In order to stay up to date in my field of research I have generated a number of automated searches at places like PubMed that alert me when my keywords appear in published work. I also have subscribed to the ‘electronic tables of contents’ for a number of journals. This makes finding papers for my work very easy but sometimes I happen to scan across something that takes my fancy.
Last week (I think it was last week anyway) one such paper took my fancy.
“A Trojan horse mechanism of bacterial pathogenesis against nematodes”. I very nearly did my honours in a nematode research lab so I have always had a soft spot for the little blighters. Plus if you can throw in nematode AND a Trojan horse example I’m sold.
Niu et al., (2010) look at novel host-pathogen interactions and do their work in a nematode called Caenorhabditis elegans due to the ease of working with an invertebrate. C. elegans also has other advantages. We have a lot of genetic tools for manipulating the organism and in some pretty inspired Nobel Prize winning work we know the cell lineage of ALL the cells in the mature worm. This makes them a fantastic model organism for any kind of research, particularly modeling bacterial infection and invasion.
Many nematode species feed on bacteria and C. elegans is no different but Niu et al. found a cool host-pathogen interaction when the worm interacts with the bacterium Bacillus nematocida B16.
Just like any organism, nematodes posses the ability to sense their surroundings, particularly food. Humans can use their nose and eyes to find the hot apple pie in the windowsill but the worm senses soluble chemicals using sense organs in its head. By following the chemicals a worm can find its food source. This is where the pathogenesis bit comes in.
It’s thought that ‘food’ bacteria throw off these chemical signals as a by-product of survival but in reality we do not know too much about these chemicals called volatile organic compounds or VOC’s. All we do know is that they are chemotatic for nematodes.
Bacteria being bacteria have found a way to exploit this sensing system. Bacillus nematocida B16 is a natural pathogen of C. elegans. In contrast to other nematoxic (kills a nematode) bacteria Bacillus nematocida B16 does not attack the worm from the outside. Rather, it releases very potent VOC’s that appear to override the other VOC’s providing a very strong stimulus for the nematode worm to follow. These VOC’s scream to nematodes I AM FOOOOOOODDDDDDDDD!!!!!! In what would appear at first to be a counterproductive strategy, as the worm consumes the bacteria, the bacteria have to quickly adapt to the nematode gut. Once in the in the nematode gut the bacteria release proteases to break apart the worm from the inside but the worm remains able to move for some time after the infection allowing the bacteria time to grow but also the worm to interact with other nematodes facilitating the spread of the bacteria. After the bacteria have made the worm into a worm shaped nematode flesh balloon the bacteria spew forth to start the cycle again.
This activity appears to be somewhat novel as most nematoxic bacteria secrete all sorts of things to kill nematode worms, but as I mentioned earlier, it’s normally done from outside the worm.
This work may lead to a couple of interesting ideas and perhaps some practical output. Firstly, is this something that other bacteria can do? The idea that bacterial pathogens which seek out host cells to live in (such as L. pneumophila) may be using chemotaxis targeted at the immune system to achieve more efficient uptake of bacterial cells doesn’t seem too far fetched and may unlock yet another pathogenic mechanism possessed by intracellular pathogens. Practically this kind of work can be put to use in designing new and more ecologically sound nematode control measures which means less broad spectrum chemicals in agriculture and at home on our plants.
Niu Q, Huang X, Zhang L, Xu J, Yang D, Wei K, Niu X, An Z, Bennett JW, Zou C, Yang J, & Zhang KQ (2010). From the Cover: A Trojan horse mechanism of bacterial pathogenesis against nematodes. Proceedings of the National Academy of Sciences of the United States of America, 107 (38), 16631-6 PMID: 20733068