A large number of human illnesses have been linked to consumption of leafy greens contaminated with STEC O157 
. Although there are multiple routes for human infection with STEC O157, outbreaks associated with leafy greens are becoming increasingly common 
. The 58 laboratory-confirmed cases of STEC O157 matching the outbreak strain are likely only a fraction of the individuals who became ill as a part of this outbreak. Under-diagnosis results from variations in medical care seeking behaviors, specimen submission, laboratory testing, and test sensitivity (1). Burden of illness calculations, by Scallan et al. (2011), estimate 26.1 unreported infections for every one laboratory-confirmed case of STEC O157 (90% credibility interval: 16.1–41.3) 
. Extrapolating these under-diagnosis estimates to this outbreak, there were an estimated 1,514 illnesses (90% credibility interval: 934–2,396) associated with this outbreak, although these estimates may have been affected by the media attention surrounding the outbreak in the St. Louis metropolitan area. However, strict microbiological criteria requiring both a two enzyme PFGE match and MLVA match excluded a number of suspected cases who sought medical attention but did not match the outbreak strain.
Based on the epidemiologic and environmental investigations, the likely source of this outbreak was contaminated romaine lettuce sold predominantly at Grocery Store Chain A salad bars in the St. Louis, Missouri metropolitan area. This outbreak demonstrates the important public health aim to decrease the burden of STEC O157 in produce products that are often consumed raw. Instituting policies to help prevent contamination may help prevent future STEC outbreaks.
Consumption of romaine lettuce in the seven days before illness onset was significantly associated with illness in both the case-control and case series studies. Eating alfalfa sprouts was not significantly associated with illness in either study. Sprouts other than alfalfa (i.e., bean, clover, broccoli, daikon radish, etc.) were significantly associated with illness only in the case-series study when comparing cases to the FoodNet population, but were not found to be associated with illness in the case-control study. Seven of 41 cases (17.1%) in the case series reported sprouts in the seven days before illness onset; such a low reporting percentage of diverse sprouts, even if more than expected compared to the FoodNet population, is unlikely to explain all of the cases involved in this outbreak, even accounting for recall error among cases not reporting sprouts. Romaine lettuce consumption accounts for a higher proportion of illnesses (64.9% in case series) and trace-back identified a common supplier linking illness in multiple states.
This study was subject to several limitations including recall bias, small sample size, comparison populations, and selection of a single E. coli colony for microbiological testing. For the hypothesis-generating questionnaire and the case-control and case-series study, cases were asked to recall food histories seven days before illness and were interviewed one to four weeks after their illness and controls were asked to recall their food histories approximately two weeks to their interviews. Throughout the investigation there were local media reports implicating different food exposures before all case-patient interviews were completed, potentially biasing case-patients’ food histories. Our comparison group in the case-series study was the FoodNet population from ten FoodNet sites, but does not include individuals from the St. Louis metropolitan area where dietary habits may be different. Additionally, our case-control study sample was limited to case enrollment and not powered to detect a single food item on the salad bar. As for the microbiological investigation, there is always a chance that another strain of O157 serotype may have been missed since MLVA results were unavailable for two isolates with PFGE matches to the outbreak strain and multiple non-sorbitol fermenting colonies typical for O157 serotype are not routinely tested due to a lack of resources. Additionally, strains of non-O157 serotype could have also been missed since laboratories testing primary samples do not routinely pick sorbitol fermenting colonies for testing if they have already identified typical colonies for the non-O157 serotype.
Despite these limitations, our investigation was able to determine that romaine lettuce harvested at Farm A was used to supply both the salad bars at Grocery Store Chain A locations and a university campus in Minnesota. Romaine lettuce served on the salad bars at all locations of Grocery Store Chain A came from a single lettuce processing facility via a single distributor. This indicates that contamination of romaine lettuce likely occurred before the product reached the multiple Grocery Store Chain A locations.
Outbreaks of bacterial foodborne illnesses associated with raw produce are becoming increasingly common and STEC O157 has been isolated from the watershed in a major produce production region in California 
. Outbreaks associated with raw produce are not likely to subside in absence of changes in agricultural practices. The dynamic contamination of the watershed in the produce-growing Salinas Valley described by Cooley et al. (2007) raises important questions about the development of novel prevention strategies to minimize the likelihood of future leafy green-associated outbreaks 
. Improved ability to trace produce from the growing fields to the point of consumption will allow more timely prevention and control measures to be implemented. Records indicate that the distributor that supplied lettuce to the university campus in Missouri received lettuce from Farm A, but records were not sufficient to determine if this lot was sent to this university campus. Improved record keeping strategies would aid in food product traceability from farm to fork and ultimately prevent additional illnesses.