Homologous Recombination

Example 1

To generate an attenuated strain of P. aeruginosa for production of alginate, the following virulence factor genes were sequentially deleted from the chromosome of the wild-type strain PAO1: toxA, plcH, phzM, wapR, and aroA. toxA encodes the secreted toxin Exotoxin A, which inhibits protein synthesis in the host by deactivating elongation factor 2 (EF-2). plcH encodes the secreted toxin hemolytic phospholipase C, which acts as a surfactant and damages host cell membranes. phzM encodes phenazine-specific methyltransferase, an enzyme required for the production of the redox active, pro-inflammatory, blue-green secreted pigment, pyocyanin. wapR encodes a rhamnosyltransferase involved in synthesizing O-antigen, a component of lipopolysaccharide (LPS) of the outer membrane of the organism. aroA encodes 3-phosphoshikimate 1-carboxyvinyltransferase, which is required intracellularly for aromatic amino acid synthesis. Deletion of aroA from the P. aeruginosa genome has previously been shown to attenuate the pathogen. Each gene was successfully deleted using a homologous recombination strategy with the pEX100T-Not1 plasmid. The in-frame, marker-less deletion of these five gene sequences was verified by Sanger sequencing and by whole genome resequencing (FIG. 1 and FIG. 8). This engineered strain was designated as PGN5. The whole genome sequence of PGN5 has been deposited to NCBI Genbank with an accession number of CP032541. All five in-frame gene deletions were detected and validated to be the deletion as designed using PCR (FIG. 7).

To verify gene deletion and attenuation of the PGN5 strain, the presence of the products of the deleted genes was measured and was either undetectable, or significantly reduced in the PGN5 strain. To test for the toxA gene deletion in PGN5, a Western blot analysis was performed for the presence of Exotoxin A in the culture medium. Exotoxin A secretion was detected in wild-type PAO1 control, but not in the PGN5 strain (FIG. 2A). To confirm the loss of plcH, hemolysis was assessed on blood agar. The hemolytic assay was carried out by streaking PAO1, PGN5, P. aeruginosa mucoid strain VE2, and a negative control, Escherichia coli strain BL21 on blood agar plates. A clear zone was observed surrounding PAO1 and VE2 cell growth, indicating complete (β-) hemolysis (FIG. 2B). In contrast, the blood agar remained red and opaque surrounding PGN5 and BL21 growth, indicating negligible or no hemolytic activity in these strains (FIG. 2B). To assess for deletion of phzM, the amount of pyocyanin secreted by PAO1 and PGN5 was extracted and measured. The amount of pyocyanin detected was significantly reduced in PGN5 (FIG. 2C). In fact, the difference in pigment production between PAO1 and PGN5 was immediately apparent on agar plates (FIG. 3A-3B). To test for wapR gene deletion, an LPS extraction was performed, followed by silver-stained SDS-PAGE and Western blot on the following strains: PAO1, PGN4 (PGN5 without aroA deletion), VE2, and PAO1_wbpL, which serves as a negative control due to a deletion in the O-antigen ligase gene, and thus produces no O-antigen. The presence of O-antigen was detected in PGN4, but the level of LPS banding was significantly reduced compared to the LPS banding profile observed in PAO1 and VE2 (FIG. 2D). Lastly, to test for aroA deletion, ELISA was performed to detect the presence of 3-phosphoshikimate 1-carboxyvinyltransferase in cell lysates prepared from PAO1 and PGN5. The ELISA results showed that the amount of 3-phosphoshikimate 1-carboxyvinyltransferase was significantly reduced in PGN5, compared to that in PAO1 (FIG. 2E). Additionally, the deletion of aroA resulted in slower growth in the PGN5 strain, a growth defect that was restored with the addition of 1 mg/mL of aromatic amino acids (W, Y, F) to the culture medium (data not shown).