The toughest cells in nature

Bacteria can produce spores, natural packages which last for hundreds of years. Can we use these to keep drugs on the shelf for longer?
10 June 2014

Interview with 

Graham Christie, University of Cambridge


Bacillus megaterium spore


For drugs to last over time and work effectively they need to be kept in pristine condition until we need them. One way to do this is to use tools that have evolved naturally already. The toughest cells in nature are actually spores, produced by fungi and some bacteria, which can lie dormant for thousands or even millions of years before being reactivated when conditions suit them. Kat Arney spoke to Graham Christie from the Department of Chemical Engineering & Biotechnology at the University of Cambridge, who is investigating whether nature's toughest exterior can be made to contain therapeutic proteins, potentially increasing a drug's shelf life exponentially...

Kat -   So, let's start off with the obvious question, what is a spore?

Graham -   Well, most bacteria will consume nutrients as they grow.  When the nutrients run out, essentially they die off or they adopt a quiescent or nonviable state.  Spore forming bacteria on the other hand can sense when nutrients are running out and they decide to develop a spore as a survival mechanism.

Kat -   So, it's a bit like kind of an egg I guess, some kind of precious container for their genes.

Graham -   It's more like a seed I guess, yeah.

Kat -   And what do spores actually look like in sort of molecular terms?  What makes a spore a spore?

Graham -   Well, spores from different species can look quite different.  They all share the same general outer structure in that they have a spore coat on the outside and then a thick cell wall.  In the middle of the spore, the spore core as well the DNA and the all the usual cellular proteins are kept.  But the spore coat can look quite different in different species.

Kat -   How does it sense when the time is right to come back to life again?

Graham -   That's a good question and that's something I've been looking at for the last decade or so.

Kat -   Any clues?

Graham -   So, yeah.  So, spores have receptors that are buried deep on the inside of a spore and again, different species have different receptors that recognise different molecules.  So generally, amino acids are good signals that...

Kat -   So, it's like bits of proteins, so food.

Graham -   Yeah, bits of proteins or sugars are good signals that the spore now finds itself in an environment that's conducive to growth again.

Kat -   And it is staggering how incredibly hardy these things are.  Just before the show, you were telling me about some of the incredibly old bacterial spores.  How old are the oldest ones that have been found and reinvigorated?

Graham -   That's right.  Most people will say, certainly, hundreds, if not, thousands of years, they'll be able to persist in an environment.  But there were papers in Nature and Science maybe 10 or so years ago, claiming that they had revived spores from amber deposits and these were supposed to be millions of years old.

Kat -   Now, these seem like incredible little packages for things, very hardy, they can be reactivated.  How are you trying to use this incredible natural thing for the benefit of packaging drugs?  Tell me about your work.

Graham -   The main focus of our work has always been on spore germination, how they wake up.  And more recently, we started thinking about the general spore structure and how it achieves dormancy and environmental protection.  When we were working on that we realised that the spore core is exquisitely designed to store proteins and DNA for long periods.  And so, the aim was if a spore can preserve a protein for a long time, a spore protein, then can it also be designed to make a therapeutic protein, a drug, and keep this in a spore core for an extended time?

Kat -   And would this be really useful because some drugs have some sort of small molecules and they're quite stable?  This presumably would be for maybe more of the biological drugs that are a bit less stable.

Graham -   Yeah.  We're thinking of proteins, antibodies and such like.

Kat -   So, how do you get them, these proteins, these useful proteins into a bacterial spore to store them?

Graham -   There are different ways of doing this, but I guess we're really focusing on engineering the spores, so a form of genetic engineering.  We modify the vegetative cell such that when it senses it's running of nutrients then we've inserted a gene.  We've inserted the information to make an antibody or a therapeutic protein and for the spore to make it and direct it to the spore core.

Kat -   So, the bugs will just go, "Oh!  We're starving here.  Quick, let's pack up everything and this thing that we've just made."

Graham -   Exactly, yeah.

Kat -   ...and turn into spores.

Graham -   Yeah, we've got this new piece of information and we'll also channel that to the spore core and look after it.

Kat -   Is there any mileage in taking some of the proteins that make up the spores and trying to turn them into like little packing cases?

Graham -   Yeah.  So, some people are quite nervous about spores and the idea of using them in medicine because spores are associated with Clostridium difficile infections and anthrax and things like that.  But in fact, most spores are fairly harmless and can be used as probiotics.  But the spore coat itself is made of up to about a hundred unique and different proteins.  So, we can actually express those individually.  We're starting to find out now that they will self-assemble into spheres and little shells that have some of the properties of spores, but lack the DNA and the genetic material that people are so scared of I guess.

Kat -   So, that could be sort of a cleaner way of doing it then.

Graham -   Yeah, a much cleaner way and the challenge is to make those non-viable shells as tough and resilient as the native spore itself.

Kat -   But then you still need to get them to uncoat in the same way because I guess when you're packaging the drugs in regular bacterial spores when they go into a person, there's loads of sugar and stuff, and they go, "Ooh!  Off you go!"  How would you get these shells to release their cargo?

Graham -   Yeah, so even with the normal spores, there's quite a challenge for them to actually release the proteins.  So normally, when they germinate, they release small molecules, but not things the size of proteins.  So, one of the challenges we're also looking at is, can we actually lyse the spore completely?  Can we get it to pop when it germinates and to release its payload?  But with the shells, we can look at things like changing the pH or the acidity and the alkalinity of the environment.  This causes the shell to expand or contract.  When it expands, it can actually start to release the payload.

Kat -   So, how much more work do you have to do because this sounds like, it's kind of a cool idea, but how close is it to actually, a reality?

Graham -   We've certainly made spores that have got drugs packaged up on the inside, and I think we're probably a year or so away from engineering spores that lyse or pop upon germination.  But in terms of getting something to market or into clinical trials, we're looking at at least 5 years.

Chris -   Can this work with any kind of drug or will there be certain chemicals that it just won't work for?

Graham -   Yeah, we're finding that we're having to take on a case by case basis.  So, we can't generalise with antibodies.  We have to take one particular antibody and see if the spore can actually make it or not.


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