Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 3, 2003
Publication Date: June 15, 2003
Citation: COLEMAN, P., TAMPLIN, M.L., PHILLIPS, J.G., MARMER, B.S. INFLUENCE OF AGITATION, INOCULUM DENSITY, PH, AND STRAIN ON THE GROWTH PARAMETERS OF ESCHERICHIA COLI O157:H7 - RELEVANCE TO RISK ASSESSMENT. JOURNAL OF FOOD PROTECTION. 2003. Interpretive Summary: Regulatory agencies require information about the factors that affect the growth of pathogenic strains of E. coli O157:H7 in ground beef so that they can provide policy and guidance on proper food handling procedures to food industries and consumers. This information is also needed by risk assessors to inform regulators and risk managers about the conditions that limit the growth of E. coli O157:H7. Risk assessors sometimes estimate th growth of E. coli O157:H7 from research studies that used microbiological broth as a growth medium. In many instances, these broth test systems used relatively high starting levels of E. coli O157:H7 and vigorous culture agitation, without examining the effects of these conditions on E. coli O157:H7 growth. Consequently, research is needed to evaluate the growth of E. coli O157:H7 under experimental conditions that are most relevant to scenarios of ground beef contamination. The present study investigated the einfluence of the following factors on the growth of E. coli O157:H7 in a microbiological broth culture system: the starting level of E. coli O157:H7, the agitation of culture broth during incubation, strain-dependent variation in growth, and incubation temperatures of 10, 19 and 37 degrees Celsius. The results showed that the initial E. coli O157:H7 level and culture agitation had significant effects on the growth rate of E. coli O157:H7 at 10 degrees Celsius, but not at higher temperatures. Variations in the growth of nine different strains of E. coli O157:H7 were observed at high levels of acid and low temperature that approached the lower boundaries of growth. This information demonstrates that models of the growth of E. coli O157:H7 must be developed under specific conditions to allow results to be applied to food.
Technical Abstract: Extrapolating data from pure culture broth-based systems to a microbial risk assessment for food can lead to unexpected results due to the effects of the food matrix and native microflora. Foods differ in at least two key variables from broth culture systems typically used in predictive microbiology: 1) initial population density of the pathogen and 2)agitation nof the culture. The present study used nine E. coli O157:H7 strains isolated from ground beef and associated with human illness.Initial experiments using brain-heart infusion (BHI) broth at pH 5.5 with one E. coli O157:H7 strain were performed in a 2x2x3 factorial design testing the effects of a low (ca. 1 to 10 colony-forming units [cfu] per milliliter) or high (ca. 1000 cfu per milliliter) initial population density, culture agitation or no culture agitation, and incubation temperatures of 10, 19 and 37 degrees Celsius. Kinetic data were modeled using simple linear regression and the sigmoidal Baranyi model. Significan effects of agitation and initial population density were identified at 10 degrees Celsius but not at 19 or 37 degrees Celsius. Similar growth patterns were observed for two additional strains. Further tests were conducted in a 96-well microtiter plate system to determine the effect of initial population density and low pH (4.6 to 5.5) on the growth of E. coli O157:H7 strains in BHI at 10, 19 and 37 degrees Celsius. Strain variability was more apparent at the boundary conditions of growth of low pH and low temperature. This study demonstrates the need for growth models which are specific to food products and environments. Further research is needed to measure the impact of the spoilage flora on the growth of low initial pathogen densities for risk assessment exposure scenarios.