Antibiotic Resistance Levels of Bacteria Harbouring more than One R Factor

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Abstract
Strains of Escherichia coli were constructed that harboured 1, 2 or 3 independent R factors, and their antibiotic resistance levels determined by a spot-testing technique on solid media. R factor-mediated resistance levels to chloramphenicol and tetracycline remained unchanged in the presence of other R factors that did not determine resistance to these antibiotics. Resistance levels of strains carrying 2 or 3 R factors mediating ampicillin resistance were at least equal to the sum of the levels conferred by the R factors individually. A similar additive pattern was found in strains harbouring R factors that inactivated streptomycin by phosphorylation. However, streptomycin resistance levels conferred by R factor-mediated adenylylation of the antibiotic were not increased on the introduction of a second R factor that also conferred streptomycin resistance by adenylylation.

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