Passengers in a Bacterial Body

Why ex-smokers commonly complain about gaining weight when they quit, and why active smokers are usually thinner on average, has been cracked by scientists in the US.

Nicotine has always been regarded as the prime suspect, because experimental animals lose weight when given the drug, but no one has ever worked out why.

Now, writing in Science, Yale scientist Yann Mineur and his colleagues have tracked the smoking gun to a region of the brain's hypothalamus concerned with appetite.

In this area, the team have found, are nerve cells activated by nicotine that make a chemical called proopiomelanocortin (POMC). When this substance is released from these cells into an adjacent, appetite-related region of the brain known as the paraventricular nucleus, it has an anorexic effect.

The team made the discovery in experimental mice by using drugs to selectively activate or block certain subclasses of the chemical docking stations, called acetyl choline receptors, that nicotine binds to in the brain.

One drug tested, cytisine, closely mimicked nicotine's appetite-blocking action. It binds selectively to acetyl choline alpha3-beta4 receptors, so the team used a modified virus to deactivate just these receptors in the hypothalami of one group of mice.

These treated animals ceased to lose weight when the nicotine-mimicking drug was subsequently given. Using a separate batch of animals, the team then made electrical recordings from nerve cells when nicotine was added, which revealed that the drug was triggering bursts of activity in nerve cells that produce POMC, suggesting that this might be the anti-appetite signal.

Consistent with this theory, mice from which POMC has been "knocked out" genetically do not show any nicotine-induced changes in appetite. And when the team used a modified virus in another group of mice to deactivate a gene called melanocortin 4R (Mc4R), which encodes the receptor for POMC, the food intake amongst these animals was also unaffected by nicotine.

Together these findings show that nicotine triggers a specific subset of POMC-producing nerve cells in the brain's hypothalamus. Acting via the melanocortin (Mc4R) receptor in the adjacent paraventricular nucleus, this damps down sensations of hunger and contributes to reduced calorie intake.

According to Mineur and his colleagues, the molecular clockwork of this pathway could be useful in producing new drugs designed to prevent weight-gain following smoking cessation, and also to tackle obesity and its related metabolic disorders.

Jane -  Hearing aids work on the general principle that if you are finding it hard to hear something you simply turn the volume up.  Fine if you are talking to one person in a quiet room, but if you are in a noisy environment then everything is amplified.  Now unique computer modelling techniques, combined with new ways of carrying out hearing tests, are revolutionising the way that hearing loss and impairment is diagnosed and treated.  Professor Ray Meddis is leading the work at the University of Essex.  

Ray -  Hearing consists of a number of stages between the sound arriving at the ear and then the signal going up to the brain. The computer model tries to represent each one of these stages separately. The test that we use involved two measurements, one concerns tuning and the other one concerns compression.  Tuning refers to the situation where you can hear one sound and tune out background sounds.  It's a bit like a radio where you can tune into one radio station, but you don't want interference from another station.  Now with normal hearing this tuning takes place naturally so one of our tests focuses on that particular problem.  Compression is concerned with how intrusive other sounds are when they become louder. So normally we can tune out another sound if it is quiet, but as it increases in intensity then we have more difficulty in tuning it out.  A person with hearing loss often has a particular difficulty in tuning out irrelevant sounds and so our compression measure is particularly concerned with increasing the intensity of the sound and then noticing how that comes to make it difficult to listen to a particular vocal sound.                               

Jane -   Strange as it may seem, it was the art of dressmaking that inspired Ray and his team to take this tailor-made approach to the diagnosis and treatment of hearing impairment.

Ray -   When generating a computer model for a patient with hearing loss, we always start off with a normal model and we try and make a single adjustment which refers to the particular pathology which we feel is causing the problem for that particular patient. When you have a dress made the tailor measures your body size then changes the shape of the dummy to represent your body size and then cuts and sews the clothing so that it fits the dummy.  The reasonable expectation is that when the customer comes back the clothing will fit the customer perfectly.  Now we believed that we could do the same thing with hearing impairment.  We already have a model of normal hearing, so by making certain adjustments we could make the dummy simulate impaired hearing and the idea was we could then use that dummy to fit the hearing aid and we would adjust the hearing aid so that we got the best possible output from the impaired hearing dummy.  Our hope is that we will be able to use the hearing dummy to make these adjustments before the hearing aid is supplied to the patient.  In other words the hearing aid should be fitted to the dummy just like a dress would be fitted to a tailors dummy and then when the patient comes back, ideally, the aid should be perfectly suited to their particular needs.

