SUMMARY: When staphylococci, resistant to 4 μg. tetracycline/ml., were grown in nutrient media at subinhibitory levels of the drug, phenotypical resistance increased until the cocci grew with 160 μg. tetracycline/ml. Resistance increased most rapidly at the highest concentration of tetracycline which did not significantly inhibit growth. Increase in resistance was also obtained by pre-incubation with β-apo-5-oxy-tetracycline. Increase in resistance could be prevented by chloramphenicol and actinomycin D, but not by nalidixic acid. When a highly resistant culture was transferred to tetra-cycline-free medium, phenotypical resistance decreased gradually; after four transfers on nutrient agar it returned entirely to the original level.


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