Detection Method for Injured Coliform Organisms in Milk and Dairy Products

Abstract

Coliform organisms have been widely used in assessing the sanitary quality of foods. However, when a population of coliforms in food is exposed to some environmental strains, a part of the cells may become injured. In this case, the use of the conventional selective media is often unsuited to detect injured coliforms. Thus, an appropriate method to resuscitate the injured cells is required. In this study, suspensions of Escherichia coli (108-109/ml) were treated by heating at 63°C for 10min or freezing at -70°C for 5min, and were used for the experiment. A comparison of the recovery of the organism was made on tryptic soy agar (TSA), violet red bile agar (VRBA), desoxycholate agar (DA), VRBA with TSA and DA with TSA. Consequently, among the selective media used, VRBA or DA with TSA gave higher counts than the VRBA or the DA alone. In addition to this, when 5 samples of commercial ice cream were studied, generally VRBA or DA with TSA have yielded a higher number of coliforms. Therefore, the surface-overlay with VRBA or DA on TSA plate was found to be a useful method for the recovery and enumeration of injured coliforms.