Trading Defense for Virulence

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Science  24 Aug 2012:
Vol. 337, Issue 6097, pp. 893
DOI: 10.1126/science.337.6097.893-a

Like larger organisms, bacteria are plagued by viruses and other mobile entities bringing in foreign DNA that may be damaging. Bikard et al. recognized that Streptococcus pneumonia (pneumococcus) lacks the CRISPR-endoribonuclease mechanism that has evolved to defend against bacteriophage infection and plasmid conjugation. Lateral gene transfer between unrelated species is a fundamental characteristic of prokaryote evolution, however, and therefore CRISPR loci could block the transfer of traits. Pneumococci rely on exogenous DNA to endow them with virulence and antibiotic resistance through direct uptake of DNA, and this may be why CRISPR is absent in this species. CRISPR sequences can be experimentally introduced into pneumococcus that prevent the pathogen from capsule-switching—a key trait that allows this bacterium to dodge host immune responses—and allow mice to survive infection with CRISPR+ pneumococci. Interestingly, during the experiments, pneumococci emerged that could inactivate the introduced CRISPR, indicating that selection pressure to avoid immune responses is strong enough to leverage reacquisition of the ability to scavenge for foreign genes in this species.


Cell Host Microbe 12, 177 (2012).

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