On 18th January 2011, I wrote about the withdrawal of salad vegetables from certain supermarkets in New Zealand because of possible contamination by Listeria (See Listeria Hysteria).
This week, I came across a paper published in Food Microbiology, describing the transfer of Listeria innocua from contaminated compost and irrigation water to lettuce leaves. (M. Oliveira et al. Food Microbiology 28 (2011) 590-596).
These authors noted in their introduction that "Fresh produce can become contaminated at any point during the primary production, processing, distribution and preparation. Primary sources of preharvest contamination include soil amended with untreated or improperly composted manure, contaminated irrigation water, the presence of wild and domestic animals, infected workers, and unclean containers and tools used in harvesting. Research has demonstrated that many human pathogens are able to survive for extended periods in soils, manure and water."
Listeria monocytogenes causes Listeriosis, a rare but serious disease in humans, with an incidence of 2-3 cases per million of the total population in England and Wales.
In their experiments, Oliveira et al used an avirulent strain - Listeria innocua. They transplanted lettuce seedlings into pots containing soil and contaminated compost; compost manually surface irrigated with contaminated water after the seedlings were transplanted; and treated a third set of seedlings by hand-spraying contaminated irrigation water onto the lettuce leaves after transplanting.
These workers reported that the lettuce leaves became contaminated with Listeria from the soil and, not surprisingly, sprinkling with contaminated water also resulted in contamination of the leaves with Listeria, which survived for some days. The bacteria survived in the compost for more than 9 weeks, giving ample opportunity for contamination of the leaves by transfer of soil.
Clearly, as these workers showed, irrigation water used for fresh salad vegetable growth must be pathogen free and manure used in compost must also be treated to remove pathogens.
I am a professional food microbiologist. I created this blog to help ordinary folk and food manufacturers understand food safety so that we can reduce the incidence of food mediated illness. I would like it to become a resource for teachers in schools. If readers send comments or questions, I'll try to incorporate them into future postings. Write to: FoodMicrobiologist.007@gmail.com I hope you find the site useful and perhaps even thought-provoking.
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- John Brooks
- Ashhurst, Manawatu, New Zealand
- Safe Food is written by John D. Brooks. The articles are my own opinions and observations. If you would like to contact me to comment or make suggestions for articles, please write to: FoodMicrobiologist.007@gmail.com
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9 comments:
Last summer, I threw a cantaloupe contaminated with listeria into my compost bin. I used that compost in my garden this year. Should I worry about contamination of my veggies in this years crop?
A difficult one to answer. Certainly, you have put Listeria into your compost bin and the organisms might not be killed by the composting process unless the temperature got up above about 70C. However, Listeria are found in the soil, rotting organic matter and surface water, so they may have been in your vegetable garden anyway.
I think the thing to do is keep vigilant about not bringing soil into the kitchen and allowing cross contamination between raw and cooked foods. Ensure that you rinse your salad vegetables thoroughly before consumption.
If you are a normal, healthy adult and not pregnant or immunocompromised, I reckon "worry" is too strong a word.
Thanks for your comment.
What about if I have plants growing from seeds in my compost? For example, in my garden I have a cantaloupe plant growing, even though we never planted it intentionally. There is no way it could get into the "blood lines" of the plant, right? I ordinarily wouldn't be so concerned, but my children do usually eat from our garden as well.
Anonymous 2:
As Oliveira et al showed, when they transplanted seedlings into soil containing contaminated compost, the seedlings became contaminated. I see no real difference between transplanting and direct seeding.
The cantaloupe melon is fairly unlikely to become contaminated internally, but the skin, which is very wrinkly, may trap soil particles and bacteria. If the skin is not thoroughly washed before cutting, the knife may transfer bacteria into the cut flesh.
We should not get this out of proportion. Provided that you and your children are healthy and you carefully wash the melon before cutting, then eat it straight away, there should be no problem. You could even add a little chlorine bleach to the wash water if you are concerned, but make sure you remove all traces of soil. Don't give the melon to very young children and don't store it for long periods (even in the refrigerator) after cutting.
I should point out that the Microbiological Reference Criteria for Foods, available on the New Zealand Government website www.foodsafety.govt.nz
(search by typing Microbiological Reference Criteria) shows that the reference criteria for Listeria in food do NOT apply to fresh fruits and vegetables. This is a recognition that it will be found on some occasions, though from a good manufacturing practice point of view, fruit and vegetables offered for sale should not routinely contain Listeria.
My home was built on a 30 degree slope and unusual torrential rains washed a large pile of bio matter (top soil, tiny twigs and rabbit/goat/sheep feces) onto one run-off point on my property. This pile looked like great "soil" to plant in so I started my first veggie patch with it, adding some very old chicken manure from my old chicken coop and a few vegetable food scraps.
Do you think there is any risk of listeria or other pathogens making the veggies (which are growing nicely) unsafe to eat? If so, is there a way to clean them or do I need to get rid of them all.
How do you make safe compost?
Thanks for your comment. It's not a straightforward question, as are many! Your vegetables are growing well and it is a common practice to use animal manure as fertiliser. This has been done for years in Europe. However, there is always a risk associated with using animal manure, as some animal pathogens can also infect humans. In the case of vegetables like peas and beans and cabbages, they are normally cooked before consumption, so Listeria etc. will be killed. Washing off all traces of soil and peeling carrots if you intend to eat them raw will also reduce the risk.
Lettuces, however, are a different situation. The best you can do is to wash the leaves carefully (if the lettuces are the type that heart up, you can discard the outer leaves). Remember that Listeria is found in the environment, including soil, so will often be found also on vegetables and you will therefore often be exposed to it.
I certainly would not get rid of your vegetables; just be careful to remove all soil before consumption and be especially careful if your or your family are pregnant.
Ideally, your compost heap should get hot and kill many bacteria. However, well-rotted compost made of leaves, lawn clippings and vegetable scraps is fine for digging into the garden.
Hello. How likely is it that Listeria contaminates flowers one buys in a supermarket, like in a bucket. They don't contain soil. Are grown on animal manure though. Thank you.
Hello Sindromesyunsiglo
Thanks for your comment. This is not an easy one either! Not all countries and flower growers use animal manure. However, as I said in the blog, Listeria is likely to be found in both vegetables and flowers grown in soil (as distinct from hydroponics). The answer is to ensure that no cross contamination occurs from your flowers to foods that will not be heated before consumption.
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