Methods Bacterial strains and growth media The rhizobia used in this study included strains UCT40a, UCT44b, UCT61a and PPRI13, which were isolated from native Cyclopia species in the Western Cape of South Africa, using yeast-mannitol agar as growth medium. The choice of these four strains
out of 39 bacterial isolates was based on their superior symbiotic performance. In general, some of the 39 bacterial isolates were faster in growth (appearing within two days of streaking and producing copious quantities of exopolysaccharide gum, e.g. UCT44b and UCT61a), while phenotypically similar strains only appeared 5 days after streaking. Antibiotic Resistance Intrinsic natural resistance to low antibiotic concentrations The intrinsic resistance of the four Cyclopia strains to the antibiotics streptomycin sulphate (Sigma Chemical Co. Ltd.) and spectinomycin selleck chemicals Capmatinib dihydrochloride pentahydrate (Fluka Biochemica Ltd.) was determined by streaking rhizobial Cell Cycle inhibitor culture onto yeast-mannitol agar (YMA52) plates containing incremental concentrations of streptomycin
(0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 and 5.0 μg ml-1) or spectinomycin (0, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 5.0, 10.0 and 20.0 μg ml-1). The antibiotics were first sterilised by filtration through a 0.45 μm Millipore filter before addition to autoclaved YMA (cooled
to < 50°C). Test strains were grown in yeast-mannitol 4-Aminobutyrate aminotransferase broth (YMB52) at 20°C to 0.6 OD600, serially diluted to 10-6 and 0.1 ml streaked onto each plate. Plates were streaked in triplicates. Colony-forming units (CFU) per plate were counted after four days of growth. A strain was considered to have intrinsic resistance to an antibiotic if it attained 50% or more growth on antibiotic plates (colony-forming units, CFU, per plate) compared to antibiotic-free control plates. Antibiotic marking To develop spontaneous antibiotic-resistant mutants, streptomycin or spectinomycin was incorporated at 10 × the intrinsic resistance level of the test strain into YMA plates. Unmarked parent strains were each grown in YMB to 0.6 OD600 and 0.1 ml (107 – 108 cells), and streaked onto five replicate streptomycin-containing YMA plates. Mutants that appeared spontaneously within five days of growth were isolated, re-streaked onto YMA containing streptomycin, and stored at 0°C. For each test strain, three streptomycin-resistant mutants were randomly selected, grown in YMB broth to 0.6 OD600 and 0.1 ml streaked onto each of five replicate spectinomycin-marked plates. To develop a double marker, the spontaneous mutants were isolated and re-streaked onto plates containing both antibiotics.