different temperatures (cold shock and heat shock proteins), and activate stress responses to deal with antimicrobials/sanitizers (efflux pumps, membrane modifications, biofilm). If conditions are particularly harsh, Salmonella can even enter a dormant/persister state to enhance survival and ensure the best opportunity to thrive once better conditions come upon them. These dormant/persister states vary based on the stress the cells are encountering, and these reversible physiologic states allow the cells to persist at a reduced metabolic state for protection – staying alive by not actively growing and then reemerging into vibrant states once the stress conditions change.   

Understanding Salmonella is the first step in being able to effectively predict how to manage it when producing food products. As we take a closer look into Salmonella in produce growing and processing, it is important to remember that bacteria in general are masters in survival and have existed much longer on this planet than humans have. In fact, bacteria’s believed origin is 3.5-4 billion years ago while humans are traced back only 6-7 million years ago (500-700 times older than humans). As a result, bacteria have had the advantage of time to figure out how to optimize survival and how to thrive in variable conditions and environments. 

Conditioned for survival 

In the food industry, Salmonella is one of the most common pathogens contributing to foodborne illness. Annually, the estimated salmonellosis cases are 1.35 million in the United States, contributing to an estimated 26,500 hospitalizations and 420 deaths. Globally, the numbers are much larger with over 93.8 million cases leading to 155,000 deaths. Within the United States, the Interagency Food Safety Analytics Collaboration calculates the salmonellosis attribution estimates for fruits, seeded vegetables, other produce, and vegetable row crop at 39.4% of all salmonellosis cases in the US per year (Figure 1).  

Figure 1: Graph from the Interagency Food Safety Analytics Collaboration (IFSAC) Foodborne Illness Source Attribution Estimates – United States, 2022 report found here. Accessed online February 19, 2025.

How does one microbe cause such a large impact? Salmonella has numerous biological strategies for survival, allowing for their successful persistence in the food system. First, they have the capacity to evolve and quickly replicate, with doubling times for many types of Salmonella being only 20-30 min at 35-37C. The capacity to rapidly double means that one cell can turn into one million cells in just over 6.5 hours at ideal temperatures. Rapid replication not only increases their numbers, but in food production, also increases the likelihood of an infectious dose making it into a serving of food (10-100 cells for higher-risk consumers, higher doses for healthier individuals). With growth comes ample opportunities for the cell to mutate as well (either on purpose or accidentally), modifying biological function and capabilities to exploit the environmental conditions it finds itself within. In the following sections some key characteristics about Salmonella are discussed so that the produce industry can better evaluate processes and facilities to control the risk from Salmonella.  

Who is Salmonella? 

Salmonella is a gram-negative, motile (flagella), and non-spore forming bacteria in the family Enterobacteriaceae, and with two species, Salmonella enterica and Salmonella bongori. Despite only two species, there are six subspecies of enterica with over 2,600 serotypes. Serotypes/serovars are additional means of differentiating Salmonella (and other bacteria) based on differences in antigens (substance that elicits an immune response) found on the surface of the cell. Unlike Salmonella enterica, Salmonella bongori is most often associated with reptiles and usually not associated with human illness. For foodborne illness, Salmonella enterica is the most common culprit and subsp. Enteritidis, Typhimurium, Newport, Heidelberg, Javiana and Montevideo are common serovars that contribute to foodborne illness. Salmonella enterica subsp. Typhi and Paratyphi cause typhoid fever and paratyphoid fever, respectively, and are not often associated with food contamination but contaminated water instead. 

Salmonella enterica and its 2,600+ serotypes have several characteristics that provide the organism an advantage over other microorganisms within environmental and food production scenarios.  

