Microbiological Surveillance Sampling: FY21 Sample Collection and Analysis of Lettuce Grown in Salinas Valley, CA
Microbiological Surveillance Sampling Main Page
In 2021, the U.S. Food and Drug Administration (FDA) conducted an assignment to collect lettuce (i.e., iceberg, romaine, red leaf, and green leaf) from commercial coolers that service the Salinas Valley growing area in California to test for E. coli O157:H7, a type of Shiga toxin-producing E. coli (STEC), and Salmonella spp. This sampling assignment began on May 18, 2021 and concluded in the end of harvest season in November 2021.
The agency’s goal in conducting this assignment was to identify potential contamination events associated with lettuce grown in the Salinas Valley and to prevent contaminated lettuce from entering commerce, when possible. This assignment is part of an ongoing preventive effort to ensure the microbiological safety of leafy greens in the U.S., the FDA’s Leafy Greens STEC Action Plan.
Foodborne illness outbreaks involving leafy greens linked to or potentially linked to the Salinas Valley region have continued to occur, with at least one outbreak occurring every year for the past four years. Following an outbreak in Fall 2020, the FDA analyzed outbreaks that had occurred each fall since 2017 in light of the findings of the 2020 investigation and found three key trends in the contamination of leafy greens by E. coli O157:H7 in recent years: a reoccurring strain, reoccurring region and reoccurring concerns with the potential impacts of adjacent lands. The FDA therefore determined the reoccurring pathogenic E. coli strain appears to be a reasonably foreseeable hazard in the California Central Coast leafy greens growing region, and specifically of concern in the South Monterey County area of the Salinas Valley growing area. This assignment is part of the agency’s ongoing surveillance of the commodity following these outbreaks of E. coli O157:H7 linked or potentially linked to romaine lettuce or other leafy greens.,
Annually, California and Arizona produce 98% of the nation’s domestically grown leafy greens, including various types of lettuce. The Salinas Valley is known as “The Salad Bowl of the World” because it produces roughly 70% of the nation’s lettuce crops. Leafy greens are the second largest source of food related STEC infections in humans. This large number of infections may be attributed in part to the popularity of lettuce in the American diet and the fact that lettuce is usually eaten without having undergone a ‘kill step,’ such as cooking, to reduce or eliminate bacteria.
The FDA planned to collect and test 500 samples of lettuce for this assignment. However, in total the agency collected 513 samples and tested 498 samples (149 iceberg, 271 romaine, 19 red leaf, 59 green leaf) for Salmonella spp. and E. coli O157:H7. In those few instances when samples were not analyzed, firms were notified promptly to minimize disruptions. Each sample consisted of 10 subsamples, with a minimum of 400 grams per subsample, for a total of 4,980 sampled lettuce plants. Romaine lettuce subsamples were made up of one package of hearts or loose leaves. Leaf and iceberg lettuce subsamples were made up of one or two heads of lettuce (trimmed, cored, and possibly wrapped). This approach – the collection and testing of samples composed of multiple subsamples – increases the probability of detecting pathogens when their abundance is low and/or not uniformly distributed.
The agency collected no more than two samples per lot. For this assignment, the FDA considered a “lot” to be product from a day’s harvest, from the same land/field, and the same harvest crew. The FDA did not collect any baby leaf lettuce types (e.g., arugula, spinach, red oak leaf, radicchio) that are typically mechanically harvested.
The agency collected and tested samples from FDA-regulated commercial coolers in the majority of cities that service Salinas Valley lettuce growers: Castroville (40), Gonzales (43), Salinas (358), San Juan Bautista (27), Soledad (12), Watsonville (18). Sample collection at commercial coolers allows the FDA to collect harvested lettuce after the field heat is removed (i.e., pre-cooled) and/or before shipping and processing. For this assignment, pre-cooled product was preferred and covered the majority of the product sampled. This approach was used to efficiently, and with minimal disruption to cooler operators, obtain samples from multiple farms at centralized locations with the intent of identifying contamination prior to release to consumers, thus protecting consumers by limiting consumer exposure and facilitating prompt traceback and follow-up if contamination was detected.
In accordance with the FDA’s Investigations Operations Manual (IOM), agency field staff collected all samples aseptically to prevent the lettuce from becoming contaminated by the sampling equipment or procedures. The agency’s aseptic sampling methods, which entail the use of sterile implements and containers, and prescribed collection procedures, are published in IOM chapter 4.3.
