P-012. Comparison of Acid and Oxidative Stress Resistance of Listeria monocytogenes Strains with Sprout Colonization

L. Gorski, D. Flaherty, J. M. Duhé;
USDA, ARS, WRRC, Albany, CA.

Seventeen strains of the foodborne pathogen Listeria monocytogenes were tested for their ability to colonize alfalfa, radish, and broccoli sprouts, as well as their capacity to withstand acid and oxidative stress, two stresses common to the sprout growth environment. Whereas large variations in different strains’ abilities to colonize alfalfa sprouts were confirmed (between <1 log CFU/sprout - > 5 log CFU/sprout), the variations with radish and broccoli sprouts were less severe. Strains colonized radish sprouts to between 5 - 6 log CFU/sprout, and broccoli sprouts to between 3 - 5 log CFU/sprout. With a few exceptions, strains that were poor colonizers of alfalfa tended to be among the low colonizers of radish and broccoli and vice versa. The strains also showed variability in their resistances to both acid and oxidative stress. The amount of culture death in response to pH 2.5 exposure among the different strains ranged from 2 - 9 log CFU/ml. There was a 2 log difference among the strains in resistance to oxidative stress by cumene hydroperoxide exposure. When the stress resistance of the strains was compared with their ability to colonize the three sprout types, statistical analysis showed that there was no correlation between acid stress and sprout colonization, but there was a positive correlation between resistance to oxidative stress and colonization of all three sprout types. More strains are being tested. Microscopy with GFP-expressing L. monocytogenes showed that the bacterium prefers to colonize the roots of the sprouts, where free radicals would be produced. While the response to oxidative stress is known to be important for L. monocytogenes virulence, it may also be important for life outside of a host.