274 ' Mary Barber 



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DISCUSSION 



Pollock: If you simply grow the Oxford staphylococci or other peni- 

 cillin-sensitive staphylococci in climbing concentrations of penicillin, 

 you can train them to penicillin resistance without the formation of 

 penicillinase. Now, in your technique you plated out the strain in a 

 penicillin ditch plate and then subcultured from the edge. Was this into 

 broth before a further subculture, or into penicillin broth? 



Barber: Into broth with traces of penicillin, 0-005 units/ml. 



Pollock: You weeded out cases which were resistant to an unknown 

 cause and not to penicillinase ? 



Barber: Yes, but I did not find many; with that very small concentra- 

 tion of penicillin they are less frequent than with higher concentrations. 



Lederberg: Have not you and Rountree both found that the peni- 

 cillinase-producing strains are fairly characteristic, at least most of them 

 of a single phage type ? 



Barber: That is not true any more, although it used to be the case. 

 When penicillin-resistant infection first became a menace in hospitals, 

 practically all penicillin-resistant strains were sensitive to phages of 

 group 3. Later, as the process went on, penicillin-resistant strains of 

 phage group 1 were isolated and now even group 2 strains may be 

 penicillin-resistant so that penicillinase-producing staphylococci now 

 encountered in clinical infection may be of any phage group. 



Eagle: Many of us who work on the development of resistance under 

 laboratory conditions are inclined to equate that phenomenon with the 

 clinical problem. The inference is that there is a constant race between 

 the development of new antibiotics and the development of resistance 



