SUMMARY: To test the capacity of different staphylococci to acquire resistance to bacteriophage in vitro, twenty phages were grown on solid medium and in fluid medium with their propagating strains of Staphylococcus aureus, different phage types and patterns beingrepresented.

Resistant secondary growth never appeared among staphylococci of the 3 A group and often appeared among members of the 6/47 group. Some members of the 29/52 group acquired resistance readily, and others never became resistant, Cross-resistance to other phages wa oornmonly acquired. Secondary growth was shown to be resistant or sensitive to the phage, or to yield a mixture of resistant and sensitive colonies.

With the exception of propagating strain PS69, all staphylococci which became resistant to phage acquired lysogenicity for the sensitive parent strain. All but one of the staphylococci which had acquired resistance to a phage appeared to be capable of absorbing that phage. A substance which caused non-specific inhibition of phage lysis on agar mediumwas present in phage lysates of a staphylococcus that had shown sensitive secondary growth. These results am discussed with reference to phage typing and to the possibilities of therapy by phage.


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