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Tag: malaria

Mosquitoes are Drawn to Your Skin Bacteria

There’s only two more days left in 2011, so time for my final paper a month post for 2011!

Like with the paper I blogged for last month, this month’s paper (from open access journal PLoS ONE) is yet again about the impact on our health of the bacteria which have decided to call our bodies home. But, instead of the bacteria living in our gut, this month is about the bacteria which live on our skin.

Its been known that the bacteria that live on our skin help give us our particular odors. So, the researchers wondered if the mosquitos responsible for passing malaria (Anopheles) were more or less drawn to different individuals based on the scent that our skin-borne bacteria impart upon us (also, for the record, before you freak out about bacteria on your skin, remember that like the bacteria in your gut, the bacteria on your skin are natural and play a key role in maintaining the health of your skin).

Looking at 48 individuals, they noticed a huge variation in terms of attractiveness to Anopheles mosquitos (measured by seeing how much mosquitos prefer to fly towards a chamber with a particular individual’s skin extract versus a control) which they were able to trace to two things. The first is the amount of bacteria on your skin. As shown in Figure 2 below, is that the more bacteria that you have on your skin (the higher your “log bacterial density”), the more attractive you seem to be to mosquitos (the higher your mean relative attractiveness).

Figure 2

The second thing they noticed was that the type of bacteria also seemed to be correlated with attractiveness to mosquitos. Using DNA sequencing technology, they were able to get a mini-census of what sort of bacteria were present on the skins of the different patients. Sadly, they didn’t show any pretty figures for the analysis they conducted on two common types of bacteria (Staphylococcus and Pseudomonas), but, to quote from the paper:

The abundance of Staphylococcus spp. was 2.62 times higher in the HA [Highly Attractive to mosquitoes] group than in the PA [Poorly Attractive to mosquitoes] group and the abundance of Pseudomonas spp. 3.11 times higher in the PA group than in the HA group.

Using further genetic analyses, they were also able to show a number of other types of bacteria that were correlated with one or the other.

So, what did I think? While I think there’s a lot of interesting data here, I think the story could’ve been tighter. First and foremost, for obvious reasons, correlation does not mean causation. This was not a true controlled experiment – we don’t know for a fact if more/specific types of bacteria cause mosquitos to be drawn to them or if there’s something else that explains both the amount/type of bacteria and the attractiveness of an individual’s skin scent to a mosquito. Secondly, Figure 2 leaves much to be desired in terms of establishing a strong trendline. Yes, if I  squint (and ignore their very leading trendline) I can see a positive correlation – but truth be told, the scatterplot looks like a giant mess, especially if you include the red squares that go with “Not HA or PA”. For a future study, I think it’d be great if they could get around this to show stronger causation with direct experimentation (i.e. extracting the odorants from Staphylococcus and/or Pseudomonas and adding them to a “clean” skin sample, etc)

With that said, I have to applaud the researchers for tackling a fascinating topic by taking a very different angle. I’ve blogged before about papers on dealing with malaria, but the subject matter is usually focused on how to directly kill or impede the parasite (Plasmodium falciparums). This is the first treatment of the “ecology” of malaria – specifically the ecology of the bacteria on your skin! While the authors don’t promise a “cure for malaria”, you can tell they are excited about what they’ve found and the potential to find ways other than killing parasites/mosquitos to help deal with malaria, and I look forward to seeing the other ways that our skin bacteria impact our lives.

(Figure 2 from paper)

Paper: Verhulst et al. “Composition of Human Skin Microbiota Affects Attractiveness to Malaria Mosquitoes.” PLoS ONE 6(12). 17 Nov 2011. doi:10.1371/journal.pone.0028991

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The Sickle Cell Salve

I might have been crazy late with April, but for May, my on-timeliness when it comes to the paper a month posts is returning with a vengeance.

This month’s paper from Cell (a journal I usually avoid because their papers are ridiculously long :-)) dives beneath the surface of one of the classic examples of genetics used in almost every intro-to-genetics seminar/class/textbook. As you probably know, living things typically receive two sets of genes: one from the mother and one from the father. If those two sets of genes result in the same protein, then the organism is said to be homozygous for that particular trait. Otherwise, the proper term is heterozygous. In classical genetics (i.e. what was painstakingly discovered by Gregor Mendel, the “father of genetics”), being heterozygous, to a casual observer, was usually something that could only be seen after multiple generations (or with a DNA test). This is because even though the individual has two different versions of the same gene, one of them is “dominant”, expressing itself more loudly than the other.

en93587For the mutation which causes the disease sickle cell anemia (see image to the right as to why the disease is called “sickle cell”), however, the truth was a little different. While heterozygous individuals did not suffer from the problems associated with sickle cell anemia, unlike individuals homozygous for the “normal” gene, they showed a remarkable advantage when it came to surviving infection with malaria. It is one reason scientists feel that sickle cell anemia continues to be endemic in parts of the world where malaria is still a major issue.

