A new study evaluating the presence of Listeria monocytogenes in a controlled environment agriculture (CEA) operation provided insights into potential contamination risks and effective environmental monitoring and sanitation approaches.

The project was conducted by researchers with the Spanish National Research Council (CEBAS-CSIC) and published in Food Control.

The findings confirmed the critical importance of environmental monitoring for detecting potential microbiological contamination risks, informing appropriate intervention measures, and confirming the effectiveness of corrective actions. 

Study Design

For the study, the researchers collected a total of 156 environmental samples from a commercial hydroponic pak choi growing operation. Samples were collected on three occasions over a period of eight months, focusing on food-contact surfaces, non-food-contact surfaces, and crop water sources (i.e., irrigation community water, nutrient solution, and drainage solution).

L. monocytogenes was only detected in two of the 156 samples, both of which were collected during the first sampling occasion.

Boot Covers Prove to be Useful Swabbing Tool for Environmental Monitoring

One L. monocytogenes-positive sample was collected from boot covers worn by personnel walking through pre-harvest, harvest, and post-harvest zones, demonstrating the effectiveness of using boot covers as a swab tool to evaluate cleaning and sanitation. The presence of L. monocytogenes on boot covers, despite no direct contact with the crop, highlights the potential for cross-contamination between operational areas.

The other positive sample came from the drainage nutrient solution, indicating a potential contamination risk to the crop. Water samples were passed through a modified Moore Swab filtration system—an approach the researchers recommend.

Sampling Showed Cleaning and Sanitation Approach was Effective

The cleaning and sanitation strategy implemented after the first sampling occasion proved effective, as no L. monocytogenes was detected in samples collected during the latter two sampling occasions, despite an increased number of samples.  The first sampling occasion occurred on the last day after 11 months of production, while the second sampling occasion occurred on the first day after a month-long production break, during which intensive cleaning and sanitation were conducted. The cleaning practices for the harvesting equipment focused on removing all visible debris via scrubbing and pressurized water prior to sanitation to prevent bacterial transfer during operations.

Minimal Dispersal of Contamination from Puddle Throughout Facility

The researchers also conducted an experiment using L. innocua to simulate contamination transfer from a puddle throughout a CEA facility via boots and wheels. In the boot trials, one hour after inoculation, levels of viable L. innocua on the boot and initial floor surfaces were low but comparable at 1.80 and 1.70 log colony-forming units per square centimeter (CFU/cm²), respectively. One hour post-inoculation in the wheel trials, a significant decrease in viable L. innocua was observed from the wheel surface (2.68 log CFU/cm²) to the first floor point (1.02 log CFU/cm²). At 24 hours post-inoculation, L. innocua counts were below the limit of detection at all floor points for both boot and wheel trials. Overall, the transfer of contamination from a puddle and its contribution to the dispersion of L. innocua were minimal under the studied environmental conditions.