Lysozyme: just an additive or a technological aid as well?

Production and Processing of Milk from Transgenic Goats Expressing Human Lysozyme in the Mammary Gland

The potential for applying biotechnology to benefit animal agriculture and food production has long been speculated. The addition of human milk components with intrinsic antimicrobial activity and positive charge to livestock milk by genetic engineering has the potential to benefit animal health, as well as food safety and production. We generated one line of transgenic goats as a model for the dairy cow designed to express human lysozyme in the mammary gland. Here we report the characterization of the milk from 5 transgenic females of this line expressing human lysozyme in their milk at 270 microg/mL or 68% of the level found in human milk. Milk from transgenic animals had a lower somatic cell count, but the overall component composition of the milk and milk production were not different from controls. Milk from transgenic animals had a shorter rennet clotting time and increased curd strength. Milk of such nature may be of benefit to the producer by influencing udder health and milk processing.

Pharmacological aspects and therapeutic applications of lysozymes

The therapeutic effectiveness of lysozyme (large scale manufactured hen egg-white lysozyme) is actually based on its ability to control the growth of susceptible bacteria and to modulate host immunity against infections and depressions of immune responses. If the former is based on the first evidence of the biological activity of this enzyme, the second is a relatively recent acquisition of extreme importance for the possibilities offered in terms of the regulation of the functioning of the host's immune system. Antibotic activity and immune stimulating effects are also used together, as in the case of the treatment of gastrointestinal infections, including those originated by therapeutical treatments. Based on these biological properties, in addition to the wide range of therapeutic activities for which lysozyme was exploited in the past, at present the most promising data concern the prevention of bacterial cariogenesis and treatment of cancer patients to improve the effectiveness of anticancer drugs or to allow the host to recover from the immune suppression caused by anticancer treatments. However, lysozyme does not yet hold a clear place as an immune modulating agent, in spite of the fact that it has been shown to stimulate immunity with no difference between experimental animals and human beings. The hope is therefore that doctors will understand its potential and that they will take advantage of the existence of this simple and useful molecule.

Mammary gland expression of transgenes and the potential for altering the properties of milk

Transgenic animals are a useful in vivo experimental model for assessing the ability and impact of foreign gene expression in a biological system. Transgenic mice are most commonly used, while transgenic sheep, goats, pigs and cows have also been developed for specific, "applied" purposes. Most of the work directed at targeting expression of transgenes to the mammary gland of an animal, by using a milk gene promoter, has been with the intent of either studying promoter function or recovering the desired protein from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk resulting in novel manufacturing properties. The properties of milk have been altered by adding a new protein with the aim of improving the milk, not of recovering the protein for other uses.

The effect of mammary gland expression of human lysozyme on the properties of milk from transgenic mice

Transgenic mice were used as model systems to evaluate the impact of human lysozyme expression in the mammary gland. We previously generated two lines of transgenic mice that express human lysozyme mRNA in the mammary gland under the tissue-specific and developmentally correct control of the bovine gene promoter for alpha s1-casein. Concentrations of human lysozyme protein in milk of transgenic mice varied from .25 to .71 micrograms/microliters of milk. Human lysozyme secreted into mouse milk retained its antimicrobial activity, as determined by a denaturing polyacrylamide gel activity assay. The physical and functional properties of the milk were also altered, because mouse milk containing human lysozyme had a 35% decrease in rennet clotting time, a smaller median micelle size (157 nm vs. 172 nm), and a 2.5- to 3-fold greater gel strength than control milk. From these results, we conclude that the use of transgenic animals producing lysozyme in the milk is feasible and potentially useful to the dairy industry.

Sensitization of heat-treated Listeria monocytogenes to added lysozyme in milk

Listeria monocytogenes was highly resistant to hen egg white lysozyme in whole milk but was sensitive in media and in phosphate buffer. Methods to sensitize the pathogen to lysozyme in milk were investigated. Treatment of whole milk by cation exchange to remove minerals, particularly Ca2+ and Mg2+, slightly promoted inactivation of L. monocytogenes by lysozyme at 4 degrees C over a period of 6 days. Heat treatment (62.5 degrees C for 15 s) strongly sensitized L. monocytogenes to lysozyme in demineralized milk and in MES [2-(N-morpholino)ethanesulfonic acid] buffer. Addition of Ca2+ or Mg2+ to the demineralized milk restored resistance to lysozyme. Cells were more rapidly heat inactivated at 55 degrees C in demineralized milk containing lysozyme, and addition of Ca2+ to the demineralized milk restored the resistance to heat. The results indicate that minerals or mineral-associated components protect L. monocytogenes from inactivation by lysozyme and heat in milk, probably by increasing cell surface stability. The heat treatment of foods containing added lysozyme can probably play a significant role in producing microbiologically safe foods.