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Streptomycin dependency in Escherichia coli K12 Whitlow, Karen J.

Abstract

Twenty-three spontaneous streptomycin dependent (Sm[sup d]) Escherichia coli K12 mutants were isolated from a log phase culture of wild-type streptomycin sensitive (Sm[sup s]) Escherichia coli K12. The metabolism of these mutants was characterized by a state of relaxed catabolite repression during growth on glucose. One of the more readily defined characteristics of these mutants is the loss of the wild-type phenotype of sensitivity of growth to inhibition by L-valine. In-studies involving both Sm[sup d] and Sm[sup s] Escherichia coli K12, it was shown that inhibition of growth by L-valine depended upon the level of activity of a regulatory enzyme in the isoleucine-leucine-valine (ilva) biosynthetic pathway, acetohydroxy acid synthase (AHAS). Hence, under conditions of catabolite derepression, as observed in the Sm[sup d] mutants or during growth of the wild-type parent in the presence of 5mM CAMP or on acetate, AHAS was derepressed and the sensitivity of growth to inhibition by valine was relieved. That this was a function of the level of activity of this enzyme was supported by the finding that the sensitivity of AHAS to inhibition by L-valine was identical in both wild-type and Sm[sup d] strains. In Sm[sup d] mutants all of the final products of the ilva pathway - valine, isoleucine, leucine and pantothenate were found to be involved in the regulation of the synthesis of AHAS. The close interrelationship observed in this work between dependency on streptomycin for growth and for the synthesis of catabolite sensitive enzymes, prompted a study of the influence of carbon source on antibiotic requirement. Subsequently, it was observed that Sm[sup d] Escherichia coli K12 grew at least as well on an acetate medium devoid of antibiotic as on a glucose medium containing dihydrostreptomycin (DHSM). The same observation was made with Sm[sup d] mutants of other strains of Escherichia coll. These observations are not easily explained on the basis of the current hypothesis of dependency based on a ribosomal site of action for streptomycin (or DHSM). However, the experimental observations may possibly be explained on the basis of an effect of the antibiotic on the bacterial membrane. Preliminary investigations showed that the character of the fatty acids in the membrane of Escherichia coli K12 varied both in relation to carbon source and streptomycin phenotype.

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