Impact of dilution ratios on Listeria monocytogenes growth during University of Vermont medium enrichment of deli meats.
In the U.S. Department of Agriculture (USDA) method for Listeria detection, a 25-g composite food sample is enriched in 225 ml of University of Vermont medium (UVM), giving a detection limit of 0.04 CFU/g. However, in a recent large-scale four-state deli meat survey for L. monocytogenes, 125-g samples enriched in 1,125 ml of UVM were requested to increase the detection limit to 0.008 CFU/g. To circumvent problems associated with large volumes of UVM, the impact on L. monocytogenes growth of lower dilution ratios used for enrichment and most-probable-number (MPN) detection was compared with the results obtained using the conventional 1:10 dilution. In this study, 125-g samples of cured turkey, uncured turkey, ham, and roast beef were inoculated with a six-strain L. monocytogenes cocktail to contain approximately 1 x 10(3) CFU/g. This cocktail was then diluted 1:3, 1:5, or 1:10 in UVM, homogenized, enriched at 30 degrees C, and periodically plated on modified Oxford agar to determine generation times during 24 h of incubation. The same enrichment protocol was also assessed in a three-tube MPN assay using 125-g samples inoculated with L. monocytogenes to contain approximately 1 CFU/g. The effects of two homogenization methods, stomaching and pulsifying, on Listeria growth were compared using oven-roasted turkey breast diluted 1:3, 1:5, and 1:10 in UVM. Overall, the growth rates, generation times, and MPN values for each of the four selected deli meats were similar (P > 0.05) using UVM enrichment ratios of 1:3, 1:5, and 1:10, with no significant (P > 0.05) differences in L. monocytogenes growth rate or generation time between experiments using pulsifying and stomaching. These findings indicate that lower volumes of UVM can be used in the USDA procedure when examining deli meats without compromising Listeria recovery.
Zhang
Lei
L
Department of Food Science and Human Nutrition, Michigan State University, East Lansing, Michigan 48824-1225, USA.
Yan
Zhinong
Z
Ryser
Elliot T
ET
D000818:Animals; D002417:Cattle; D015169:Colony Count, Microbial / Q000379:methods / Q000592:standards*; D003470:Culture Media / Q000737:chemistry*; D005506:Food Contamination / Q000032:analysis*; D005516:Food Microbiology; D008089:Listeria monocytogenes / Q000302:isolation & purification*; D008461:Meat Products / Q000382:microbiology*; D012680:Sensitivity and Specificity; D013552:Swine; D014422:Turkeys
D003470:Culture Media