1949b The consequences of mutation during the growth of biochemical mutants of Escherichia coli. IV. The mechanism of inhibition of histidine-independent bacteria by histidineless bacteria

that the adaptation of unshaken histidineless (h-) cultures is limited by the acid produced during growth. The various pH's developed at intermediate his-tidine concentrations prevent the further growth of the histidine-independent (h+) mutants produced in these cultures. The sensitivity of the h+ bacteria to the various pH's is conditioned by something produced by h- bacteria during their growth. It was thought that the acid production might be a function of the partially anaerobic metabolism that must occur in the depths of the 10 ml of medium that were contained within the 15-by-150-mm pyrex test tubes (Stokes, 1949). Aeration might prevent this acid production and allow the phenomenon of adaptation to be studied without the complication of a continually changing pH. For this reason an analysis of shaken cultures of h- bacteria was undertaken. EXPERIMENTAL PROCEDURES The majority of the methods used in this research are described in the first two papers of this series (Ryan and Schneider, 1948, 1949a). In addition, in order to create aerated conditions of growth, the 10-ml aliquots of medium were place