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United States Department of Agriculture

Agricultural Research Service

Research Project: MOLECULAR MICROBIOLOGY AND CONTROL OF ENTERIC PATHOGENS THAT CONTAMINATE FRESH PRODUCE Title: Sigma B is a determinant of fitness for listeria monocytogenes serotype 4b strain in soil

Authors
item Gorski, Lisa
item Duhe, Jessica -
item Flaherty, Denise -

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 2010
Publication Date: May 27, 2011
Repository URL: http://DOI: 10.1089/fpd.2010.0752
Citation: Gorski, L.A., Duhe, J.M., Flaherty, D. 2011. Sigma B is a determinant of fitness for listeria monocytogenes serotype 4b strain in soil. Foodborne Pathogens and Disease. 8:699-704.

Interpretive Summary: In nature the foodborne pathogen Listeria monocytogenes lives on decaying plant material in the soil from where it can contaminate growing produce. This environment can present many stresses such as ultraviolet light, variations in temperature and humidity, and oxidative stress from growing plant matter in the soil. The alternative sigma factor Sigma B, encoded by sigB, controls the response to most stresses in L. monocytogenes. Fitness in soil and on radishes sown and grown in contaminated soil was measured in a wild type and an isogenic sigB mutant strain to determine if the sigma factor was necessary for life in these niches. Levels of wild type and mutant strains were monitored in soil over the course of radish gestation from seed to mature tuber, and levels on mature radishes were determined. The wild type strain was able to survive in soil over the 4 weeks of the experiment at a level of 10-100 times greater than the mutant strain. The mutant was affected within the first 24 hours after inoculation in the soil. When the radishes were harvested the mutant was 100 times reduced from the wild type strain, and neither strain could be washed or wiped from the radish surfaces. The conclusions are that Sigma B is necessary for initial adaptation to the soil environment, and plays a role in maintaining the population, but probably does not play a role in attachment or colonization of the radish tissue.

Technical Abstract: In nature the foodborne pathogen Listeria monocytogenes lives as a saprophyte where it can contaminate pre-harvest produce. This environment can present many stresses such as ultraviolet light, variations in temperature and humidity, and oxidative stress from growing plant matter in the soil. The alternative sigma factor Sigma B, encoded by sigB, controls the response to most stresses in L. monocytogenes. Fitness in soil and on radishes sown and grown in contaminated soil was measured in a wild type and an isogenic sigB mutant strain to determine if the sigma factor was necessary for life in these niches. Levels of wild type and mutant strains were monitored in contaminated soil over the course of radish gestation from seed to mature tuber, and levels on mature radishes were determined. The wild type strain was able to survive in soil over the 4 weeks of the experiment at levels of 4-7 log CFU/g soil, and the levels of the sigB mutant were reduced by 1-2 log from the wild type. The mutant showed reduced levels in soil by 6 h after inoculation, and stayed at a reduced level through 4 weeks of incubation. Upon harvest of the radishes, 3 – 4 log CFU/g of wild type L. monocytogenes was detected on radishes, and the bacteria could not be washed off of the vegetable by vigorous handling under running water. The mutant strain was 1-2 log CFU/g reduced from the wild type on mature radishes. The levels on mature radishes reflected the levels in the soil at 4 weeks. The conclusions are that Sigma B is necessary for initial adaptation to the soil environment, and plays a role in maintaining the population, but probably does not play a role in attachment or colonization of the radish tissue.

Last Modified: 10/24/2014
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