The Effects of Changes in Sodium and Potassium Concentration on the Growth of Mastitogenic Bacteria In Vitro

Mastitis is characterized by increased sodium and decreased potassium in milk, and the effect of this on bacterial growth was examined. Colonies of Staphylococcus aureus, Streptococcus uberis, and Escherichia coli were grown in tryptone broth containing 0−75 mM Na or 0−40 mM K. For S. aureus, the total viable count (TVC) growth rate decreased as Na increased from the normal level to that typical of mastitic milk. When Na was increased, time to peak TVC and lowest pH both increased, suggesting that the fatty acid utilization phase was retarded, and final pH decreased while the time to reach it increased, suggesting that the protein utilization phase was also retarded. For E. coli and S. uberis, there were less clear effects of varying Na or K. It is concluded that an increase in milk Na, such as normally occurs during a mastitic event, reduces the growth of one mastitogenic bacteria, S. aureus, but not E. coli or S. uberis. Moulton Agriculture College University College Northampton Northamptonshire, United Kingdom School of Veterinary Science University of Queensland