Milk-deteriorating exoenzymes from Pseudomonas fluorescens 041 isolated from refrigerated raw milk

The practice of refrigerating raw milk at the farm has provided a selective advantage for psychrotrophic bacteria that produce heat-stable proteases and lipases causing severe quality problems to the dairy industry. In this work, a protease (AprX) and a lipase (LipM) produced by Pseudomonas fluorescens 041, a highly proteolytic and lipolytic strain isolated from raw milk obtained from a Brazilian farm, have been purified and characterized. Both enzymes were purified as recombinant proteins from Escherichia coli. The AprX metalloprotease exhibited activity in a broad temperature range, including refrigeration, with a maximum activity at 37 °C. It was active in a pH range of 4.0 to 9.0. This protease had maximum activity with the substrates casein and gelatin in the presence of Ca+2. The LipM lipase had a maximum activity at 25 °C and a broad pH optimum ranging from 7.0 to 10. It exhibited the highest activity, in the presence of Ca+2, on substrates with long-chain fatty acid residues. These results confirm the spoilage potential of strain 041 in milk due to, at least in part, these two enzymes. The work highlights the importance of studies of this kind with strains isolated in Brazil, which has a recent history on the implementation of the cold chain at the dairy farm.

The proteolytic activity of Pseudomonas fluorescens 07A isolated from milk is not regulated by quorum sensing signals

The proteolytic activity of Pseudomonas fluorescens 07A was investigated, and was optimal on tryptone-calcium medium. N-acyl-homoserine lactones (AHLs) were not detected on supernatants of late-exponential and stationary-phase culture broths. Synthetic AHLs or bacterial cell extracts added to the medium did not influence growth or proteolytic activity suggesting that quorum sensing might not regulate protease production in this strain.