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1.  Altruism can evolve when relatedness is low: evidence from bacteria committing suicide upon phage infection 
High relatedness among interacting individuals has generally been considered a precondition for the evolution of altruism. However, kin-selection theory also predicts the evolution of altruism when relatedness is low, as long as the cost of the altruistic act is minor compared with its benefit. Here, we demonstrate evidence for a low-cost altruistic act in bacteria. We investigated Escherichia coli responding to the attack of an obligately lytic phage by committing suicide in order to prevent parasite transmission to nearby relatives. We found that bacterial suicide provides large benefits to survivors at marginal costs to committers. The cost of suicide was low, because infected cells are moribund, rapidly dying upon phage infection, such that no more opportunity for reproduction remains. As a consequence of its marginal cost, host suicide was selectively favoured even when relatedness between committers and survivors approached zero. Altogether, our findings demonstrate that low-cost suicide can evolve with ease, represents an effective host-defence strategy, and seems to be widespread among microbes. Moreover, low-cost suicide might also occur in higher organisms as exemplified by infected social insect workers leaving the colony to die in isolation.
PMCID: PMC3619501  PMID: 23516238
altruistic suicide; host defence; parasite transmission; structured population; Hamilton's rule; kin selection
2.  Defying bacteriophages 
Adaptations by hosts in response to parasitism are generally believed to reduce the susceptibility of the adapted individual. However, recent work on Escherichia coli showed that bacteria can fight deadly phage attacks by committing altruistic suicide upon infection, in order to prevent parasite transmission to nearby relatives. Here, we compare the efficiency of suicidal host defense with individual-based resistance. We show that in unstructured environments suicidal host defense is futile since suicide cannot preferentially protect relatives, whereas individual-based resistance is highly efficient in defying phages. In contrast, we found that in structured environments suicidal host defense and individual-based resistance were both efficient in withstanding phages, with the latter type performing slightly better. We propose that the putative lower efficiency of suicidal host defense might be compensated by the fact that suicidal systems usually do not bear pleiotropic costs of resistance, as it is usually the case for individual-based resistance mechanisms.
PMCID: PMC3829904  PMID: 24255741
altruistic suicide; resistance; structured population; parasite transmission; host-parasite interactions; E. coli; T4 phage
3.  Within-host competition determines reproductive success of temperate bacteriophages 
The ISME Journal  2011;5(9):1451-1460.
Within-host competition between parasites is frequently invoked as a major force for parasite evolution, yet quantitative studies on its extent in an organismal group are lacking. Temperate bacteriophages are diverse and abundant parasites of bacteria, distinguished by their ability to enter a facultative dormant state in their host. Bacteria can accumulate multiple phages that may eventually abandon dormancy in response to host stress. Host resources are then converted into phage particles, whose release requires cell death. To study within-host competition between phages, I used the bacterium Escherichia coli and 11 lambdoid phages to construct single and double lysogens. Lysogenic bacterial cultures were then induced and time to host cell lysis and productivity of phages was measured. In double lysogens, this revealed strong competitive interactions as in all cases productivity of at least one phage declined. The outcome of within-host competition was often asymmetrical, and phages were found to vary hierarchically in within-host competitive ability. In double infections, the phage with the shorter lysis time determined the timing of cell lysis, which was associated with a competitive advantage when time differences were large. The results emphasize that within-host competition greatly affects phage fitness and that multiple infections should be considered an integral part of bacteriophage ecology.
PMCID: PMC3160688  PMID: 21412345
multiple infection; within-host competition; polylysogeny; evolution of virulence; Escherichia coli; bacteriophage λ
4.  Combining forces to target bacteria 
Evolutionary Applications  2012;5(6):537-539.
PMCID: PMC3461137  PMID: 23028395
5.  Real-time quantitative PCR to discriminate and quantify lambdoid bacteriophages of Escherichia coli K-12 
Bacteriophage  2012;2(2):98-104.
Quantification of bacteriophages by real-time quantitative PCR (qPCR) is an interesting alternative to the traditional plaque assay. Importantly, the method should in principle be able to discriminate between closely related phages that are indistinguishable by most other means. Here, a method is presented that employs qPCR to discriminate and quantify ten closely related lambdoid phages of Escherichia coli str. K-12. It is shown that (1) treatment of samples with DNase efficiently removes non-encapsidated DNA, while the titer of plaque forming units is not affected, (2) individual phage types can be accurately quantified in mixed lysates, and (3) the detection limit corresponds to that of a plaque assay. The method is used to quantify individual phage types that are released from lysogens that carry up to three different prophages.
PMCID: PMC3442831  PMID: 23050220
multiple infections; real-time quantitative PCR; bacteriophages; Escherichia coli; bacteriophage lambda; lambdoid bacteriophages; detection; discrimination; quantification; polylysogeny
6.  Variation in phenoloxidase activity and its relation to parasite resistance within and between populations of Daphnia magna. 
Estimates of phenoloxidase (PO) activity have been suggested as a useful indicator of immunocompetence in arthropods, with the idea that high PO activity would indicate high immunocompetence against parasites and pathogens. Here, we test for variation in PO activity among clones of the planktonic crustacean Daphnia magna and its covariation with susceptibility to infections from four different microparasite species (one bacterium and three microsporidia). Strong clonal variation in PO activity was found within and among populations of D. magna, with 45.6% of the total variation being explained by the clone effect. Quantitative measures of parasite success in infection correlated negatively with PO activity when tested across four host populations. However, these correlations disappeared when the data were corrected for population effects. We conclude that PO activity is not a useful measure of resistance to parasites or of immunocompetence within populations of D. magna. We further tested whether D. magna females that are wounded to induce PO activity are more resistant to infections with the bacterium Pasteuria ramosa than non-wounded controls. We found neither a difference in susceptibility nor a difference in disease progression between the induced group and the control group. These results do not question the function of the PO system in arthropod immune response, but rather suggest that immunocompetence cannot be assessed by considering PO activity alone. Immune response is likely to be a multifactorial trait with various host and parasite characteristics playing important roles in its expression.
PMCID: PMC1691701  PMID: 15306368

Results 1-6 (6)