Root colonisation experiments indicated that Pseudomonas sp. strain CT 364 was successfully able to colonise the roots of Arabidopsis. The next step was to develop Pseudomonas sp. as an endophytic chassis, so that roots could be colonised by a Pseudomonas sp. strain that had been genetically engineered to express gene(s) of interest. This involved three main objectives: identifying suitable selectable markers, establishing a transformation protocol for Pseudomonas sp., and the design, assembly and characterisation of a new genetic part.

Pseudomonas sp. (CT 364) was obtained from DSMZ, Germany (DSM No. 25356). The strain arrived freeze-dried and was revived according to the protocol recommended by DSMZ.As Pseudomonas sp. is a soil bacterium it has an optimum growth temperature of 28 °C. Pure cultures were obtained following revival after overnight incubation (Figure 1).

The first step in the chassis development was to identify antibiotics that Pseudomonas sp. was susceptible to. This would indicate which selectable markers were suitable for use in transformation procedures. Five antibiotics were tested, chloramphenicol, kanamycin, gentamicin, carbenicillin and streptomycin. Kanamycin, gentamicin and streptomycin are all aminoglycosides with similar but independent modes of action interfering with protein synthesis via ribosomal binding. Chloramphenicol prevents protein chain elongation and carbenicillin interferes with cell wall synthesis. Many Pseudomonas species are known to be resistant to chloramphenicol but this antibiotic was tested to determine compatibility with pSB1C3. Carbenicillin and the aminoglycosides have been identified as active against many Pseudomonas species.

Antibiotic screening on tryptone soy agar (TSA) showed Pseudomonas sp. to be resistant to chloramphenicol, kanamycin and carbenicillin. Antibiotic concentrations of 50 and 100 µg/ml were tested with two replicates for each concentration. Lawns formed on agar containing 100 µg/ml of each antibiotic (Figures 2, 3 & 4) after 24 hours incubation at 28 °C.

Screening found Pseudomonas sp. to be susceptible to both streptomycin (Figure 5) and gentamicin (Figure 6). Antibiotic concentrations of 50 and 100 µg/ml were again tested with two replicates for each concentration. No colony forming units (CFUs) were visible on tryptone soy agar containing 50 µg/ml of either antibiotic inoculated with Pseudomonas sp. after 24 hours incubation at 28 °C.

After identifying which antibiotics were active against Pseudomonas sp. The next step was to identify working concentrations of these antibiotics to be used when selecting transformants. This was done by carrying out minimum inhibitory concentration (MIC) experiments where growth was tested against a range of antibiotic concentrations. To minimise variation between replicates MIC experiments were automated using an Opentrons OT-2 pipetting robot.

The results of MIC experiments showed a clear dose response between antibiotic concentration and growth of Pseudomonas sp. for both streptomycin and gentamicin (Figures 7 and 8). Gentamicin was found to be the more effective antibiotic with a concentration of 1.5 µg/ml sufficient to prevent growth. A minimum concentration was clear for gentamicin with a significant drop in growth rate between concentrations of 1.0 µg/ml and 1.5 µg/ml. The minimum inhibitory concentration for streptomycin was less clear, with a small drop in growth rate for each increase in concentration. A working concentration of 6.0 µg/ml was determined for streptomycin. A slight increase in absorbance was observed in the positive control for both antibiotics. This is likely due to release of compounds by bacterial cells upon death.

After identifying the MIC in liquid culture, the MICs in agar were determined for gentamicin as this was the antibiotic being taken forward for use in transformation. Concentrations of 2.0 µg/ml, 4.0 µg/ml (Figure 9) and 6.0 µg/ml (Figure 10) were tested as the working concentration was expected to be slightly higher than the 1.5 µg/ml MIC in liquid culture due to lower availability of antibiotic in agar. It was necessary to establish a working concentration in agar as cells would be plated onto agar following transformation. Gentamicin was selected as the working antibiotic as it is known to stunt Arabidopsis growth. Treatment of Arabidopsis with a gentamicin resistant transformant could prevent stunting of growth by breaking down gentamicin in the rhizosphere. Demonstrating that a plant colonising microbe can be used to change the physiology of the plant.

Agar assays showed that concentrations of 2.0 µg/ml and 4.0 µg/ml of antibiotic were insufficient to prevent growth of Pseudomonas sp. on agar. A lawn formed on agar containing 2 µg/ml and some colonies formed on agar containing 4.0 µg/ml (Figure 9). A gentamicin concentration of 6.0 µg/ml was sufficient to prevent growth of Pseudomonas sp. as no colonies formed at this concentration (Figure 10).

Once working antibiotic concentrations had been characterised for both antibiotics, transformations could be carried out. Addgene plasmid number 79813 was selected for transformation protocol optimisation due to its Pseudomonas origin of repolication and gentamicin resistance gene. Two minipreps were carried out on the plasmid containing E. coli strain, one of which was aliquoted and spun down giving three DNA solutions of varying DNA concentration (Table1).

Four electroporation experiments were carried out, one for each DNA solution and a negative where water was added in place of a DNA solution. Gentamicin resistant colonies formed for each of the DNA solutions with solution 1 giving the highest number of CFUs (Figure 11) and miniprep 3 the lowest (Figure 13). No colonies formed on agar inoculated with the negative control (Figure 14). Colony forming units obtained from each miniprep are displayed in Table 2.

Table 2. Colonies formed from electrocompetent Pseudomonas sp. inoculated with solutions containing Addgene plasmid #79813

An additional MIC experiment was carried out to characterise gentamicin resistance of the transformant (Figure 15). The transformant was resistant to gentamicin at the highest concentration tested (50 µg/ml).

A composite part containing the antibiotic resistance gene aadA, an anderson promoter, a strong RBS and a native terminator was designed. This part conferred resistance to streptomycin, the second antibiotic found to be active against Pseudomonas sp. This part was assembled into pBS1C3 for characterisation and submission to the registry. The Gibson assembly reaction mix is shown in Table 3.

Following assembly, chemically competent E. coli cells were transformed using each reaction mix. CFUs were obtained for both the streptomycin resistance Gibson assembly reaction mix (Figure 16) and the positive control (Figure 17). No colonies formed on agar inoculated with the negative control (Figure 18).

Following transformation, colonies were re-streaked on agar containing streptomycin (50 µg/ml) (Figure 19) confirming resistance and on agar containing chloramphenicol (25 µg/ml) as resistance was confirmed by the pBS1C3 backbone (Figure 20).

Streptomycin resistance of the transformed E. coli was compared to the wild type (Figure 21). The transformant was resistant to streptomycin at the highest concentration tested (64 µg/ml).

Pseudomonas sp. was found to be susceptible to two antibiotics, streptomycin and gentamicin. Using a Pseudomonas origin plasmid containing a gentamicin resistance gene electroporation protocols have been optimised for Pseudomonas sp. The new streptomycin resistance part confers strong resistance to streptomycin in E. coli.