Limitations of colony morphology and antibiotic resistance in the identification of a Bradyrhizobium sp. (Lotus) strain in soil |
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| Authors: | H H Lochner B W Strijdom P L Steyn |
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| Institution: | (1) Plant Protection Research Institute, Private Bag X134, 0001 Pretoria, South Africa;(2) Present address: Tobacco and Cotton Research Institute, Private Bag X82075, 0300 Rustenburg, South Africa;(3) Present address: Department of Microbiology and Plant Pathology, University of Pretoria, 0002 Pretoria, South Africa |
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| Abstract: | Summary We investigated the reliability of antibiotic resistance and colony morphology of clones of a Bradyrhizobium sp. (Lotus) strain for strain identification in nodulation competitiveness experiments in soil. There was no difference in nodulation competitiveness between the wild type strain and each of five mutants resistant to streptomycin and spectinomycin at the time of their isolation from antibiotic-containing media. However, these mutants were significantly less competitive when tested 4 months later. The apparent instability of the newly isolated mutants and their subsequently decreased nodulation competitiveness show that mutants must be examined carefully after being allowed time to stabilize. Two clones of the Bradyrhizobium sp. (Lotus) strain that differed in colony morphology on yeast mannitol medium did not differ in antigenic properties, whole cell protein electrophoresis profiles, mean cell generation times in yeast mannitol medium, N2-fixing ability, nodulation of Lotus pedunculatus in growth pouches, or in nodulation competitiveness. Both clones retained their colony morphology after numerous transfers on yeast mannitol agar over 3 years and after at least 6 months in soil. A limiting factor, which may restrict the use of colony morphology as a marker for strain identification in competition experiments, is the problem of detecting double-infected nodules when the small colony type comprises a relatively small portion of the total nodule population. |
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| Keywords: | Antibiotic resistance Colony morphology Strain identification Bradyrhizobium sp (Lotus) |
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