Jane -   Supported by the Engineering and Physical Sciences Research Council, this cutting-edge work has also led to a prototype design for a new type of hearing aid.

Ray -   Because our primary interest was in developing computer models we came to realise that hearing aids respond differently to sounds compared to a normal hearing individual.  So it seemed to us that we could build a hearing aid that simulated normal human hearing so it compressed the sound in the same way, tuned the sound in the same way and dealt with variations in level in the same way. We have designed a new type of hearing aid which uses something called instantaneous compression.  Currently in hearing aids sound is compressed, but it takes a little while for the compressor to respond to the sound levels and this gives rise to some complications.  But by simulating normal human hearing we've been able to produce instantaneous compression without the distortion which everybody used to believe was an unavoidable accompaniment of instantaneous compression.

Jane -   So a number of aspects of this research are cutting-edge.

Ray -   The research is a world first in a number of respects.  First of all this will be the first model to represent a number of different kinds of hearing impairment.    It is also the first attempt anybody has made to go through the complete cycle that is measuring the hearing loss, designing the hearing dummy, adjusting the hearing aid using the hearing dummy and then modifying the hearing aid to suit the patient.  

Jane -   The new approach has simplified the complicated laboratory testing process, significantly cutting down on the time it takes and the number of trials involved, along with the level of expertise required.   Within four years we could see this system of testing in widespread use, along with the new type of hearing aid.

Kat -   Now, you can't turn on the telly without seeing some woman drinking yoghurt and then dancing around for all that she's worth.  We're very familiar with the idea of friendly bacteria. Foods, drinks, and supplements that promote healthy gut flora could be found in all the supermarkets.  But what are all these about?  What are friendly bacteria?  How are they different from bad bacteria and what do they actually do?  To find out, we're joined by Dr. Karen Scott from Aberdeen University.  Hi, Karen.

Karen -   Hello.

Kat -   Thanks for coming on the Naked Scientists.  Now let's get bacterial with this.  Tell us about the sort of bacteria that are in our gut.  What are they?  What are they doing there and how did they get there?

Karen -   Well, there's actually a large number of bacteria in the human gut.  In fact, we could almost be said to be carrying more bacteria in our bodies than there are actually human cells. These bacteria are very important in helping us to digest the foods that we eat all the time.

Kat -   Where do they actually come from?  Are we born with these bugs in us?

Karen -   Newborn babies are colonised as they're born effectively.  So although our guts are actually sterile when babies are in the womb, as soon as they're born they start to pick up bacteria from their mothers or from their environment and we rapidly become colonised by our gut bacteria.  The bacteria that are in an infant consuming milk are actually different from those that are in a developed adult. It's around the time of weaning when the types of bacteria that are present start to change. This is because the bacteria composition depends very much on the food that we eat.  By the time a child is aged say, 2, 3, 4, they probably have a very similar gut bacteria to an adult.

Kat -   Now I'm sure any parents listening will be aware that their child's digestion changes quite significantly at that kind of age, but what are these bacteria actually doing for us in our gut?  Presumably, you're talking about the kind of the good bacteria that are helping us out.

Karen -   Yeah.  I should've said that.  Although the bacteria that we hear a lot about in the news are the bad bacteria, the pathogenic bacteria, the vast majority of bacteria that live in our gut are very good for us. They're actually very important to maintain a healthy gut.  They help us because, when we eat, much of the food that we eat we are unable to digest ourselves.  We don't have the enzymes that are required to degrade much of this food material. When it reaches the colon, the plant fibres for example haven't been digested at all higher up in the gastrointestinal tract, and there we completely depend on our bacteria in our colon to digest these fibres for us, and gain some energy from them.

Kat -   So, some of the questions that this throws up to me is how much of our energy is actually coming from bacteria?

