SUMMARY: Fifty strains of resistant to penicillin or tetracycline or both were examined for loss of these two resistances as the result of growth at 43-44°. Twelve strains showed a loss of penicillin resistance under the experimental conditions, and of these twelve, three showed a loss of tetracycline resistance. The two resistances were lost independently; in strains in which both resistances were lost most of the sensitive variants had lost one or other resistance but not both. However, penicillin resistance was lost only in strains that were also tetracycline-resistant and All strains in which a loss of resistance occurred had similar phage-typing patterns and all belonged to the ‘52, 52A, 80, 81 complex’ of strains. Not all strains in the complex, however, showed a loss of resistance. Both penicillin and tetracycline resistance were transduced into suitable sensitive recipients. The results showed that the heat-sensitivity of the transduced resistance was the same as in the donor in which the transducing phage was propagated. The results are consistent with the hypothesis that these two resistances are carried by two separate plasmids, at least in certain strains of

Nine multiply-resistant strains of were examined for loss of resistance to antibiotics other than penicillin and tetracycline as the result of growth at elevated temperatures. These other antibiotics included streptomycin, erythromycin, novobiocin, oleandomycin, neomycin and bacitracin. In no case was any loss of resistance observed.


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