FDA and Dole investigated multistate Listeria outbreak linked to Dole packaged leafy greens. Dole detected Listeria on equipment at Yuma, AZ facility. FDA WGS analysis confirmed isolate matched the outbreak strain. Dole later found the strain on harvesting equipment used for iceberg lettuce, issued expanded recall covering products from Ohio and California facilities. Harvester was permanently decommissioned and dismantled. Link →

Detailed root-cause reporting: Dole swabbed 13,000+ times at processing facilities in Georgia and Michigan — no Listeria found. Traceback revealed a single bulk lettuce harvester moved from California to Arizona between seasons was the source. Dirt and debris trapped in the machine harbored the outbreak strain. Dole scrapped the harvester, redesigned conveyor systems, replaced plastic parts with stainless steel, and revised cleaning protocols. Link →

Retrospective analysis of two concurrent Listeria outbreaks (A and B) linked to packaged salads. Outbreak A: root-cause analysis revealed outbreak strain on a harvest rig used across multiple facilities. Outbreak B: clinical isolates matched strains previously found in Salinas Valley watersheds. Combined: 30 illnesses, 27 hospitalizations, 4 deaths over an 8-year period (strain persisted). Link →

2015–2016 multistate listeriosis outbreak linked to packaged leafy green salads processed at a single US facility. 19 hospitalized, 1 death. FDA inspection found the facility had detected Listeria in environmental samples 11 times over 19 months but failed to properly investigate. Link →

Multiple public health alerts for lettuce-containing ready-to-eat salads and wraps contaminated with Listeria monocytogenes, spanning 2017–2025. Recurring pattern of contamination at processing and packaging level. Link →

Summary of CDC retrospective analysis: outbreaks "underscore the long-term persistence of Listeria monocytogenes contamination in leafy green production and processing environments." Highlights that contamination can persist outside of processing environments (i.e., on harvest equipment in the field). Link →

Listeria monocytogenes can replicate at refrigerator temperatures. This is a key virulence factor that makes it difficult for food industries to control. Growth and division continue at refrigerator temps, allowing bacteria to reach infectious doses despite standard food storage. Link →

L. monocytogenes can grow at temperatures as low as −0.4°C and survive at freezing temperatures (−18°C). Adapts via changes in cell membrane composition, cold shock proteins, and cryoprotective compound uptake. Refrigeration is insufficient to control Listeria in long-term storage products. Link →

Listeria grows from −0.4°C to 45°C, tolerates salt up to 20%, pH range 4.6–9.5. Forms biofilms on food processing equipment surfaces that are difficult to remove and allow long-term persistence. Mortality rate of 20–30% worldwide for listeriosis. Link →

"Unlike most bacteria, refrigeration will not keep Listeria from growing, and freezing will not kill the pathogen." The longer food is stored in the refrigerator, the more Listeria can grow and potentially contaminate other food. Link →

Listeria thrives in wet and cold processing environments. Forms biofilms (compared to plaque on teeth) that protect bacteria from routine cleaning and can periodically break open, re-contaminating seemingly clean surfaces. Multiple outbreaks traced to packing and processing facilities. Link →

Survey of 100 lettuce samples (bagged and un-bagged) from supermarkets in Palmerston North, New Zealand. 7% overall prevalence of L. monocytogenes. Every positive sample was bagged/pre-cut lettuce. Zero un-bagged whole heads tested positive. Concludes: "postharvesting and preretail handling may be a source of produce contamination" and bagged salads carry higher microbial risk. Link →

Because packaged salad greens are processed at a small number of facilities across the US, bacteria like Listeria can easily spread from one batch to many. Whole heads of lettuce may be safer — inner leaves are less exposed to contamination sources and handled less than bagged greens. Hydroponic lettuce is less likely to be contaminated by animal droppings. Link →

Listeria monocytogenes is widely distributed in agricultural environments (soil, manure, water) and survives in food-processing and produce-packing environments and on equipment. Several listeriosis outbreaks have been linked to fresh produce contamination at the processing level. Studies confirm L. monocytogenes can multiply on lettuce under proper storage conditions — unlike most other foodborne pathogens. Link →

Listeria illnesses most often linked to dairy products, vegetable row crops, and fruits. Over 75% of Listeria illnesses attributed to these three categories. Based on 60 Listeria outbreaks in the 1998–2022 dataset. Link →