Growth of a rifampicin-resistant strain of a proteolytic psychrotroph, Pseudomonas fluorescens, in raw and ultra heat-treated milk

Pseudomonas fluorescens UQM2490, resistant to 250 μg rifampicin/ml, was derived from P. fluorescens JC1, a proteolytic psychrotroph isolated from raw milk. Growth of UQM2490 was followed in raw and ultra heat-treated milk, by viable counting on rifampicin-containing agar medium. The growth curves obtained demonstrate slower growth in raw milk than in treated milk and the variation in growth with change in inoculum level. Generation times in ultra heat-treated milk ranged from 9.5 to 14.1 h compared with 9.6 to 33 h in raw milk.