Platypus milk has protein with potential to fight superbugs

The milk of the duck-billed platypus has a unique protein with antimicrobial properties that Australian scientists believe could be a new lead in creating antibiotics effective against superbugs.

Australian researchers find monotreme lactation protein has unusual structure that helps it kill bacteria

The duck-billed platypus is a monotreme that lays eggs but produces milk for its young. (Laura Romin and Larry Dalton Wildlife Reflections/CSIRO)

The milk of the duck-billed platypus has a unique protein with antimicrobial properties that Australian scientists believe could be a new lead in creating antibiotics effective against superbugs.

The platypus is already a strange creature — a venomous mammal with a beaver-like tail and duck bill. Because it is a monotreme, it lays eggs and hatches its young outside its body, but the mother lactates milk to feed them through a mammary pad in its abdomen.

Scientists at Deakin University in Geelong, near Melbourne, isolated the milk proteins in platypus milk as part of their study of lactation among monotremes and marsupials.

All mammals have antimicrobial proteins in their milk, developed as a way of protecting and strengthening the very young. But the platypus milk had a monotreme lactation protein unlike anything the researchers had seen before, says Janet Newman, a bioscientist with the Commonwealth Scientific and Industrial Research Organization (CSIRO) based in Melbourne.

Molecular view of milk protein

Newman is lead author of a paper in Structural Biology Communications that describes the unusual crystalline structure of the protein, which has a deep fold that could be related to its effectiveness in killing bacteria.

To study its properties, researchers grew a crystal of this protein and examined it using crystallography, which allows them to see down to the molecular level.
Monotreme lactation protein comprises a monomer of 12 helices and two short strands. Its unique structure also includes a deep fold. (CSIRO)

"How the structure is arranged gives it its properties," Newman told CBC News.

"It tells us this protein is not like any other protein that has been studied over time."

Almost every living organism has tens of thousands of types of proteins, but Newman says it may be worth studying the proteins associated with monotremes further because of the antimicrobial properties found in the milk protein.

Kills bacteria

The researchers tested that by exposing the protein to Staphylococcus aureus and Enterococcus faecalis, both ubiquitous bacteria that can cause infections. The monotreme milk protein killed them.

Deakin University's Julie Sharp said platypus babies are vulnerable to bacteria around them in the environment because they hatch from an egg and are nurtured outside their mother's body. The milk itself is exposed to air before they can suckle.

Researchers believe the antibacterial properties of the milk were an adaptation by monotremes for the survival of the young.

Newman said it's also a promising lead in developing new antibiotics that could defeat superbugs, the new strains of bacteria that have become resistant to known antibiotics.

The World Health Organization released a report in 2014 warning of the global danger of increasing antibiotic resistance. In 2015, it got health and agriculture ministers from around the world to agree to work to reduce the use of antibiotics, including antibiotics fed to food-producing animals.

But scientists remain on the lookout for new ways of fighting bacteria as antibiotic resistance grows.

There are many questions to be answered before researchers know if the proteins in platypus milk can be developed in this direction, Newman said.

Among them are, "Where does the bacteria and the fold [in the protein] interact?" she said.

The researchers at CSIRO and Deakin did their early research on platypus milk "because it was cool," she said, but didn't have dedicated funding.

Now they are hoping their published findings will attract backers and collaborators who can take the discovery further.