By Alison Cranage, science writer at the Wellcome Sanger Institute
With an immune system like ours, but no central nervous system, caterpillars provide an ideal way to study how pneumonia-causing bacteria work. Their usefulness also offers the promise that caterpillars might be able to replace animals in a wide range of medical research experiments in the future
Humans share 90 per cent of their genes with every other species of mammal. We all have the same vital organs – a brain, heart, liver, lungs and kidneys. We also share one of our most essential features with caterpillars. These squidgy grubs don’t have a nervous system or any organs that resemble ours, but they do have a similar innate immune system.
Using caterpillars, instead of mice, to investigate dangerous Klebsiella pneumoniae bacteria, won first prize in our annual 3Rs awards.
The 3Rs – Replace, Refine and Reduce
The 3Rs are the principles of Replacing, Refining and Reducing animals in research and testing. Using animals in research is important for many studies – especially those to develop and test new medicines. We wouldn’t have many cancer treatments or an Ebola vaccine without animal research.
While research using animals is important, it is illegal in the UK if there is any alternative, and their use is tightly regulated by the Home Office.
Here at the Wellcome Sanger Institute, our researchers utilise alternatives including human cells, organoids, or computational methods wherever possible. Our animal facility is home to mice and fish which are used in studies to understand cancer, malaria and other pathogens when research in a whole living organism is needed. Each year we run a competition for all our staff who work with animals, based on the principles of the 3Rs.
This year’s winning entry was submitted by Francesca Short who has developed a method that uses the caterpillar Galleria mellonella to study disease-causing bacteria. She told us about her work.
“I wanted to investigate which genes in Klebsiella pneumoniae are responsible for it being able to infect its human host and cause disease. They might be genes that allow it to attach to cells or evade the immune system – there are a whole range of functions that can make bacteria harmful.”
Klebsiella pneumoniae is one of the most common causes of hospital-acquired infections worldwide. Many strains are resistant to antibiotics, making it a serious public health concern. Infection poses a serious threat to those who are immunocompromised. Some strains of the bacteria can infect otherwise healthy people too – causing life-threatening liver abscesses, pneumonia or meningitis.
“To find the genes important for infection I created over 100,000 mutants of the bacterium, each with a gene knocked out – this was our library. The next step would be to infect an animal with the mutant library to see which bacteria, out of the thousands present, take hold.
“A specialised genome sequencing technique called TraDIS [which was developed by Sanger Institute scientists / Dr Who fans] can be used to tell which mutant bacteria made it, and were able to successfully infect the animal.
“A lot of practical factors have to be met to get meaningful results using TraDIS. For infection studies like this one, it would mean using a lot of animals. And I wasn’t even sure it would work, because the hospital strain of Klebsiella I was working with doesn’t cause disease in mice. So I started looking for an alternative.”
Call in the caterpillars
The use of caterpillars is growing in popularity for research. Because they don’t have a central nervous system or any pain receptors, their use isn’t regulated. They can be used in a normal laboratory, without any specialised equipment. Research-grade caterpillars are grown and supplied by commercial companies and can be easily shipped as and when they are required. They’ve been used before to compare how different strains of bacteria behave, so Francesca wanted to see if they could be used on a bigger scale.
“I was surprised! I’m a microbiologist, and I had no idea the caterpillars have an immune system similar to ours. They make a good representation of what is happening in other animals when a bacteria invades. They can be kept at 37 degrees, body temperature, and directly injected with bacteria which go into their blood stream.”
“We did some preliminary studies to see if we could infect the caterpillars with Klebsiella pneumonia – it worked very well.”
“The final step was to see which of the bacteria mutants were able to infect the caterpillars. In total I infected around 200 caterpillars with my set of mutant bacteria, then used TraDIS to see which genes were needed for infection.
“Our results are promising. We found many of the genes already known to be involved in Klebsiella pneumoniae’s ability to infect and persist in its host. We found some new ones too.”
The newly discovered genes are an important finding; they have never been linked to infection in any bacteria before. The knowledge advances the understanding of Klebsiella pneumoniae biology and in turn, ways it might be treated.
Francesca plans to continue using the caterpillars in her research, and has trained others in the methods too.
Other 3Rs awards winners
Other proposals that were highly commended in this years 3Rs awards were to enhance the welfare of mice in the facility, reduce the numbers needed for breeding programmes and to find alternative uses for mice when they are no longer needed for experiments.
While mammals remain important for many studies, the use of alternative methods, including caterpillars, holds promise for replacing them in many experiments. It’s possible they could be used to study a wide range of bacteria that cause human diseases, as well as to develop new drugs and understand antibiotic resistance.