Salads and Internalized Pathogens: What are They? Should we Worry?

Recently on Examining Food, we reviewed a research paper that reported using PCR to detect a common foodborne pathogen, salmonella, that had internalized within leaves of growing lettuce plants¹. Following surface contamination of growing plants, the researchers could isolate viable colony forming units within plant tissues including the edible leaves, and confirmed this by PCR.

What started as a simple review of PCR options for food safety testing and their application sparked a lively debate in our Comments section. Most comments were about quantifying the risk to human health posed by these internalized pathogens – whether or not a positive finding by PCR automatically translates into risk to human health.

So what does it mean? Is there a risk of foodborne illness from consuming pathogens hidden in the leafy goodness of your bagged salads?

The answer is inconclusive. From an online review of literature dealing with foodborne disease due to pathogens internalized in fresh produce, there doesn’t appear to be a definitive answer to the risk consumers face from internalized human health pathogens in bags of salad. Microbes hiding within lettuce leaves or inside succulent cherry tomatoes, immune to careful washing prior to landing on a plate or in someone’s mouth, may or may not be harmful.

What researchers have shown, though, is that human pathogens such as salmonella and E. coli can colonize plants, reaching internal structures via the plant’s vascular system or by creeping in through leaf stoma². Internalization also happens with surface injury such as bruising, cuts and insect damage. Microorganisms localize in the roots, through growth in contaminated growing media or by infected seeds, and can also spread to edible parts such as leaves or sprouts³. The important fact is that once there, they escape the normal hygiene decontamination measures such as rinsing and wiping.

Unfortunately, wide variation in experimental design means that results are difficult to interpret. Does contamination only occur with hydroponic cultivation or in sterile soil? What about competing soil microorganisms under normal growing conditions? Does the age or health status of the plant make a difference? Why does internalization happen in lettuce but not cabbage?²

In summary, some studies say yes, the pathogens do reach the edible portions and are thus available for consumption while others say no, it doesn’t happen. Is it relevant that most studies take place under experimental conditions rather than out in the field?

Furthermore, the risk to human health has not been stated since apparently no studies have shown a direct link between a disease outbreak and ingestion of raw foods such as salads, fruit or vegetables containing internalized pathogens.

What is certain is that with interest in healthy eating growing, more and more consumers are turning to salads and other raw foods as a bigger part of their daily diets. Current nutritional wisdom says that fruits and vegetables should form a high proportion of daily intake. For convenience with today’s busy lifestyles this means that more households are consuming bagged salads and other ready-to-eat washed and sliced raw foods. Along with this rise in consumption, the CDC reports an increase in cases of foodborne illness caused by these convenience raw fruit and salad food preparations. In the 1970s only 0.7% of all cases were due to salad ingredients; in the 1990s this had risen to 6% of reported cases⁴.

Until a definitive answer on the role that internalized pathogens in fresh produce plays in human disease is available, good hygiene practices, routine monitoring and attention to advances in food safety are required to keep consumers both informed and safe.

References:

1. Ge, C. et al. (2014) “Impact of phytopathogen infection and extreme weather stress on internalization of Salmonella Typhimurium in lettuce,” International Journal of Food Microbiology 168–169(pp.24–31)

2. Warriner, K. et al. (2003) “Internalization of human pathogens within growing salad vegetables”, Biotechnology and Genetic Engineering Reviews 20 (pp.117-34)

3. Wright, K. M. et al. (2013) “The endophytic lifestyle of Escherichia coli O157:H7: Quantification and internal localization in roots”, Phytopathology 103 (pp.333-340)

4. Council for Agricultural Science and Technology (CAST). 2009. Food Safety and Fresh Produce: An Update. CAST Commentary QTA2009-1. CAST, Ames, Iowa

 

Further Reading

• Berger, C.N. et al. (2010) “Fresh fruit and vegetables as vehicles for the transmission of human pathogens”, Environmental Microbiology 12(9) (pp.2385–2397)           doi:10.1111/j.1462-2920.2010.02297.x
• Hirneisen, K.A. et al. (2012) “Human Enteric Pathogen Internalization by Root Uptake into Food Crops”,  Foodborne Pathogens And Disease 9 (pp. 396-405
• Summer Meeting 2007–The Problems with Fresh Produce: An Overview Journal of Applied Microbiology 105(2):317-30 eds. M.P.Doyle and M.C.Erickson
• Potential for Infiltration, Survival, and Growth of Human Pathogens within Fruits and Vegetables http://www.fda.gov/Food/GuidanceRegulation/HACCP/ucm082063.htm