2.2.6. Listeria monocytogenes

L. monocytogenes is best known as a serious human pathogen that is transmitted to humans via consumption of contaminated food products. However, it is also a bacterium that occurs commonly in soils, associated with decaying plant matter. Thus, it can be naturally associated with any plant product grown in contact with soil. Unusually for human pathogens, it can use plant sugars for growth and hence, the organism is often a problem associated with minimally processed fruit or vegetables. Specifically, when vegetables or fruit are cut, sugars are released which the organism can utilize for growth.

Listeria in plant tissue. It is still unclear whether L. monocytogenes can become internalized in plant tissue.

The information currently available on the internalization of Listeria is from very limited studies and some of the results are contradictory. It has been shown that, like E. coli, Listeria can enter the stomata of leaves but whether this is an active process or a passive entrapment of bacteria within the structure (which is far larger than the bacterial cell) has not been determined. However, bacteria able to enter natural openings in the plant can be protected from various sanitizers following closure of the guard cells. Since the organism is commonly found in soil, the entry of Listeria into leaf tissue is more likely to occur post-harvest rather than during growth. The fruit and vegetable crops can be contaminated post-harvest when there is a big enough temperature differential between the crop and wash water to allow movement of water into the plant tissue. Following entry, it is likely that the bacteria are transported via the mass flow of water entering the plant and moving within it.

The exception to this is when Listeria-contaminated manure has been used as a fertilizer, as occurred in the Maritime coleslaw outbreak in 1981. This was the first outbreak of L. monocytogenes that could be definitely linked to food and was caused by commercially prepared coleslaw in Canada and resulted in at least 41 cases and 7 confirmed deaths. In this case, it was found that manure from Listeria-infected sheep had been used as an organic fertilizer when growing the cabbages used to prepare the salad, leading to heavy leaf contamination.

Growth of Listeria on fresh produce. Produce commodities can differ in their ability to support growth of L. monocytogenes. Some studies have been carried out to compare growth on intact and damaged or cut produce. Growth has been reported on intact tomatoes, but studies on carrots, cabbage, peppers, and cantaloupe melon reported growth only on cut, shredded or damaged produce. In general, Listeria grows well on shredded lettuce, with populations increasing during storage. Survival and growth patterns on vegetables have been found to vary with strain, product type, and packaging atmosphere. It has been suggested that packaging under modified atmosphere increases the ability of L. monocytogenes to grow by inhibiting competing microflora, but this remains to be proven, and the effect may differ by commodity, atmosphere composition, and storage conditions. However, modified atmosphere packaging (MAP) evidently does not inhibit the growth of L. monocytogenes, and the prolonged shelf life may in itself increase the listeriosis risk.

Significant differences have been observed in terms of the ability of Listeria to survive in heat and acidic conditions and on packaged vegetables. Cooked or pasteurized produce usually supports rapid growth and may represent a risk after post-processing contamination, especially if not refrigerated and consumed without heating. It is now accepted that most produce commodities support the growth of L. monocytogenes.