• Successful at a broad temperature range: Salmonella enterica is an enteric bacterium living and associated with the intestines of animals and humans. While Salmonella loves to live within the intestine of animals, it is also able to live in the environment such as in ponds, soils, etc. It can survive and grow at a wide range of temperatures, from 5°C all the way to 47°C (or 41°F to 116°F). The speed of growth at those temperatures varies, with the slowest rate of growth being at cold temperatures, and optimal growth temperatures at 35-37°C (95–98.6°F). Unlike Listeria that can grow at cold temperatures, Salmonella will not grow, but it survives for weeks and months at cold temperatures. Salmonella is also particularly heat tolerant, growing well at the hot end of the temperature range as well.  
• Conditioned for survival: Salmonella has a multitude of stress responses that allow the cells to survive challenging environments. One response is to enter into a dormant state called a persister or Viable But Not Culturable (VBNC) state. The persister state is a physiologic response and leads to cells that are metabolically active but not growing/replicating and therefore less susceptible to threats. These cells are able to revert into actively growing cells as conditions change and the stresses are removed. Salmonella’s toolbox for physiologic modification and adaption allows the cells to survive particularly challenged conditions such as low water activity, temperature and pH (4-9).   
• Salmonella can live in many, if not most, agricultural and food processing environments: Salmonella is a very common organism in the food system and environment. As such environmental monitoring programs should assess the likelihood of Salmonella being introduced into production areas/fields. Studies have also shown that Salmonella in the environment is impacted by rainfall patterns, increasing temperatures and climate change. 

Salmonella in Fresh Produce 

In fresh produce environments, Salmonella is a common inhabitant. While Salmonella is often spoken about and discussed as coming from the intestines of animals and humans, it is also able to survive in other natural environments like soil and water. Salmonella can move throughout the environment through animal vectors, wind, water, rain, dust, etc. As such, the presence of Salmonella in produce or agricultural environments does not mean direct fecal contamination occurred since Salmonella can naturally reside in numerous ecological niches and move via many means throughout the ecosystem. As examples of the environmental prevalence, two relevant produce environmental studies are briefly discussed below. One study regarding Salmonella in the Salinas Valley, a predominant leafy green growing in the United States, and another study regarding the Eastern Shore of Virgina, a major agricultural area in Virginia.   

Example 1: Salinas Valley Watershed 

A 2011-2013 longitudinal study of the Salinas Valley Watershed included bi-monthly water samples (Moore swabs w/ 24hr exposure) in 30 locations up and down the valley. The Salinas Valley Watershed water was testing, not irrigation water from wells (the most common agricultural source of water in the valley). This study helped provide insight into how common Salmonella, shiga-toxin producing E.coli, generic E.coli, and Listeria monocytogenes are in this agricultural area. This study found very high positive rates within the six watershed sampling sites in the Salinas Valley – with Salmonella nearly always present in most water sources, and sometimes up to 100% of the time.  

Figure 2: Graph recreated from data from Prevalence of shiga toxin producing E.coli, Salmonella enterica, and Listeria monocytogenes at public access watershed sites in a California Central Coast agricultural region. Front. Cell. Infect. Microbiol., 03 March 2014. The positivity rate is displayed to show Salmonella prevalence (positive rate) by month at six sampling locations during 2011-2013.

Example 2: Eastern Shore of Virginia 

The Eastern Shore of Virginia is an agricultural area in Virginia that has historically grown crops such as tomatoes and has been associated with some Salmonella outbreaks. In 2014 and 2015 researchers set out to better understand the prevalence of Salmonella in agricultural settings and the broader environment by taking weekly from pond and well water used in irrigation in the area and sampling environmental water sources (non-agricultural water from creeks and the Chesapeake Bay) in the area.  

The study found the prevalence of Salmonella below:  

Table 1: Recreated with data from Diversity and Dynamics of Salmonella enterica in Water Sources, Poultry Litters, and Field Soils Amended With Poultry Litter in a Major Agricultural Area of Virginia. Front. Microbiol., 16 December 2019.

