An Investigation of the Nature of Certain Adaptive Changes in Bacteria

Four strains of bacteria which, until trained, show a long lag before they will utilize certain new carbon or nitrogen sources, have been examined by methods involving the transfer of inocula at widely varying dilutions to liquid or solid media containing the new substrate. The strains and sources studied are (a) Bact. lactis aerogenes and D-arabinose, (b) Bact. coli mutabile and lactose, (c) Bact. coli and ammonium sulphate, (d) a coli-aerogenes intermediate and sodium citrate. With (a) and (b) in solid and liquid media and (d) in liquid media all or most of the dilutions which are viable in a glucose-amino-acid medium (which supports growth without lag) also show growth, though after considerable delay, in the new medium. With (c) the proportion of cells which grow is small and, even for parallel inocula, very sensitive to minute changes in the test medium. But in general the reason why many cells fail to grow is found to be that they have died before they have traversed the lag phase which necessarily precedes their development. The bearing of these observations on theories of mutation and adaptation is considered.