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 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.