I know some people who loudly proclaim milk to be unnatural and eat rice pudding instead of yoghurt, but for most people, cow's milk is part of the daily diet. So it is of no surprise whatsoever when the dairy industry tries to use transgenic technologies to improve its product.
Attempts to make transgenic animals were made ever since the 1970s, but it wasn't until 1980 that Gordon et al. developed the now widely used DNA microinjection technique in mice that the creation of such animals became more feasible. By 1985, scientists were able to make transgenic rabbits, pigs, sheep, and cattle. Nowadays, there are three methods for making transgenic animals. DNA microinjection is the most straightforward method--your genetic construct of choice is directly injected into an embryo at the one-cell stage. In a retrovirus-mediated gene transfer, a retrovirus is used as a vector to carry genetic material to the target cell. The drawback for this particular method is that it generates chimeras and you would have to waste time inbreeding those chimeras for several generations before you get an animal that's carrying your gene of interest in every cell. The third method is embryonic stem cell-mediate gene transfer where stem cells are genetically manipulated and then inserted in the embryo--again, this creates a chimera.
Aside from obvious medical and industrial applications for transgenic animals, there are also agricultural reasons as well--such as the creation of disease resistant animals. For instance, the creation of an influenza-resistant pig would not only halt the spread of a disease through a pig herd but also prevent the virus from using the pig as a vessel to develop into a strain that might infect people. In the April issue of Nature Biotechnology, Wall et al. had this in mind when they made transgenic cows to combat a Staphylococcus aureus infection called mastitis.
Mastitis, infection of the cow udder, is quite difficult to treat due to the location of the infection. Moneywise, it's quite costly to the dairy industry--$2 billion a year in the U.S. Infected cows have to be culled, because not only can the disease be transmitted to other animals, but bacteria can also get into the milk supply. But if conventional antibiotics and vaccines can't help, what can? One possibility is to try to increase the resistance to S. aureus in the cow. Wall et al. generated transgenic cows that were able to fight off infection. The transgene chosen for the task was lysostaphin, an enzyme that targets the cell wall of S. aureus and chops up the glycine residues holding it together.
Normally, staphylococci infect the mammary gland and use milk as a nutrient. As they multiply and spread, they trigger an inflammatory response. In transgenic cows, lysostaphin is also secreted in the mammary gland. When these cows were experimentally infected with S. aureus, no bacteria could be recovered from the milk. The researchers point out that the bacteria were most likely killed upon contact of the secreted lysostaphin before they even had a chance to gain a foothold in the mammary gland to cause infection.
Creating transgenic cows is certainly a way to make more disease resistant animals and the work of Wall et al. prove that this is possible--but is it the way to go about doing this? Aside from the public's qualms about transgenic animals in general--are there any problems concerning this approach? For one, lysostaphin is secreted in the milk. What sort of effect will its consumption be on a person? And more importantly--will this lead to the rise of lysostaphin-resistant strains of S. aureus? This is not an idle worry. S. aureus isn't called the superbug for nothing.
I know some people who loudly proclaim milk to be unnatural and eat rice pudding instead of yoghurt, but for most people, cow's milk is part of the daily diet. So it is of no surprise whatsoever when the dairy industry tries to use transgenic technologies to improve its product.
