Wednesday 17 August 2016

Elegant against Zika

I'm on a bit of a microbe jag this month and there's more to come. There are two aspects which interest me
  1. the diversity of the invisible microbiome and its interactions with the nearby bigger-than-a-breadbox world. The technology (Next Generation Sequencing NGS) for documenting this has only developed in the last 10 years. Before than we really hadn't a clue because most microbes cannot be grown on a Petri dish, so we could not characterise, name and catalogue them.
  2. precision use of particular microbes to carry out beneficial tasks instead of a blood-bath kill them all approach which takes out a lot of innocent bystanders. Like Eleftheria terrae's novel antibiotic; or Monsanto's plant-growth promoting bacteria trials.
Wolbachia pipientis is a member of the Rickettsiae a subset of the alpha-proteobacteria. We've met Rickettsia before as the agent of typhus on emigrant ships and as the probable ancestor of mitochondria. They have really small genomes, and Wolbachia is no exception: managing quite nicely with only about 800 genes on a chromosome about a third the length of E. coli's. One of the consequences of a small genome is that Wolbachia needs a bit of molly-coddling.  It wouldn't survive for months in a cold pasture like Mycobacterium tuberculosis, for example. Contrariwise, it can only grow and multiply in the the cosy environment inside eukaryotic cells. See [R] a Drosophila embryo filled, like a cake with currants, with Wolbachia false-stained in blue. W. pipientis has a preference for arthropods so you don't need to worry about catching a dose yourself. In particular it seems to zero in on the reproductive organs of flies, beetles and spiders. There it has a variety of bizarro effects on the behaviour and capability of the flies:
  1. selective killing of males
  2. a switch to parthenogenesis (reproduction in the absence of males)
  3. feminisation of males
  4. restrictive incompatability: Wolbachia infected males cannot mate successfully with uninfected females
If you're going to use Wolbachia as an agricultural or medical intervention you want to be double-plus sure you know which of these scenarios will play out with the particular strain of Wolbachia, the particular insect species and the particulars of the environmental conditions that pertain. Get it wrong and you may reap a whirlwind: turning all the males into blood-sucking females for example.. It may work fine in the lab but eventually you have to carry out a field trial and that will have a more complex system of interaction terms than you can begin to model in a laboratory or as a computer simulation.

In the free back-issues of Nature I picked up in Dublin earlier in the month, there was an article which describes a new proposal to control the spread of Zika. Published in May, it was unaware of the cases of Zika more recently reported in Florida. The Center for Disease Control CDC in Atlanta, Georgia cannot help but get more engaged when the threat is only one state line and 500km away. ANNyway, a biotech startup called MosquitoMate is proposing to infect male Asian tiger mosquitoes Aedes aegypti with a strain of Wolbachia pipientis that will ensure their sterility. When these doctored males mate in the wild with incompatible females, the eggs fail to develop properly like in case 4 above. The descendantless males will be in competition with wild-type males so the company or their customers are going to have to release containers-full of the infected males to make a difference in the reproduction of this species of mosquito. Aedes aegypti is not the only vector of Zika virus, so I guess the argument is that any intervention that reduces transmission of the latest virus should be considered. Field trials by the company with a different mosquito species have been claimed to reduce the population of the flies by 70%. The reservoir of 30% mean that the protocol is set to keep on giving value to the shareholders. If the customers (state health boards, the UN, whoever) don't keep on throwing out doctored flies, everyone is back to the status quo ante except for being several million dollars poorer. It won't be like fatal hammer given to screwworm in Libya in 1990.

MosquitoMate  makes me think of Fianna Fail's slogan in the 2002 General Election:

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