Bacteria fight dirty: competition vs reproduction in a bacteria symbiont of a nematode
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Symbiosis can be defined as “an interaction (positive, negative, or neutral) in which two species exist in intimate physical association for most or all of their lifetimes and are physiologically dependent on each other” . This is the interaction between the bacteria Xenorhabdus bovienii and its host, the nematode Steinernema affine, on which it depends to colonize insects . They infect the insect (caterpillars, for instance), get separated, and then both contribute to kill it. Once dead, the insect provides resources that allow reproduction of both species. When these resources are depleted, the bacteria and the nematode reunite and emerge from the insect.
Xenorhabdus is able to produce a compound (bacteriocin) that inhibits bacteria from different strains inside the insect. Inhibiting strains of the bacteria show a clear advantage over non-inhibiting ones when they reproduce together inside the insect. However, non-inhibiting strains of Xenorhabdus are observed isolately, but generally not together with inhibiting strains. So how can both types persist in nature? That is the question that the authors of , and perhaps yourselves, try to answer.
They hypothesize that there may be a reproductive cost for the bacteria associated with inhibiting other strains, and experimentally test it. Their results suggest that, indeed, on a non-competitive environment, non-inhibiting strains of Xenorhabdus are able to kill and reproduce faster than inhibiting ones.
Figure: Top - Nematode (Steinernema affine). Middle - Bacteria (Xenorhabdus bovienii). Bottom - Caterpillar (Malacosoma americanum). Source: 
Propose and analyse a mathematical model that incorporates the interactions and describes the dynamics of the species in this system.