U.S. flag An official website of the United States government
  1. Home
  2. Food
  3. Compliance & Enforcement (Food)
  4. Sampling to Protect the Food Supply
  5. Microbiological Surveillance Sampling: FY21 Sample Collection and Analysis of Lettuce Grown in Salinas Valley, CA
  1. Sampling to Protect the Food Supply

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.[1]


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.[2],[3]

Annually, California and Arizona produce 98% of the nation’s domestically grown leafy greens, including various types of lettuce.[4] The Salinas Valley is known as “The Salad Bowl of the World” because it produces roughly 70% of the lettuce grown in California.[5] Leafy greens are the second largest source of food related STEC infections in humans.[6] 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.

[1Leafy Greens STEC Action Plan
[2] Outbreak Investigation of E. coli: Romaine from Salinas, California (November 2019)
[3] Outbreak Investigation of E. coli - Leafy Greens (December 2020)
[4] Lettuce | Agricultural Marketing Resource Center (agmrc.org)
[5] USDA: 2017 Census of Agriculture – Table 29
[6] 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.
[7] A culture of microorganisms isolated for study
[8] 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.
[9] 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.
[10] 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.
[11] Animal fecal droppings

Back to Top