Karen -   It's thought that about 10% of our energy might come from these gut bacteria. That's not to say that the bacteria are making us fat because the energy is in the form of short chain fatty-acids. These bacteria are carrying out fermentation reactions and these acids that they produce are actually extremely essential for the health of the colon.  One of the acids in particular, a compound called butyrate, is protective against colon cancer. In general, having a slightly acidic colon actually prevents the growth of the pathogenic bacteria like E. coli because these bacteria can't grow under acidic conditions.  So by maintaining the fermentation that happens by your good bacteria, you can keep out the bad guys that you don't really want in there.

Kat -   So what sort of foods are we talking about?  Fruit and veg, and that kind of thing?

Karen -   Yeah.  I mean, it's plant material, carbohydrates and resistant starches. All these sorts of materials reach the colon and are digested by the bacteria that are there.  You can actually manipulate the composition of your gut bacteria if you want, by changing your diet.  You can do this for better or for worse, depending on which way we change it.

Kat -   Now I live with three guys, and I'm well aware of the effects for example, of when they eat things like beans on their guts' activities. So most of the gas that we produce in our guts, is that down to the bacteria?

Karen -   Yeah, that's right.  I mean, these short chain fatty acids for example are volatile compounds and they are pretty smelly really.  Some of the gases that are produced are released in flatulence or oral belching, but that's not to say it's a bad thing. What happens, which is very interesting, is that some of the population have certain bacteria that make smellier farts if you want, whereas other people have a different type of bacteria and they don't tend to be as smelly.  So they might be producing just as much gas, but you just can't detect it quite as well it seems.

Kat -   I'd say that was the clearly the female half obviously.  So you mentioned that we can change the composition of our gut bacteria, say switch it from being some nasty ones to being more good bacteria, and we are bombarded with these ideas that you just drink this little yoghurt drink and that's going to give you all the stuff for your friendly bacteria.  What's in these yogurt drinks and do people actually need them?  Are they doing us any good?

Karen -   Well that's an interesting one.  The yogurt drinks actually contain a bacterium themselves.  So, when you consume those yogurt drinks, the manufacturers are actually giving you their bacterium. All these yogurts have slightly different bacteria in them because they're all patented, and each manufacturer has a slightly different bacterium that they put in their drinks.  Now these bacteria have all been categorised and they're all very safe.  They're not going to do you any harm, but in general terms, you're putting - say one pot of yogurt might contain around 100 million bacteria - and you're putting them in your gut, up against a million, million bacteria.  So, they really are outnumbered and I think, in my opinion anyway, it's much better to try and encourage the growth of the bacteria that are already there, so your normal healthy bacteria.  You can do that by increasing the intake of carbohydrates and fibre, but the other way that food companies are trying to do this is by introducing what they call prebiotics.  The yogurt and stuff that contains the bacteria in the yoghurt are called probiotics.  Prebiotics are actually carbohydrate-like compounds that again, we can't digest.  They go straight through to our gut and our bacteria that are always there can grow on these prebiotics. They're sort of targeted to help the growth of the good bacteria in our guts.

Kat -   So it's pretty obvious that we have evolved to have these bacteria and they're helping us digest our food and keep us healthy.  Do most people need help with their gut bacteria?

Karen -   I think as long as you consume a generally healthy diet and a varied diet, because there are many many different types of bacteria in the gut.  There are over 500 different species of bacteria and they all have slightly different 'eating habits' if you want, so as long as you have a generally good healthy diet, then yes, your gut bacteria will be functioning adequately.  But sometimes you may inadvertently cause a shift in your bacteria and they may then need a little help to re-establish a good bacterial profile. That's when these probiotic products are actually quite good for you, and I'm talking probably antibiotics is the main thing really there.

Kat -   We've started to hear some stories lately on the Naked Scientists about how our gut bacteria may be having a much bigger impact say, on our immune system.  Do you think that this kind of area is going to become much more important in the future?

Karen -   I mean, it's certainly been established that in a very early life your immune system is sort of primed by your gut bacteria.  The immune system is in contact with our normal gut bacteria all the time and it's constantly sampling these bacteria, and it knows not to react to them.  It knows not to get inflamed and cause a very big immune response to our normal bacteria. That's how it knows when it encounters something it's not used to, then it can react to that. We need that to happen in order to stop getting sick.  This happens from very early life and is part of the hygiene hypothesis, which is where perhaps if you try to stay too clean then you don't get this constant priming of your immune system. In some cases, that may mean that you over react to what should not normally cause a bad reaction.

