Editors' ChoiceInfectious Disease

Sequence-Specific Antibiotics

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Science Translational Medicine  22 Oct 2014:
Vol. 6, Issue 259, pp. 259ec183
DOI: 10.1126/scitranslmed.aaa0896

Deemed a national security priority by President Obama on 18 September 2014, antibiotic resistance threatens domestic and international health. Contributing to the problem is an inadequate pipeline of new antibiotic development to keep pace with rising rates of drug resistance. Now, Bikard et al. present a customizable molecular tool to combat infections with surgical precision.

The strategy uses Cas9, a double-stranded DNA nuclease present in bacteria as part of an acquired immune defense mechanism. Cas9 activity is directed by 20-nucleotide sequences derived from crRNA (CRISPR RNA; clustered, regularly interspaced, short palindromic repeats). Cas9-induced cleavage is thought to result in irreparable DNA lesions that ultimately result in cell death. The final ingredient in this system is the use of phagemids to deliver the system to desired cells.

The authors test this system in several experiments, highlighting its effectiveness and specificity. First, they showed that targeting a kanamycin- or methicillin-resistance gene in Staphylococcus aureus by CRISPR-Cas9 selectively killed only those cells harboring that gene. This selective killing means that any normal members of the microbiota would be unaffected. The authors also showed that CRISPR-Cas9 can target plasmids within cells, which frequently harbor and spread antibiotic resistance genes. In contrast to chromosomal targets, these cells did not die. Instead, the cells were cured of the plasmids, providing a mechanism to target resistance so that existing, conventional antibiotics would regain their effectiveness. Last, the authors used a mouse skin infection model to show that topical application of this CRISPR-Cas9 system resulted in targeted decolonization of the desired bacterial population.

However, there are important limitations; delivery of the CRISPR-Cas9 system is not absolute. Some targeted cells were not infected, whereas others received a defective CRISPR system. In addition, the necessary dose must be significantly larger than the target population, presenting challenges for large-scale production. Moreover, targeting infection sites that are more complex than mouse skin will need to be demonstrated. Despite these limitations, the CRISPR-Cas9 system is a new approach to antibiotic development, one that offers high selectivity and the potential to resensitize organisms to existing but currently ineffective antimicrobials.

D. Bikard et al., Exploiting CRISPR-Cas nucleases to produce sequence-specific antimicrobials. Nat. Biotechnol. 1038/nbt.3043 (2014). [Full Text]

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