Strain Specific Differences in the Attachment of Listeria Monocytogenes to Alfalfa Sprouts

Strain-Specific Differences in the Attachment of Listeria Monocytogenes to Alfalfa Sprouts

November 2004

Journal of Food Protection Volume 67 Number 11 p. 2488-2495
Lisa Gorski,a Jeffrey D. Palumbo,a and Kimanh D. Nguyena

aProduce Safety and Microbiology Research Unit, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan Street, Albany, California 94710, USA

Interpretive Summary: Food-borne illness due to Listeria monocytogenes can lead to systemic disease with the potential for death. Outbreaks and sporadic cases of disease have occurred from contamination of produce. L. monocytogenes is a bacterium that lives in the soil decaying plant material. However, the fundamental physiology of L. monocytogenes as it relates to plants has not been well studied. We observed that there were strain specific differences in the ability of L. monocytogenes to colonize alfalfa sprouts. Some strains colonized very well, were resistant to washing off the sprouts, and competed well with normal sprout-related bacteria for growth on sprouts. Other strains colonized poorly, and were easily washed off the sprout tissue. We determined with the 8 strains studied that there were 5 levels of colonization efficiency ranging from <10 cells/sprout to >100,000 cells/sprout when the sprouts were grown for 3 days at room temperature with daily changes of the irrigation water. Experimental evidence suggested that these differences were due to differential attachment of the strains to the sprout tissue. Microscopic analysis with L. monocytogenes expressing the Green Fluorescent Protein showed that the bacteria associated preferentially with the root hairs of the alfalfa, disregarding the leaves of the plant.

Technical Abstract: Contamination of fresh produce with Listeria monocytogenes has resulted in outbreaks of systemic listeriosis and febrile gastroenteritis. Recalls of alfalfa sprouts have occurred due to contamination with L. monocytogenes. Alfalfa sprouts were used as a pre-harvest model for the interaction between this human pathogen and plants. Eight strains were assessed for their capacity to colonize alfalfa sprouts, and strain specific differences were revealed when the sprout irrigation water was changed daily. Two of the strains colonized and attached to the sprouts very well, reaching levels of >5 log cfu/sprout. The remaining strains varied in their final levels on sprouts between <1log ‘ 4.7 log cfu/sprout. All of the L. monocytogenes strains grew to equivalent levels on the sprouts when the irrigation water was not changed, suggesting the differences observed upon regular changing of the water resulted from differences in attachment. Further analysis of the best colonizing strains indicated that only between 0.3 ‘ 1 log cfu/sprout could be removed by additional washing of the sprout, and the presence of normal sprout bacteria did not compete with the L. monocytogenes strains on the sprouts. The poorest colonizing strain was able to grow in the irrigation water during the course of the experiment, but could not attach to the sprouts. Microscopic examination of the sprouts with L. monocytogenes expressing the Green Fluorescent Protein indicated that L. monocytogenes associated with the root hairs of the sprouting alfalfa, with few to no cells visible elsewhere on the sprout.

Note from the SproutNet:

It is not a common practice for commercial alfalfa sprout producers to reuse their water and change it daily.  We recommend theRota-Tech System and rinsing the sprouts every three minutes with fresh water.  This not only reduces the chance of bacteria clinging to sprouts, it virtually eliminates the roots, as the plant has no need to search for water.  It should also be noted however that, to my knowledge, there has not been any  research undertaken to determine if this eliminates or reduces the ability of Listeria monocytogenes to attach to alfalfa sprouts.