Milk Plasmin, N-Acetyl-β-D-Glucosaminidase, and Antitrypsin as Determinants of Bacterial Replication Rates in Whey

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Abstract
Quarter milk samples were collected monthly on a selected herd of 80 Ayrshire cows having a high frequency of subclinical mastitis. Analysis of bacterial growth rates in milk showed that whey prepared from infected or inflamed quarters stimulated bacterial growth. Milk N-acetyl-beta-D-glucosaminidase, antitrypsin, and plasmin activities all showed positive correlations with bacterial replication rates (Staphylococcus aureus and Escherichia coli) in respective whey samples as determined by a turbidometric micro-technique. Increased bacterial replication rates in mastitic whey represent an increased yield of the key nutrients for bacteria. Bacterial growth enhancement can be partly explained by proteose-peptone originating from plasmin activation and casein degradation. However, as multiple regression analysis showed that a combination of the predictor variables: antitrypsin, N-acetyl-beta-D-glucosaminidase and plasmin explained enhancement of bacterial growth better than plasmin alone, other factors connected with inflammation should be sought when searching for growth-enhancing factors in whey. Milk plasmin activities showed increasing activities toward end of lactation (before drying off) as well as during later lactation (age of cow in years minus 2). Bacterial replication was enhanced in parallel with these changes in plasmin activities.

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