Influence of culture media on the recovery of psychrotrophic Clostridium spp. associated with the spoilage of vacuum-packed chilled meats

Abstract

This study was undertaken to determine the influence of culture media on the quantitative recovery of vegetative cells and spores of psychrotrophic Clostridium spp. associated with the spoilage of chilled meats. For recovery of vegetative cells and spores (presumptive counts), 24 h or 48 h broth cultures in Peptone Yeast Extract Glucose Starch (PYGS) broth were used; for spore counts, concentrated spore suspensions derived from 35-day cultures were used. For presumptive counts, seven non-selective and eight selective media were tested. Recovery of psychrotrophic clostridia with optimum growth temperatures between 15 degrees C and 20 degrees C was best with non-selective media, such as Peptone Yeast Extract Glucose Starch (PYGS) agar with lysozyme; recovery of clostridia with growth optima between 25 degrees C and 30 degrees C was best with selective media, such as Shahidi Ferguson Perfringens (SFP) agar. For organisms with the lower optimum growth temperature (heat-sensitive group) spore recovery after heat treatment (80 degrees C for 10 min) was best if a 2 M (pH 10) thioglycollate treatment (10 min at 45 degrees C) was used, followed by plating onto a lysozyme- or egg-yolk-containing medium. For organisms with the higher optimum growth temperature (heat-resistant group), spore recovery was best on Glucose Starch agar without added lysozyme. The diversity among psychrotrophic Clostridium spp. associated with chilled meat spoilage precludes the identification of a single 'best' recovery medium or technique. Consequently, a variety of complementary selective techniques and media must be used if comprehensive recovery is to be assured.