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United States Department of Agriculture

Agricultural Research Service

Title: Measurement of the growth and control of Listeria species

item Covert, Brittany
item Salley-guydon, Judith
item Arnold, Judy

Submitted to: American Society for Microbiology
Publication Type: Abstract Only
Publication Acceptance Date: 10/21/2006
Publication Date: 11/2/2006
Citation: Covert, B., Salley-Guydon, J., Arnold, J.W. 2006. Measurement of the growth and control of Listeria species. American Society for Microbiology. #11, p 17.

Interpretive Summary:

Technical Abstract: The bacterial pathogen, Listeria monocytogenes, with a 20 percent death rate among its victims, can cause diarrhea, fever, and chills. Most of the food product recalls in the last two years occurred because of contamination with L. monocytogenes. Because of the detrimental effect that these microbes have on consumers as well as the poultry and meat industries, the present study was performed to determine how the growth of these microbes could be inhibited before they reach the public. Listeria strains, isolated from meat and poultry products, were grown on brain heart infusion agar and confirmed by the Gram stain and growth on Chromagar. Staphylococcus aureus, Salmonella cholerasius, Escherichia coli, and Pseudomonas aeruginosa, as representatives of bacterial pathogenicity and spoilage activity, were compared with the Listeria species. A germicide test was performed on all the bacterial species to determine the minimum inhibitory concentration of disinfectant needed to kill each pathogen. Each strain was spread onto a trypticase soy agar plate. In each of six sections on the plate, a test compound was applied in a series of concentrations. The plate was incubated at 37°C for 24 hours, and the zone of inhibition for each concentration was recorded. Measurement of the zone of inhibition from the applied compound indicated the surface area on which the disinfectant killed bacteria. The zones for the species in the experiment ranged from 8.5 to 12.2 mm. The zone was indicative of the minimum inhibitory concentration for each species. There was some inhibitory activity for all species, and the minimum concentration of the test compound that inhibited the growth of Listeria strains ranged from 25 ppm to 300 ppm. To mimic growth of the bacteria in real-world conditions, we evaluated the growth of L. monocytogenes in a biofilm. A biofilm reactor was used to grow biofilms under flow conditions similar to food processing areas. The biofilm formed on stainless steel coupons in the reactor was used to quantify L. monocytogenes. The biofilm growth on a portion of coupons at the completion of each 24 and 48 h was scraped off, placed into trypticase soy broth, and homogenized. A serial dilution and an aerobic plate count were performed to assess the initial presence of the bacteria in the reactor as well as on the coupons after each 24 h. Because of the urgent need for control of this pathogen, we will continue our studies to test an improved inhibitory compound that is targeted against the six Listeria spp. grown as biofilms in the reactor.

Last Modified: 06/23/2017
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