Isolation of Listeria spp. from Feces of Feedlot Cattle

MATERIALS AND METHODS Healthy feedlot beef cattle were surveyed for the presence of Listeria spp. in fecal grab samples taken over 3 months. Composite samples were made from 224 individual animals each month. Listeria monocytogenes was isolated from one composite sample (4%) from the first sampling and not from the subsequent two. Listeria innocua was found in composite samples from all three samplings at levels of 17, 9, and 35%, respectively. From the individual samples comprising the Listeria spp.—positive composites, L. monoytogenes was isolated from one sample (3%) in the second sampling but not in the first or third samplings. L. innocua was found in 9, 8, and 10% of the individual samples comprising Listeria—positive composites in the first, second, and third samplings, respectively. The two L. monocytogenes isolates were pathogenic to mice. Further characterization of these isolates revealed atypical rhamnose fermentation patterns. These results indicate that the frequency of isolation of L. monocytogenes from feedlot beef cattle is low. The bacterium Listeria is widespread in nature. These organisms are frequently isolated from a large variety of environmental, food, plant, and animal sources (4,7,12). Because recent outbreaks of foodborne listeriosis have been linked to dairy and meat products, attention has been directed to identifying animal reservoirs (Table 1) of listeriae in order to better understand the transmission of the disease (7). Several workers have screened dairy and beef cattle for fecal carriage of the organism (2,9,14,15,17,26). Listeria screening has been reported for swine (2,26,27), sheep (13,20,21), poultry and goats (2,5,9,78). Fecal material is considered to be a major source of pathogen contamination of the final product as well as the processing environment (1). Because data are available on the incidence of L. monocytogenes and Listeria spp. in dairy cows and other red meat animals, this study was done to screen, specifically, feedlot cattle for fecal carriage of L. monocytogenes and Listeria spp. 'Mention of a trade name, proprietary product, or specific equipment is necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. Test animals A total of 224 cattle (Hereford and Angus crossbred steers) were penned in 22 groups of 10 and one group of four in the U.S. Meat Animal Research Center's (MARC) outdoor feedlot complex located in Clay Center, NE. All test animals were clinically healthy and were 9-12 months old at the start of the study which began in March and was carried on for 3 months. Animals were fed a diet of corn-silage (25%), corn (70%), and protein-mineral concentrate (5%). The feed concentrate was supplemented with the ionophore antibiotic monensin. Experimental design and sampling plan The animals were sampled once a month for 3 months beginning in March. At the time of each sampling, a fecal grab sample was taken from each animal as it was caught in a chute. Disposable plastic gloves were changed between each animal. The samples were stored in sterile Whirl-Pak bags and held frozen at -20°C until analyzed. Samples were thawed and 10 g of feces from each animal within the pen was combined into a 100-g composite sample in a sterile plastic wide-mouth bottle. Nine hundred milliliters of University of Vermont medium (UVM)-2 broth (BBL Microbiology Systems, Becton Dickinson and Co., Cockeysville, MD) were poured over the sample, mixed by vigorous shaking, then incubated for 24 h at 35°C. Listeria assays A sample was considered positive if a Listeria spp. was isolated from either the composite enrichment or the individual enrichment (i.e., culture confirmed). Samples were screened for the presence of Listeria spp. using the Gene-Trak Listeria gene-probe and assay system (GeneTrak Systems, Framingham, MA). Since this system was not designed for fecal analysis, the initial enrichment was modified by enriching the samples directly in UVM-2 broth as described above. An 0.1-ml aliquot from each enrichment was spread plated onto LiCl-phenylethanol-moxalactam (LPM) agar (BBL Microbiology Systems) and incubated for 48 h at 37°C. Growth from this plate was harvested with 1.0 ml of phosphate-buffered saline (PBS) and processed according to the manufacturer's instruction. Culture confirmation on the composite samples, which were positive for Listeria spp. in the gene-probe assay, was conducted as follows: (i) an aliquot from the PBS suspension of growth from the LPM plates was streaked onto LPM and/or Listeria selective isolation agar (Oxford formulation, Oxoid Unipath Co., Ogdensburg, NY) and incubated for 24-48 h at 37°C; (ii) at least JOURNAL OF FOOD PROTECTION. VOL. 56, FEBRUARY 1993 LISTERIA FROM CATTLE FECES TABLE 1. Frequency of Listeria isolation from domestic animals feces by various workers. 103 Species Status" L spp. Lm Other L spp. Ref. Cattle Cattle Cattle Cattle Cattle