Genetic Engineering Publications - GEG Tech top picks

Two independent groups have now developed a new class of antimicrobials that act on specific bacterial populations, while leaving others unharmed. These new antimicrobials are based on the Streptococcus pyogenes type II CRISPR gene-editing system, which directs the Cas9 nuclease to cleave genomic target sites that can be specified in CRISPR guide RNAs (crRNAs). Lu and colleagues targeted enterobacterial genes encoding β-lactamase enzymes that conferred extended-spectrum or pan–β-lactam antibiotic resistance, whereas Bikard, Marraffini and colleagues studied the specific elimination of kanamycin-resistant or MRSA cells. Both groups showed that transforming bacteria with plasmids bearing Cas9 and crRNAs that targeted specific antibiotic-resistance factors was able to promote killing of the intended bacterial populations without affecting cells that were not carrying the targeted sequences. Lu and colleagues also demonstrated that Cas9–crRNA modules could be introduced into target bacterial cells through conjugation with engineered donor bacteria containing mobilizable plasmids or by infection with M13 phagemids. The latter approach was used to modulate the composition of a complex microbial community in vitro and was also efficient in treating Escherichia coli O157:H7 infection in an insect larva model. Bikard, Marraffini and colleagues used a phagemid-based approach to target kanamycin-resistant S. aureus mixed with kanamycin-sensitive bacteria and found that the nontargeted cells outcompeted any residual targeted cells for growth. The group also showed that a single crRNA construct could successfully be programmed against two separate virulence plasmids in an MRSA strain. Phagemid treatment of antibiotic-sensitive S. aureus could immunize the cells against the transfer of antibiotic-resistance genes from infection with phage grown on the MRSA strain. Selective targeting of kanamycin-resistant bacteria was also demonstrated in a mouse skin colonization model. Notably, both groups found that Cas9-targeted escapees that arose after treatment were due to defects in the CRISPR constructs rather than to host-adaptive mutations that created resistance to the new drug, thus supporting the concept of CRISPR-based treatments as an alternative to traditional drug therapies.

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