Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2008
Publication Date: 11/1/2008
Citation: Berry, E.D., Wells, J. 2008. A direct plating method for estimating populations of Escherichia coli 0157 in bovine manure and manure-based materials. Journal of Food Protection 71(11):2233-2238.
Interpretive Summary: There has been an increase in E. coli O157:H7 foodborne illness outbreaks due to the consumption of produce in recent years. One potential source of this pathogen on fruits and vegetables is livestock manure or manure-based products such as compost, which are often used to fertilize the fields in which the produce is grown. While methods to detect the presence of E. coli O157:H7 have been available, accurate procedures for determining the levels of this pathogen had not been developed and tested for manures and related products. Thus, we developed procedures for determining the numbers of E. coli O157:H7 in manures and compost to verify that they are safe for use in fields used for produce production. The method effectively enumerated known numbers of the pathogen in cattle manure, compost, feedlot surface material, and soil to which manure had been applied. In addition, the method successfully estimated the numbers of E. coli O157:H7 in feedlot surface material that was naturally contaminated with the pathogen. This method can be used to determine E. coli O157:H7 numbers in manures and composts prior to their use as fertilizer, and can also be used for measuring the effectiveness of manure treatment processes to reduce this pathogen.
Technical Abstract: Escherichia coli O157:H7 outbreaks associated with produce consumption have brought attention to livestock manures and manure-based soil amendments as potential sources of pathogens for the contamination of these crops. Procedures for enumeration of E. coli O157:H7 are needed to assess the risks of transmission from these manures and their by-products. A direct plating method employing spiral plating onto CHROMagar O157 was investigated for enumeration of E. coli O157:H7 in feedlot surface material, aged bovine manure, bovine manure compost, and manure-amended soil. In studies utilizing samples spiked with a five-strain cocktail of E. coli O157:H7 at levels ranging from 102 to 105 CFU/g of sample, there were strong correlations between the observed and predicted levels of this pathogen. While the addition of 2.5 mg/l potassium tellurite and 5 mg/l novobiocin made the medium more restrictive, these amendments enhanced the ability to identify and enumerate E. coli O157:H7 in feedlot surface material, which contains a higher proportion of fresh feces than the other three sample types, and therefore higher levels of interfering bacterial microflora. The spiral plating method was further assessed to determine its ability to enumerate E. coli O157:H7 in naturally contaminated feedlot surface material. Comparison of E. coli O157:H7 counts in feedlot surface material obtained by the spiral plating method and a most probable number technique were well correlated. We conclude that direct spiral plating onto CHROMagar O157 is effective for estimating E. coli O157:H7 levels in a variety of manures and manure-containing sample types to a lower detection limit of 200 CFU/g. The method has application for determining E. coli O157:H7 concentrations in manures and composts prior to their sale and use as soil amendments, and for measuring the effectiveness of manure treatment processes to reduce or inactivate this pathogen.