Kat -   Thanks very much for that, Karen.  That is Dr. Karen Scott from the Microbial Ecology Group at the University of Aberdeen.

Meera -   For this week's Naked Engineering, Dave and I are exploring the field of fluid mechanics, so the forces and movements of fluids such as liquids and gases.  But Dave, this is still quite a big area so what are we focusing in on?

Dave -   Yes.  Fluid dynamics is a huge area, ranging from aircrafts to things which are much smaller scale and actually things that you come across in everyday life.  One of the very difficult bits of fluid mechanics is when you have more than one phase - so a solid and a liquid, or two different types of liquid, or also liquid and gases - all mixed together. Studying how these mix together and how they interact is very, very important at all sorts of scales, not least when you're cooking, or on a much larger industrial scale doing chemical reactions, or even biological reactions.

Meera - To find out a bit more about this mixing process on an industrial scale biologically,  we've come along to University College London this week to meet Martina Micheletti who's a lecturer in Biochemical Engineering.  Now Martina, your particular area is bioreactors?

Martina -   Yes.  A bioreactor basically refers to reactors in which drugs are actually produced by lab organisms, for example microbial yeast or a mammalian cell culture.  They need an aseptic environment and have a lot of sterilisation issues.

Dave -   Essentially, it's a tank where you can grow lots and lots of cells which have been bred or engineered to produce a useful product.  Quite often a drug.

Martina -   Yes.  So, the kind of geometry they will use, the most common one, is a sort of cylindrical tank with a rotating impeller in the middle.  The mixing is achieved by stirring and there is an impeller, which can be of different designs, rotating at a certain speed.

Meera -   So a bioreactor is essentially a vessel where you're trying to culture cells.  What are you actually doing inside this, or what are the conditions inside it, to allow these cells to culture and be happy?

Martina -   Definitely you need some sort of a food. So nutrients - usually in the form of powders in an aqueous medium. But cells, in order to live and grow, they need oxygen.  We usually have a pre-filtered air inlet inside the bioreactor.  Also, live organisms have got required temperatures and pH.  Usually, temperature can be 37 degrees and pH can be pH 7.  So temperature and pH have to be controlled as well.

Dave -   I guess if you've got quite a big vat full of cells, a really major issue is to make sure all the cells are happy at the same time.

Martina -   Definitely.  So, what you need mixing for is, first of all, to actually maintain a suspension of cells so they don't sediment.  You also need to mix the nutrients, so again they can be taken up by the cells.

Meera -   So you've got gases and you've got nutrients in there, all of these need mixing, for which you've got a propeller.  Do you essentially just keep this going very fast to get things mixed?

Martina -   Well you know, you'd think so.  However, there are a number of issues.  A lot of these organisms are quite fragile.  Mammalian cells for example don't have a cell wall.  Some impellers produce quite high shear rates and that can be detrimental.

Dave -   So the shear is just the rate of change of velocity. So if you've got one side of the cell going a lot faster than the other side of the cell it will get ripped apart.

Martina -   Yes.  When we take an individual cell inside a bioreactor, that's going to be subjected to different forces and force stresses.

Meera -   So how do you set about monitoring the flow through the vessel, how things are mixing, and the forces in there?

Martina -   What we really want to get is a flow pattern of the impellers as well as have a quantified turbulence level and also get an idea of the sort of energy which is dissipated by the impeller.  There are a number of techniques to do these.  I would say that the most advanced one is laser based.  One of them is for example, particle image velocimetry.

Meera -   So we've got a laser here which is pointing at a scaled down bioreactor vessel.  It's about 15 centimetres in diameter and it's got a propeller going through the middle of it. It also has a misty fluid in that vessel.

Martina -   It's an image technique essentially that works with seeding particles that follow the fluid.

Dave -   So the problem with seeing how the fluid is moving is that the fluid is transparent and you can't see it. So you've added seed particles which are very very small and will move with the fluid. So, if you know where they're moving, you'll know how the fluid itself is moving.

Martina -   Exactly.  So in fact, I can turn it on now and show you.

[Laser turned on]

Meera -   So you set it going now and this clicking sound that we can hear is the laser firing quite frequently at the vessel in front of us.

