Antibacterial discovery has become a pressing global challenge. Rising and mean we urgently need new and measures. Food production problems such as and are driving demand for too.
Authors
Tim Cushnie
Lecturer, Medical and Health Science Programmes, Mahasarakham University
Darren Sexton
Associate Professor in Immunology, Liverpool John Moores University
Vijitra Luang-In
Associate Professor of Biotechnology, Mahasarakham University
Many antibacterial products in current use come from nature, two examples being the antibiotic and the food preservative . Scientists think antibacterial production evolved for self-defence, helping small organisms and large organisms . In small organisms inhabiting a larger one, for example friendly gut microbes, antibacterial molecules may serve of these functions.
This inspired us to consider scavengers such as vultures, hagfish and blowflies. Scavengers feed on decomposing animal carcasses or carrion, a diet you might expect to contain disease-causing bacteria. We were curious if this expectation is supported by scientific data. We also wanted to know if scavengers or their gut microbes have evolved any special defences. And if, like streptomycin or lysozyme, these defences could be developed for human use.
, published in Critical Reviews in Biotechnology, set out to answer these questions. We began by cataloguing the diets of over 600 species of scavenger animals, then cross-referenced this information with current scientific research on infectious disease, immunology and biotechnology.
Exposure to disease-causing bacteria
We identified three factors that make carrion a high-risk food. First, some animal carcasses enter the food chain due to bacterial disease. Animals succumb to many types of infection and can harbour , and disease-causing bacteria after death.
Animals that die from noninfectious causes also pose a risk because , dormant in the soil and seabed, can colonise their carcasses.
Contact with fellow scavengers represents yet another risk as some, for example feral pigs, blow flies and muscid flies, carry , and infectious bacteria. Interactions between scavengers are likely during feeding, especially when a carcass is large.
Scavenger defences
Our review found that scavengers have multiple antibacterial defences. At a basic level, these include risk-reduction behaviour. For example, and avoid particularly rotten carrion. avoid carrion that’s been sitting in the warm summer sun. And prefer predator-killed animals to those that have died from unknown causes.
The physiological defences scavengers have evolved are even more interesting. For example, hold food in their stomachs for up to 12 hours, twice as long as . This gives their stomach acid longer to kill bacteria before they reach the gut. In and other insects, the gut is protected by a special lining made from the antibacterial material chitin.
Surveillance is another important aspect of defence. And new molecules specialising in bacterial recognition have been discovered in the immune systems of , and . These molecules differ structurally from those identified in other animals and the bacterial recognition molecules in non-scavenging animals.
New chemical defences have been discovered in scavengers too. These are molecules of different size and structure that inhibit or kill invading bacteria. For example, blow flies produce , and that protect their outer surface and circulatory system. Also, friendly microbes in griffon vultures and common sexton beetles produce antibacterial molecules called and that protect their host’s gut.
Antibacterial product development
Some of the above defences, by virtue of their ability to recognise or damage bacteria, may offer opportunities for product development. This type of research has already begun in , , the and other countries.
Small and medium-sized molecules that inhibit or kill bacteria, for example and , could potentially be developed as antibacterial drugs. Of particular interest are AMPs that are active . Biofilms are sticky layers of bacteria that infect and are very difficult to treat.
Larger molecules called are good at recognising bacteria and could be useful in to guide treatments to sites of infection. Also, and molecules are being studied as antibacterial construction materials for and . Chitin can be obtained from various organisms, but hold promise as a new eco-friendly source.
In agriculture, scavenger-sourced molecules may offer an alternative to antibiotics in animal feed. For example, the antimicrobial lipid inhibits disease-causing bacteria in poultry and animal growth and immunity. Antimicrobial lipids work differently to and are to generate resistance.
Scientists are also investigating scavenger AMPs as a way to improve in food crops. And bacteriocins are being tested for possible use as . With the world’s population due to exceed , studies like these are needed to increase .
Hope for the future
Our review shows that scavengers encounter lots of dangerous bacteria through their diet and dining companions, and have marshalled many defences to protect themselves.
Efforts to develop these defences into new antibacterial products have already begun. And, with over 90% of scavenger species still unstudied or understudied, it is likely that additional useful defences remain to be found.
Contrary to their reputation as “harbingers of death”, we think scavengers could help breathe new life into the challenging field of antibacterial discovery.
