Identification and proteolytic activity quantification of Pseudomonas spp. isolated from different raw milks at storage temperatures

Abstract

Commercial milk products worldwide come not only from cows, but also from goats, buffaloes, camels, and yaks. Milk from non-bovine animals is important culturally and economically. Pseudomonas spp. are frequently linked to milk spoilage under storage temperatures. The objectives of this study were to identify Pseudomonas spp. isolated from goat, buffalo, camel, and yak milks, and to measure proteolytic activity of Pseudomonas spp. under different storage temperatures. Raw milk samples of goat (n = 50), buffalo (n = 25), camel (n = 25), and yak (n = 25) were collected from 5 provinces in China. Pseudomonas spp. were analyzed by Pseudomonas-specific 16S, universal 16S rRNA, and rpoB gene sequence analyses. Proteolytic activity on milk agar, quantification via the trinitrobenzenesulfonic acid assay at 2°C, 4°C, 7°C, 10°C and 25°C, as well as alkaline peptidase gene (aprX) identification were performed to ascertain the proteolytic activity of these isolates. Pseudomonas spp. were found in 46 samples out of total 125 samples. A total of 67 Pseudomonas spp. were identified. Of Pseudomonas isolates, we obtained extracellular peptidase activity in 7 (10.4%) at 2°C, 17 (25.4%) at 4°C, 24 (35.8%) at 7°C, 39 (58.2%) at 10°C, and 41 (61.2%) at 25°C. The results revealed that a wide diversity of Pseudomonas spp. were present in different non-bovine raw milks, with the ability to produce peptidases at storage temperatures. However, proteolytic activity varied widely among the peptidase-positive isolates. A majority of isolates from yak milk had high proteolytic activity.

Keywords: Pseudomonas spp.; non-bovine raw milk; proteolytic activity; spoilage.