Bacteria Strive Not to Kill You Very Fast

The ultimate horror-movie star has to be the parasite: from the flesh-eating bacteria, to suicide-inducing worms, to castrating parasites. Such effects can be the consequences of infection, but nonetheless usually it isn't in the parasite's interest to kill the host. Or at least not too quickly - the host has to transmit the parasite to the next host.

Thus there is a trade-off: how quickly a parasite kills its host, host exploitation and parasite reproduction are balanced to maximize the parasite's lifetime transmission success. This sounds logical but "lifetime transmission success" has proven difficult to measure, making it difficualt to actually prove the trade-off theory.

But in a new study published in PLoS Biology, Knut Helge Jensen, Dieter Ebert, and colleagues have now provided empirical evidence that such a trade-off exists.

The authors worked with water fleas (Daphnia magna) and the castrating bacterium Pasteuria ramosa to investigate the relationship between parasite fitness and virulence. Castrating parasites divert resources from host reproduction toward their own reproductive ends. In the case of P. ramosa this means generating spores - that's how the parasite spreads.

According to the tradeoff hypothesis, the parasite should castrate early to optimize the appropriation of host resources, and produce intermediate levels of virulence to keep the host alive long enough to maximize spore production.

To determine the relationship between virulence and lifetime production of transmission-stage parasites, the researchers exposed a Daphnia clone to bacterial spores and tracked individual host mortality. Infected Daphnia sustained far more casualties than either unexposed controls or exposed but uninfected individuals, with deaths beginning at 23 days old and ending at 74 days old. (The first control died at 96 days old.)

Early host death (high virulence) was bad news for P. ramosa, since the parasite needs several weeks to produce spores. But it also didn't fare terribly well with a long-lived host (low virulence), suggesting that the bacteria in these hosts grew too slowly to reach the optimal time of killing.

The highest spore production was detected in Daphnia expiring at middle age, likely reflecting the benefits of using host resources for spore production - which can be impressive (one clutch of host eggs corresponds to an estimated 4.5 million P. ramosa spores, according to a recent study). Thus, researchers concluded that maximum parasite fitness derives from an intermediate level of virulence. In other words the parasites do their best not to be too deadly.

Photo credit: Jensen et. al.