The mechanism of the training ofBact. lactis aerogenesto D-arabinose

Abstract

Further experimental studies of the training of Bact. lactis aerogenes to utilize the pentose D-arabinose yield evidence against the presence in unadapted or partially adapted strains of the two distinct classes of normal and mutant cells. It has been found that the lag preceding colony formation by an unadapted strain on a solid medium is never less than the lag observed under comparable conditions in a liquid culture, but decreases continuously with the time of previous exposure to D-arabinose; that the presence of untrained cells does not inhibit the multiplication of trained cells; that the growth curve is not that characteristic of a simple mixture of normal cells and mutants; that at the end of the long lag phase the bacterial mass increases considerably before any detectable increase in cell number occurs, and that in favourable cases every cell present can become adapted. During the earliest stages of growth on D-arabinose no stably trained cells can be detected, all undergoing rapid reversion when grown in the presence of glucose under conditions where the selection of reverse mutants or of untrained cells could not be important. In mixtures of fully trained and normal cells there is no selective advantage in favour of the latter when prolonged subculture is made in glucose. The absence of reversion with fully trained cells cannot, therefore, be explained in this way (a matter relevant to a previous study). Co-operative effects result in a variation in lag according to the conditions of culture, and show themselves in the influence of sterile filtrate from grown cultures. The statistical variations in the length of the lag phase have been examined. It is concluded that no simple theory of mutation and selection will account for the observations, which, on the other hand, are adequately explained by a hypothesis of direct adaptation, i.e. the development on growth of an alternative metabolic route in the normal cell.