Impact of Extreme Climatic Events on Microbial Safety of Leafy Greens: Flooding

Introduction: Growing field used for leafy greens may periodically be subject to flooding, which affect the safety of fresh produce mainly through the spread of faecal contamination. However, information on the impact of flooding on the microbial safety of vegetables is still needed. Within Europe, the south-east region of Spain represents the main vegetable producer. This area is frequently affected by extreme climatic conditions such as flooding at the end of the summer season.

Purpose: The purpose of this study was to evaluate the impact of environmental flooding on the microbial safety of lettuce.

Methods: Last year, according to the State Meteorological Agency (AEMET), there was a rainfall of 105 L/24 hours the last two days of September, which is almost 800% above the historic average of the Southeast of Spain. One week after this flooding event, samples were taken to evaluate the microbial safety of irrigation water, soil and lettuce in four different growing fields. Samples were taken up to 45 ays after the climatic event. The microbial parameters evaluated were Escherichia. coli spp., Enterococcus spp., E. coli O157:H7, VTEC and Salmonella.

Results: The analysis of the soil and lettuce after one week of the flooding event revealed that E. coli spp. was found in all the tested samples, including lettuce (3.3 ± 0.2 log CFU/g). Higher values were observed for irrigation water and soil samples (3.6 ± 0.1 and 4.0 ±  0.1 log CFU/g, respectively). Enterococcus were found in water samples (3.5 ± .1 log CFU/g). Several samples (water, soil and plant) were positive for Salmonella and VTEC. However, samples taken after 15 or more days of the flooding event were negative for E. coli spp. or pathogenic microorganisms.

Significance: These data suggest that flooding represents a main source of contamination but, depending on the climatic conditions, survival of pathogenic microorganisms might be very low.