But, how the sickle cell disease mutation did this in heterozygotes was not well-understood. The authors for this month’s paper tried to probe one possible explanation for this using mice as an experimental system. The interesting thing that they found was that mice that were heterozygous for the sickle cell trait (HbSAD), despite having better survival against malaria than those which were homozygous for the “normal” gene (HbWT) (see the Kaplan-Meier survival curve in Figure 1A below, showing the proportion of surviving mice over time), did not have a significantly different amount of infected red blood cells (see Figure 1F below).

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So if the sickle cell gene wasn’t reducing the number of infected cells, what was causing the improvement in survival? The researchers knew that red blood cells which are heterozygous for the sickle cell trait will often “leak” an iron-containing chemical called heme into the blood stream. Because heme just floating around is toxic, the body responds to this with an enzyme called heme oxygenase-1 (HO-1) which turns toxic heme into the less toxic biliverdin and carbon monoxide (CO). The researchers considered whether or not HO-1 was responsible for the improved ability of the mice to avoid cerebral malaria. In a creative experiment, they were able to show that the sickle cell mice needed HO-1 to get their better survival – mice which were genetically engineered to be missing one copy of HO-1 (Hmox1+/-), even if they were heterozygous for the sickle cell disease, did not survive particularly well when infected (see Figure 2B below, left for the survival data).

In fact, they were even able to show that if you took mice which did not have any sickle cell trait gene (the HbWT group), and replaced their blood system using irradiation and a bone marrow transplant from a heterozygous sickle cell mouse (HbSAD), you only improve survival if the cells come from a mouse with its HO-1 genes intact (Hmox1+/+) (see Figure 4A below, right).

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So, we know HO-1 is somehow the source of the heterozygous mice’s magical ability to survive malaria. But, how? As I stated earlier, the researchers knew HO-1 produced carbon monoxide (CO) and, they were able to show that heterozygous mice with a defective HO-1 response were able to survive when given carbon monoxide (see Figure 6E below). Interestingly, exposure to carbon monoxide reduces the amount of heme floating around in the bloodstream, something which gets kicked into overdrive when malaria starts killing red blood cells left and right (see Figure 6G below), something the researchers validated when they were able to neutralize the protecting power of carbon monoxide by adding more heme back into the mouse (see Figure 6H)

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So, overall, what did I think? First the positive: these are extremely clearly designed and well-controlled experiments. I could only show a fraction of the figures in the paper, but rest assured, they were very methodical about creating positive and negative controls for all their figures and experiments which is fantastic. In particular, the use of bone marrow transplantation and genetically engineered mice to prove that HO-1 plays a key role in improving survival were creative and well-done.

What leaves me unsettled with the paper is the conclusion. The problem is that the trigger for HO-1, what the authors have shown is the reason mice which are heterozygous for sickle cell anemia survive malaria better, is heme, which happens to also be what the authors say is the cause for many of the survival complications. It’s like claiming that the best way to cure a patient of poison (heme) is to give the patient more poison (heme) because the poison somehow triggers the antidote (HO-1).

In my mind, there are two possible ways to explain the results. The first is that the authors are right and the reason for this is around the levels and timing of heme in the bloodstream. Maybe the amount of heme that the sickle cell heterozygotes have is not high enough to cause some of the malaria complications, but high enough so that HO-1 is always around. That way, if a malaria infection does happen, the HO-1 stays around and keeps the final level of heme just low enough so that problems don’t happen. The second explanation is that the authors are wrong and that the carbon monoxide that HO-1 is producing is not reducing the amount of heme directly, but indirectly by reducing the ability of the malaria parasites to kill red blood cells (the source of the extra heme). In this case, sickle cell heterozygotes have chronically higher levels of HO-1.

Both are testable hypotheses – the first can be tested by playing around with different levels of heme/HO-1 and observing how the amount of free-floating heme changes over time when mice are infected with malaria. The second can be tested by observing test tubes full of red blood cells and malaria parasites under different amounts of carbon monoxide.

In any event, I hope to see further studies in this area, especially ones which lead to more effective treatments for the many millions who are affected by malaria.

(Image Credit – Sickle Cell) (Figures from paper)

Paper: Ferreira et al., “Sickle Hemoglobin Confers Tolerance to Plasmodium Infection.” Cell 145 (Apr 2011) – doi: 10.1016/j.cell.2011.03.049

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Making the Enemy of your Enemy

What? Another science paper post? Yup, I’m trying to get ahead of my paper-a-month deadlines by posting February’s while actually still in February!

