A Study of D-arabinose Fermentation

The stability of D-arabinose adaptation ofBact. lactis aerogenes

Bact. lactis aerogenesfirst subcultured into D-arabinose has a long lag phase, in one strain about 30 hr. This is shown by cultures freshly grown from a single colony, and is not significantly changed by repeated subculture in a variety of media free from D-arabinose. According, therefore, to the theory that mutations and reverse mutations lead to an equilibrium and that the lag is the time required for the small proportion of mutants to multiply in the D-arabinose, this proportion must be very small, and the equilibrium rapidly established. Differential equations can be set up to express the rate of establishment of equilibrium starting either with normal cells (not utilizing D-arabinose) or with mutant cells. From the experimental observations an estimate can be made of the minimum rate of establishment of equilibrium in the forward direction, and the equations then can be applied to calculate the rate at which reverse mutation should occur. The calculations can be compared with the experimental results on the stability of D-arabinose adaptation during subsequent culture of the mutant cells in glucose. Reversion does not occur in the predicted manner. The theory would also fail to account for the generally observed influence of the length of training of bacteria on the ease of reversion. Theories of direct adaptation do not present these difficulties (though they do not themselves make any general prediction about the rate of reversion).

Selective population shifts in mixtures of D-arabinose positive and D-arabinose negative strains of coliform bacteria

Normal strains of Bacterium lactis aerogenes and of Escherichia coli only utilize d-arabinose after a long lag (A - ), but can become adapted to use it freely (A + ). In a medium not containing d-arabinose the A + and A - types can compete for predominance as more or less independent strains. The result of the competition in the examples studied never depended upon the initial proportions of the two types (as certain selective mechanisms would suggest) but simply upon the relative extent to which each strain had been previously acclimatized to the specific minimal medium used for the competitive growth. According to the previous history either type could be initially selected, or an even balance preserved for a considerable time. Superposed on the selection of the A + or of the A - cells there was a certain tendency for the A + cells to lose their adaptation to d-arabinose. It was very slight with Bact. lactis aerogenes and much more marked with E. coli .