Replica Plating Joshua and Esther Lederbergs
REPLICA PLATING AND INDIRECT SELECTION OF BACTERIAL MUTANTS
JOSHUA LEDERBERG AND ESTHER M. LEDERBERG Department of Genetics, ' College of Agriculture, University of Wisconsin, Madison, Wisconsin Received for publication August 31, 1951
Elective enrichment is an indispensable technique in bacterial physiology and genetics (van Niel, 1949). Specific biotypes are most readily isolated by the establishment of cultural conditions that favor their growth or survival. It has been repeatedly questioned, however, whether a selective environment may not only select but also direct adaptive heritable changes. In accord with similar discussions in evolutionary biology (Huxley, 1942), we may denote the concepts of spontaneous mutation and natural selection in contrast to specific induction as "preadaptation" and "directed mutation", respectively. Many lines of evidence have been adduced in support of preadaptation in a variety of systems (Luria and Delbruick, 1943; Lea and Coulson, 1949; Burnet, 1929; Newcombe, 1949; Lewis, 1934; Kristensen, 1944; Novick and Szilard, 1950; Ryan and Schneider, 1949; Demerec, 1948; Welsch, 1950; also reviewed: Braun, 1047; Luria, 1947; Lederberg, 1948, 1949). This paper concerns an approach to this problem that makes use of a replica plating technique which facilitates the handling of large numbers of bacterial clones for classification on a variety of media.
METHODS
Replica plating. A frequent chore in bacteriological work is the transfer of isolates from one substrate to other selective or indicator agar media. In place of an inoculating needle, one might imagine a device consisting of many needle tips in fixed array, so that one operation would substitute for repeated transfers with a single needle. The requirements of this design are met by pile fabrics such as velvet or velveteen. The pile provides space in a vertical plane for moisture that might otherwise cause lateral smearing of any impression. (According to Dr. N. Visconti, in a
JOSHUA LEDERBERG AND ESTHER M. LEDERBERG Department of Genetics, ' College of Agriculture, University of Wisconsin, Madison, Wisconsin Received for publication August 31, 1951
Elective enrichment is an indispensable technique in bacterial physiology and genetics (van Niel, 1949). Specific biotypes are most readily isolated by the establishment of cultural conditions that favor their growth or survival. It has been repeatedly questioned, however, whether a selective environment may not only select but also direct adaptive heritable changes. In accord with similar discussions in evolutionary biology (Huxley, 1942), we may denote the concepts of spontaneous mutation and natural selection in contrast to specific induction as "preadaptation" and "directed mutation", respectively. Many lines of evidence have been adduced in support of preadaptation in a variety of systems (Luria and Delbruick, 1943; Lea and Coulson, 1949; Burnet, 1929; Newcombe, 1949; Lewis, 1934; Kristensen, 1944; Novick and Szilard, 1950; Ryan and Schneider, 1949; Demerec, 1948; Welsch, 1950; also reviewed: Braun, 1047; Luria, 1947; Lederberg, 1948, 1949). This paper concerns an approach to this problem that makes use of a replica plating technique which facilitates the handling of large numbers of bacterial clones for classification on a variety of media.
METHODS
Replica plating. A frequent chore in bacteriological work is the transfer of isolates from one substrate to other selective or indicator agar media. In place of an inoculating needle, one might imagine a device consisting of many needle tips in fixed array, so that one operation would substitute for repeated transfers with a single needle. The requirements of this design are met by pile fabrics such as velvet or velveteen. The pile provides space in a vertical plane for moisture that might otherwise cause lateral smearing of any impression. (According to Dr. N. Visconti, in a
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