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Genome editing in non-model bacteria is important to understand gene-to-function links that may differ from those of model microorganisms. Although species of the Burkholderia cepacia complex (Bcc) have great biotechnological capacities, the limited genetic tools available to understand and mitigate their pathogenic potential hamper their utilization in industrial applications. To broaden the genetic tools available for Bcc species, we developed RhaCAST, a targeted DNA insertion platform based on a CRISPR-associated transposase driven by a rhamnose-inducible promoter. We demonstrated the utility of the system for targeted insertional mutagenesis in the Bcc strains B. cenocepacia K56-2 and Burkholderia multivorans ATCC17616. We showed that the RhaCAST system can be used for loss- and gain-of-function applications. Importantly, the selection marker could be excised and reused to allow iterative genetic manipulation. The RhaCAST system is faster, easier, and more adaptable than previous insertional mutagenesis tools available for Bcc species and may be used to disrupt pathogenicity elements and insert relevant genetic modules, enabling Bcc biotechnological applications. [ABSTRACT FROM AUTHOR]

ABSTRACT The use of surrogate organisms can enable researchers to safely conduct research on pathogens and in a broader set of conditions. Being able to differentiate between the surrogates used in the experiments and background contamination as well as between different experiments will further improve research efforts. One effective approach is to introduce unique genetic barcodes into the surrogate genome and track their presence using the quantitative polymerase chain reaction (qPCR). In this report, we utilized the CRISPR-Cas9 methodology, which employs a single plasmid and a transformation step to insert five distinct barcodes into Bacillus thuringiensis, a well-established surrogate for Bacillus anthracis when Risk Group 1 organisms are needed. We subsequently developed qPCR assays for barcode detection and successfully demonstrated the stability of the barcodes within the genome through five cycles of sporulation and germination. Additionally, we conducted whole-genome sequencing on these modified strains and analyzed 187 potential Cas9 off-target sites. We found no correlation between the mutations observed in the engineered strains and the predicted off-target sites, suggesting this genome engineering strategy did not directly result in off-target mutations in the genome. This simple approach has the potential to streamline the creation of barcoded B. thuringiensis strains for use in future studies on surrogate genomes.IMPORTANCEThe use of Bacillus anthracis as a biothreat agent poses significant challenges for public health and national security. Bacillus anthracis surrogates, like Bacillus thuringiensis, are invaluable tools for safely understanding Bacillus anthracis properties without the safety concerns that would arise from using a virulent strain of Bacillus anthracis. We report a simple method for barcode insertion into Bacillus thuringiensis using the CRISPR-Cas9 methodology and subsequent tracking by quantitative polymerase chain reaction (qPCR). Moreover, whole-genome sequencing data and CRISPR-Cas9 off-target analyses in Bacillus thuringiensis suggest that this gene-editing method did not directly cause unwanted mutations in the genome. This study should assist in the facile development of barcoded Bacillus thuringiensis surrogate strains, among other biotechnological applications in Bacillus species.

Genome editing in non-model bacteria is important to understand gene-to-function links that may differ from those of model microorganisms. Although species of the Burkholderia cepacia complex (Bcc) have great biotechnological capacities, the limited genetic tools available to understand and mitigate their pathogenic potential hamper their utilization in industrial applications. To broaden the genetic tools available for Bcc species, we developed RhaCAST, a targeted DNA insertion platform based on a CRISPR-associated transposase driven by a rhamnose-inducible promoter. We demonstrated the utility of the system for targeted insertional mutagenesis in the Bcc strains B. cenocepacia K56-2 and Burkholderia multivorans ATCC17616. We showed that the RhaCAST system can be used for loss- and gain-of-function applications. Importantly, the selection marker could be excised and reused to allow iterative genetic manipulation. The RhaCAST system is faster, easier, and more adaptable than previous insertional mutagenesis tools available for Bcc species and may be used to disrupt pathogenicity elements and insert relevant genetic modules, enabling Bcc biotechnological applications., Importance: Species of the Burkholderia cepacia complex (Bcc) have great biotechnological potential but are also opportunistic pathogens. Genetic manipulation of Bcc species is necessary to understand gene-to-function links. However, limited genetic tools are available to manipulate Bcc, hindering our understanding of their pathogenic traits and their potential in biotechnological applications. We developed a genetic tool based on CRISPR-associated transposase to increase the genetic tools available for Bcc species. The genetic tool we developed in this study can be used for loss and gain of function in Bcc species. The significance of our work is in expanding currently available tools to manipulate Bcc., Competing Interests: The authors declare no conflict of interest.