Thursday, March 3, 2011

More on Listeria contamination of vegetables

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 daysThe 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.


Anonymous said...

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?

John Brooks said...

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.

Anonymous said...

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.

John Brooks said...

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.

John Brooks said...

I should point out that the Microbiological Reference Criteria for Foods, available on the New Zealand Government website
(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.

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