Martina -   This is basically a high power laser that is producing two pulses at a known time interval.  I set it at 1 millisecond from each other and it is basically producing a light sheet inside the bioreactor.

Dave -   So you're using a sheet rather than just illuminating the whole bioreactor.  So you're only illuminating the particles in that sheet and you get a sort of 2-dimensional picture.

Meera -   How does putting a sheet of light over this allow you to see the velocity of these seeded particles?

Martina -   There is a high speed camera just in front of the bioreactor which is focused exactly on the actual plane where the laser is firing. The cameras can actually take two images and the camera is synchronised with the laser, so it's going to take the two images at the same time intervals as the laser pulses.  The two images are then cross correlated to actually find out the average space travelled by a particle.

Meera -   But now, on a computer screen here, we can see quite a close up image of the propeller and the seeded particles moving around it.

Martina -   Based on the images that you see now, the computer is then able to actually get a lot of information about the velocity of each of the particles at that location and at that time.

Dave -   A bit like how a speed camera works, it will flash twice and look at how your car has moved in between, so you have then got to use a computer to work out how far these particles have moved.

Martina -   Yes, exactly.

Meera -   How are you then using this information about the velocity of the particles?

Martina -   I'll give you an example.  We found recently in these type of bioreactors that there were a number of instabilities.  Basically, we found the existence of a vortex processing around the impeller shaft at a known frequency. By actually locating the feeding pipe, which is used in a bioreactor for example to feed glycerol to the cells when they need it etc, exactly inside the vortex it would decrease the mixing time (the time needed to achieve full mixing) by 50%. Which is quite a lot of energy saved.

Meera -   And I guess also, when it comes to the manufacturer of say bioreactors, you've got a 15-centimetre scaled vessel here but then hopefully you'll be able to scale this up for the industrial level of pharmaceutical companies?

Martina -   Scale up studies are something that are very sought of after by industry.  This is a 2-litre vessel but we go even lower down to 2ml scaled vessels.  All the fluid dynamics information that is small scale would allow a pharma company for example to actually come and scale up the production of a drug from a 2-litre scaled to a 200-litre possibly.

A video of this research can be seen at
thenakedscientists.com/engineering.

The best way to see them using a microscope is by staining them. You can buy some specific stains that will make the bacteria appear as pink or purple rods or Cocci, depending on what bacterium it is, and then it is very easy to see them through a normal light microscope.

I would say that the microorganisms in your gut are the most helpful, but then I would say that since I work on them. But in terms of specifics then there are two main species that we are most interested in. They are very helpful in producing butyrate, which is protective against colon cancer, and are called Roseburia and Faecalibacterium prausnitzii. Faecalibacterium prausnitzii has also been shown to not be present in people who have Crohn's disease. So there's currently a lot of work going on in various countries around the world to try and see if you can re-introduce that specific bacterium back into patients who have Crohn's disease and whether you can avoid or alleviate the symptoms.

Karen - A good question. I mean, the really important thing about the EHEC is that it requires very, very few bacteria. If your food is contaminated then you basically have to bleach it to get rid of them and make it really safe. We really don't want to do that because the toxins and the bleach would be harmful in themselves. Washing as thoroughly as possible as well, and hoping I suppose. I mean, that's a really difficult question because as you say, you can't cook salad vegetables, and you don't necessarily not want to eat them because of the chances of them being contaminated. As long as they're not from a source where this contamination has been found the risk is actually quite low. I don't know the answer to that question.

Chris - Because when people make motorway service station sandwiches and things like that, the salad in these pre-prepared sandwiches has already been washed in a weak solution of bleach, hasn't it? And I guess the problem with these bugs is if they get into small imperfections in the surfaces of the vegetable, so little nooks and crannies there, the bugs can lurk. The infectious dose is so low, it's just a handful of organisms at most that actually cause disease, and as you peel it, boil it, or leave it, then you're pretty likely to get it.

Karen - I mean, that's exactly right. Yeah, I mean, even washing them in a weak bleach as you say, they'll be lurking. If they're there, they can lurk right down in bits that will not be penetrated by the bleach and you still eat them. Yes, 6 to 10 will actually cause an infection of these particular bacteria.