As a regulatory agency, the FDA’s mission is to protect and promote public health. Where possible, the agency tries to minimize the disruption to industry while maintaining regulatory oversight. Prior to the start and throughout the execution of this assignment, the FDA engaged with state regulatory partners and industry to provide informational updates and to listen to their feedback. The FDA also monitored the assignment as it progressed, considering real time input, and making adjustments as appropriate to enhance the assignment overall.
After carefully considering the circumstances of this assignment, the agency implemented assignment updates to reduce food waste and minimize the number of “cannot rule out” (CROs) findings by narrowing the focus of STEC targets. The three key changes to this assignment were related to pre-notifications, testing, and test result notifications. The FDA implemented an advance notice of collection visits, referred to as pre-notifications. The agency narrowed the scope of the assignment to test for Salmonella spp. and E. coli O157:H7, instead of the full range STEC strains. In collaboration with industry and cooling operation management, the FDA slightly adjusted the test result notification process to provide rapid result notifications to cooler operations and other additional relevant parties in the distribution chain.
The FDA remains committed to reducing notifications turnaround times for all results and strengthening communication with industry while maintaining the integrity of the assignment and accomplishing the public health mission.
Taking into consideration the COVID-19 pandemic, the agency took additional precautions to ensure the safety of cooling operation employees and FDA investigators, improve the efficiency of sample collection, and maintain sample integrity. Agency field staff pre-announced their visits to cooling operations, were equipped with personal protective equipment (PPE), and followed local, state, and applicable CDC guidance. The agency offered FDA investigators the opportunity to get the COVID-19 vaccination, on a voluntary basis, and provided access to COVID-19 testing.
This assignment resulted in three positive findings, out of the 498 samples tested, for a human pathogen. The FDA shared these findings with the farm and relevant interested parties.
Pathogen Findings: Salmonella
The FDA detected Salmonella enterica subspecies diarizone in one green leaf sample grown in King City and collected from a cooling operation in Gonzales. This subspecies is rarely associated with clinical illness and is most often associated with reptiles and amphibians. The FDA conducted whole genome sequencing (WGS) on the organism and determined that it did not match any known human illnesses.
Pathogen Findings: STEC
The agency detected STEC in two samples – one red leaf sample and one iceberg sample.
Upon further analysis, the red leaf sample collected from Salinas was found contaminated with a non-O157 STEC that was determined to be of low risk to public health.
After further characterization of the iceberg sample collected from Watsonville, the agency reclassified it from positive for E. coli O157:H7 to inconclusive. As a result of WGS analysis, the FDA found that isolates from this sample genetically matched one of the laboratory control strains used for internal quality procedures. The agency conducted a quality assessment of the analytical procedures used to test the sample and did not find any evidence of cross-contamination. However, since the isolates genetically matched clinical and environmental isolates and the control strain, the FDA reclassified the initial positive finding to inconclusive.
Upon detecting a positive sample result, the agency assessed whether further follow-up actions were needed to identify potential routes and sources of contamination.
Follow-Up Actions: Salmonella
Upon identifying the Salmonella enterica subspecies diarizone in a sample of green leaf lettuce, the FDA notified the grower, shipper, and cooler about the positive sample. The farm destroyed the lot associated with the positive finding and none of the potentially contaminated lettuce entered commerce. The identified subspecies of Salmonella was not found to have any clinical illness matches. At FDA’s request, state partners initiated an on-farm inspection during the 2022 harvest season. This inspection included an assessment of animal intrusion, soil amendments, adjacent land use, and water use.
Follow-Up Actions: STEC
Upon identifying the E. coli O157:H7 in a sample of iceberg lettuce, the FDA notified the grower, the harvester, and the owner/shipper/cooler about the positive sample and that the isolate was undergoing further characterization. The owner/shipper/cooler informed the agency that the entire lot of lettuce associated with the positive sample had been destroyed and none of it entered commerce.
The FDA and state partners initiated an on-farm, follow-up investigation at the farm identified as the grower of the contaminated iceberg lettuce sample to determine potential sources and routes of contamination. This on-farm investigation sought to help farm management identify microbial risks so they could improve the microbiological safety of their operation(s) and potentially prevent contaminated product from reaching consumers.
