Inhibition of Enteropathogenic Escherichia coli by Homofermentative Lactic Acid Bacteria in Skimmilk

Abstract

Inhibition of enteropathogenic Escherichia coli in skimmilk at 21 and 32 C by 0.25 and 2.0% of added Streptococcus lactis, Streptococcus cremoris, or a mixed strain starter culture was studied. After 15 h of fermentation, fermented milks were refrigerated at 7 C and then were tested periodically for survival of E. coli. Three methods for enumeration of E. coli during these fermentations were compared. They included trypticase soy agar (TSA) pour plates, violet red bile agar (VRB) pour plates, and TSA surface plating with a VRB overlay. Lactic cultures had similar inhibitory properties at 32 C, but there were differences at 21 C, with S. lactis being least inhibitory and the mixed strain culture most inhibitory. The VRB pour plate method gave poorest recovery of E. coli when fermentation was at 32 C and when fermented milks were refrigerated. The TSA surface plating method apparently allowed for recovery of injured E. coli cells and gave results similar to the TSA pour plate method.