This month’s paper comes from Science and is a topic which is extremely relevant to global health. As you probably know, malaria kills close to 1 million people a year, with most of these deaths in areas lacking in the financial resources and public infrastructure needed to tackle the disease. In addition to the socioeconomic factors, the biology of the disease itself is extremely challenging to deal with because the malaria parasite Plasmodium falciparum not only rapidly shifts its surface proteins (so the immune system can’t get a good “fix” on it) it also has a very complex multi-stage life cycle (diagram below), where it goes from being carried around by a mosquito as a sporozoite, to infecting and effectively “hiding inside” human liver cells, to becoming merozoites which then infect and hide inside human red blood cells, and then producing gametocytes which are picked up by mosquito’s which combine to once again form sporozoites. Each stage is not only difficult to target (because the parasites spend a lot of their time “hiding”), but the sheer complexity of the lifecycle means the immune system and drugs humans come up with are always a step behind.

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So, what to do? While there is active work being done to build vaccines and drugs to fight malaria, the “low-hanging fruit” is getting the upper-hand on the mosquito transmission phase. Unfortunately, controlling mosquitos has become almost as bad a nightmare as dealing with the Plasmodium parasite. The same socioeconomic factors which limit medical treatment for the disease also make it difficult to do things like exterminate mosquitos. Furthermore, pesticides not only have adverse environmental impacts (i.e., DDT) but will ultimately have limited lifetimes as the mosquito population will eventually develop resistance to them.

imageWell, enter the enterprising scientist. I can’t say for sure, but I have to believe that the scientists here must have read comic books like Spiderman or Captain America as a kid because the approach they chose feels like it came straight out of the comic book world. But, instead of building a monstrosity like the Scorpion (pictured to the right), the researchers built a super-fungus super-soldier to control malarial transmission.

Instead of giving the powers of a scorpion to smalltime thief Mac Gargan (who then named himself, appropriately, The Scorpion), the researchers engineered a fungus which naturally infects mosquitos called Metarhizium anisopliae to:

  • kill the infected mosquito more slowly (as to not push mosquitos to become resistant to the fungus)
  • coat the infected mosquito’s salivary glands with a protein fragment called SM1 to block the malaria parasites from getting there
  • produce a chemical derived from scorpions called scorpine which is extremely effective at killing malaria parasites and bacteria

Pretty cool idea, right? But does it work? Figure 3 of the chart below shows the results of their experiments:

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Mosquitos were fed on malaria-infected blood 11 days before they were dosed with our super-fungus. Typically sporozoites take about 2 weeks to build in any reasonable number in a mosquito’s salivary glands, so 14-17 days after exposure to malaria, the researchers checked the salivary glands of uninfected mosquitos (the control [C] group), mosquitos infected with non-super-fungus (the wild-type [WT] group), and mosquitos infected with the super-fungus (transgenic [TS] group). As you can see in the chart above, the TS parasite count was not only significantly smaller than both the control and wild type groups, but the control and wild type groups behaved exactly as you would expect them to (the parasite counts went up over time).

So, have we discovered a super-soldier we can count on to stop mosquito-borne illnesses? I would hold off on that for a number of reasons. First, on an experimental level, the researchers only looked at 14-17 days post-infection. To be confident, I’d like to see what this looks like with different doses of fungus and over longer periods of time and a wider range of mosquitos (as nearly 70 species of mosquito transmit malaria and I don’t even know what the numbers look like for other diseases). Secondly, its not clear to me what the most effective way to dose large populations of mosquitos are. The researchers maintain that you can spray this like a pesticide and the fungus will adhere to surfaces and stay effective for long periods of time – but that needs to be validated and plans need to be drawn up to not only pay for this (I have no idea how expense this is) but also to deploy it.

Lastly, and this is something that almost any naturalist or economist will tell you: human actions always have unintended consequences. At a first glance, it looks like the researchers covered their bases. They build what looks like a strategy which avoids mosquito resistance (and, because it uses at least two ways of controlling the parasite, is probably less vulnerable to Plasmodium resistance than drugs/vaccines). But, more research needs to be done to ascertain if there are other environmental or economic impacts of using something like this.

All in all, however, this looks like a promising start for what could be an interesting and inspired way to help control malaria.

(Image credit – Malaria lifecycle) (Image credit – the Scorpion) (Figure 3 from paper)

Paper: Fang et al., “Development of Transgenic Fungi that Kill Human Malaria Parasite in Mosquitos.” Science 331: 1074-1077 (Feb 2011) – doi:10.1126/science.1199115

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