Environmental Prevalence of Salmonella 

Both research studies provided are just two examples of many that have been completed over the past decades on Salmonella prevalence in agricultural and non-agricultural environments. These types of studies give insight into how common it can be to find microbial pathogens like Salmonella in the world around us. Importantly, it also helps provide a better understanding that environmental prevalence does not immediately mean that there is a produce risk for items grown in these areas – its presence in the environment is clear and generally consistent. However, what is also clear is that we must continue to try to better understand other necessary elements/factors/mechanisms that contribute to produce contamination from these environmental sources. In short, we know that the hazard is present, but we still need means to better understand how/when that translates to produce food safety risk, and then what we can do to manage that risk within our food safety practices. Establishing proactive monitoring of growing and processing environments can provide insight into when risks are increasing by better understanding the non-agricultural areas around produce production fields. These efforts can then help better identify when and where additional risk mitigation efforts may be needed to ensure the safety of the crop or product (in the case of fresh-cut produce processing). 

Within processing plants, controls for Salmonella must constantly be evaluated and monitored. It is common to hear discussions on environmental monitoring to include that Listeria species is what should be looked for in wet environments, and Salmonella in dry ones. However, as was mentioned in the previous article on Listeria, the wet/dry conditions are so simple and, in fact, both Listeria and Salmonella can be found and thrive in both wet and dry facilities. Each organism has the capacity to survive and outwit our cleaning, sanitation and process conditions. For food processing, it is best to remain adaptable and active in evaluating environments for both Listeria and Salmonella since either organism (or both) can make its way into the production facility. 

Conclusion 

Salmonella is the leading bacterial pathogen contributing to foodborne illness worldwide, annually causing almost 100 million infections worldwide. Within the United States, fruits, seeded vegetables, other produce and vegetable row crop are estimated to contribute to 39.4% of all salmonellosis cases in the US per year (Figure 1). Given the ubiquitous nature of Salmonella in both agricultural and non-agricultural environments, the Produce industry has an ongoing and important role to play in identifying means to reduce the risk of Salmonella in the food supply chain. Designing ongoing, effective and efficient means to identify when risks may be increased are important efforts for the fresh produce industry, and for neighboring industries and activities that occur nearby agricultural production areas. Within processing plants, Salmonella should be considered in well-designed environmental monitoring programs (EMPs) as the organism’s capacity to survive a large range of temperatures, pH, desiccation conditions, etc. allow for the bacteria to thrive in facilities where food is produced. Taking the time to understand the capacity for survival of Salmonella (and other foodborne hazards) is important to ensure food safety programs return optimal results and minimize risks to the consumer and the business.  

UC Agriculture and Natural Resources (UCANR) is offering free pathogen testing to growers. This program is supported by the California Leafy Greens Research Board and provides no-cost testing for pathogens submitted to the UC Cooperative Extension (UCCE) Monterey County office in Salinas. 

Soilborne pathogens, Pythium and INSV Infections

Soilborne pathogens pose a significant threat to the lettuce industry, causing diseases such as Fusarium wilt, Verticillium wilt, lettuce drop, Botrytis crown rot, black root rot, and others. The increase of pathogens like Pythium and INSV has complicated disease management in the Salinas Valley, especially as these diseases often co-occur. Pythium wilt in lettuce, for example, has become more common in recent years, sometimes complicating the diagnosis since it often occurs alongside INSV. Symptoms of INSV on lettuce include brown to dark brown necrotic spots which sometimes resemble burn damage caused by pesticide or fertilizer. A diagnostic service that provides precise detection of INSV in lettuce is critical. This highlights the need for reliable, rapid diagnostics like those offered through the UCANR program, which helps growers distinguish between different diseases and apply the correct management practices. Read more on Pythium wilt in lettuce here. 

No-Cost Diagnostic Service 

The free testing program aligns with ongoing efforts to better understand and control these diseases. With the support of industry partners, the diagnostic lab continues to innovate and expand its capabilities, offering a valuable resource to the agricultural community. 

For more information on how to submit an anonymous sample for disease diagnostics, please click here. Contact Yu-Chen Wang ([email protected]) at UC Cooperative Extension for more questions.