Assessment of antibacterial activity of donkey milk lysozyme : safety and hygiene issues

Donkey milk contains as much lysozyme (LZ) as equine milk (6000 times higher than bovine milk). LZ shows antibacte- rial activity against Gram-positive and some Gram-negative bacteria. LZ activity in donkey milk is very high and some authors supposed that donkey resistance to mammary infec- tion is probably due to high amount of LZ. There is scarce literature on this topic; the aim of the present study is thus to evaluate the antibacterial activity of 3 different donkey bulk milk (DBM) samples, on a total of 42 bacterial strains. The microorganisms were isolated from donkey and bovine milk. Lysozyme activity was evaluated by EnzCheck Ly- sozyme Assay Kit; the milk antibacterial activity was tested by Minimal Inhibitory Concentration (MIC) method, in micro- titer plates. The first milk sample inhibited only one Pseudo- monas sp strain (MIC of LZ = 5800 U/ml); the second sample had no antibacterial effect (MIC > 2740 U/ml), whereas the third sample prevented the growth of 50% of Staphylococcus aureus strains from DBM (MIC = 3400 U/ml). LZ showed no activity against bovine strains. Finally, the authors underline the importance of further studies for a deeper understanding of LZ antimicrobial activity, both for veterinary health purpo- ses and to protect a vulnerable group of consumers, such as in- fants affected by intolerance to some milk proteins.