SUMMARY: Morphological mutants of , which undergo extensive stalk elongation when grown on complex medium, were isolated by an indirect selection technique. Wild-type is known to produce a similar morphological defect when grown under conditions of severe phosphate limitation. The maximum stalk length of the mutants was 20 μ, as compared with 2 to 3 μ for the parent strains. In these mutant strains, stalk synthesis proceeded in individual bacteria as long as they remained viable. This was in contrast to the wild-type bacteria cultivated on complete medium, where stalk growth reached completion within the first few generations. The mutants had no organic growth factor requirements, but their growth rates were considerably slower than those of the wild-type strains. Two of the mutants showed conditional phenotypic reversion to normal stalk formation when cultivated in the presence of relatively high inorganic phosphate concentrations (10 ). No ultrastructural defects in the mutants were noted, other than the excessive stalk length. Genotypic reversion rates for back-mutation to short stalk formation were estimated.


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