This on-farm investigation was limited because the field where the sampled lot of iceberg lettuce was harvested was fallow. FDA investigators interviewed farm management and searched for potential sources and routes of contamination, including consideration of possible animal intrusion, adjacent land use, employee health and hygiene, soil amendments, sanitation (of tools, equipment, and buildings), and the condition of agricultural water. The agency collected swabs from the surfaces of field and irrigation equipment and samples of soil, well water, vegetative material, and apparent bird droppings from nearby equipment storage. All samples collected for purposes of the follow-up investigation tested negative for E. coli O157:H7 and other strains of STEC.
While the FDA found a contamination rate of less than 1% in the samples collected and tested, contamination of lettuce from the California central coast region continues to pose a public health concern given the multistate outbreaks of foodborne illness in recent years. Given the relatively small scope of this assignment compared to the volume of lettuce produced in this region the FDA cautions against making inferences more broadly about contamination in lettuce grown in this region based solely on this assignment’s findings.
As stated in the “Assignment Updates” section of this report, the agency narrowed the scope of the assignment from all STEC of public significance to E. coli O157:H7. Per the “Findings” section, the FDA detected Salmonella in one sample and non-O157 STECin one sample out of the 498 collected and tested samples. Upon further review by the agency, the non-O157 STEC finding was deemed to be a low risk to public health. Initially, E. coli O157:H7 was detected in one additional sample but upon further analysis it was reclassified as inconclusive.
Lettuce is one of the most widely consumed vegetables in the United States. Due to the high level of lettuce consumption, even a low rate of contamination with Salmonella or STEC could result in a significant foodborne illness outbreak. The FDA closely tracks and investigates all current foodborne illness outbreaks. Considering the reoccurring foodborne illness outbreaks associated with leafy greens consumption, ensuring the microbiological safety of leafy greens remains a top priority of the FDA. The agency is working on several initiatives to help prevent microbial contamination of leafy greens and minimize future outbreaks. Foremost among these efforts is the FDA’s Leafy Greens STEC Action Plan, which features public health and regulatory approaches related to response, prevention, and addressing knowledge gaps.
As part of the action plan, the FDA launched two multi-year studies in an effort to determine the cause(s) of foodborne illness outbreaks linked to romaine lettuce produced in California and Arizona. These studies are designed to increase understanding of how human pathogens survive, move, and possibly contaminate produce prior to harvest. Research teams collect and examine environmental samples, including adjacent land, well and surface waters, and soil amendments that may contain compost, dust, and scat to name a few types of samples routinely collected. These studies are being conducted in partnership with state and local governments, academia, and other local agricultural stakeholders. Results from these collaborations are anticipated to lead to improved practices to prevent or mitigate food safety risks, and ultimately enhance the safety of produce grown in the regions.
To reduce the risk of foodborne illness, consumers can thoroughly rinse lettuce and other leafy greens under running water before eating them. Although thoroughly rinsing lettuce isn’t as effective as a ‘kill step,’ such as cooking to reduce or eliminate bacteria, it can still remove some pathogens, if present. For additional information, visit the Centers for Disease Control and Prevention’s (CDC) webpage on lettuce and leafy green food safety.
 Leafy Greens STEC Action Plan
 Outbreak Investigation of E. coli: Romaine from Salinas, California (November 2019)
 Outbreak Investigation of E. coli - Leafy Greens (December 2020)
 Lettuce | Agricultural Marketing Resource Center (agmrc.org)
 USDA - National Agricultural Statistics Service - California
 Herman, K.M., A.J. Hall and L.H. Gould. 2015. Outbreaks attributed to fresh leafy vegetables, United States, 1973– 2012. Epidemiol Infect.143(14): 3011–3021.
 A culture of microorganisms isolated for study
 Animals trespassing on or near a far, contaminating the land and/or water sources. Types of animal intrusion one might find include animal tracks, crop damage, and animal feces.
 Any material added to a soil to improve its physical properties (e.g., soil structure, water retention, water infiltration, drainage) which indirectly affects plant growth.
 Uncultivated land or farmland that is plowed and harvested but left unsown for a period of time to restore its fertility as part of a crop rotation or to avoid surplus production.
 Animal fecal droppings