Comparison of Sorbitol MacConkey and Hemorrhagic Coli Agars for Recovery of Escherichia coli 0157:H7 from Brie, Ice Cream, and Whole Milk

Abstract

Abstract The relative efficacies of hemorrhagic coli (HC) agar and several formulations of sorbitol Mac-Conkey (SorMac) agar, with and without 0.1 % (w/v) 4-methyllumbelliferyl-ß-D-glucuronide (MUG), in recovering unstressed and heat-stressed Escherichia coli 0157:H7 from Brie cheese, ice cream, and whole milk were determined. Recovery of unstressed E. coli 0157:H7 was determined quantitatively by spread-plating diluted samples onto different agars and performing plate counts. Recovery of stressed E. coli 0157:H7 was determined qualitatively by enriching samples in modified trypticase soy broth, streaking the incubated enrichments, and isolating E. coli 0157:H7 colonies from the agars. HC agar and the SorMac agar formulations did not differ significantly in their ability to recover unstressed E. coli 0157:H7 from ice cream and whole milk; however, HC agar recovered significantly more unstressed E. coli 0157:H7 from Brie cheese than did the SorMac agar formulations. Bacteriological Analytical Manual and Oxoid SorMac agar formulations made from individual ingredients, did not differ significantly in recovering unstressed E. coli 0157:H7 from Brie cheese. The efficiency of the commercially available Oxoid SorMac agar could not be determined because of overgrowth by indigenous microflora. HC and SorMac agars did not differ significantly in recovering stressed E. coli 0157:H7 from Brie cheese, ice cream, and whole milk. MUG had no apparent effect on recovery of either stressed or unstressed E. coli 0157:H7 from the dairy foods examined.