ADAPTATION 16-7 



They do not, however, correspond to the times of initiation of 

 growth. Traces of yeasts, much less than measurable with the 

 instrument, are generally observable earlier, usually within three 

 to four days after inoculation on the mixtures of lactate and glu- 

 cose. Occasionally, as is illustrated by some curves in figs. 16-2A 

 and 16-5A and checked by loop transfers to glucose medium, con- 

 siderable yields are obtained before adaptation to glucose occurs. 



The time required from the beginning of rapid growth to the 

 attainment of maximum rate (flex points of curves) is difficult to 

 estimate exactly but is certainly also very variable from one cul- 

 ture to the next. The average values are shown under the heading 

 "Periods of increasing growth rates." Perhaps the length of time 

 required increases with the concentration of lactate in the medium, 

 but this tendency is certainly less marked than the extreme dif- 

 ferences exhibited between individual cultures of a given set, which 

 will be discussed below. 



The later portions of the curves show that with the addition of 

 glucose alone (fig. 16-2) to the medium the growth approaches the 

 final yields very quickly once it has started, whereas with mix- 

 tures of lactate and glucose (figs. 16-3, 16-4, 16-5) the growth de- 

 creases very markedly as a rule long before the final yields are 

 attained. This decrease in growth rate is evidently due to a num- 

 ber of different causes which operated to different extents in in- 

 dividual cultures. In the first place where adaptation to glucose 

 has occurred before the lactate has been depleted subsequent 

 growth may result from further utilization of remaining lactate. 

 See curves 1 and 3 in Fig. 16-3A; curves 1 and 2 in fig. 16-4A; and 

 curves 1, 2, 4, and 6 in fig. 16-5. Secondly, it is clear from a com- 

 parison of the figures with increasing lactate concentrations that 

 there is a correspondingly slower rate of utilization of the glucose 

 and that the growth rates frequently begin to decrease long before 

 the glucose has been depleted, i.e., inhibition occurs when the 

 amount of growth is only a fraction of the final yield, so that glu- 

 cose as well as the lactate present must be required for the sub- 

 sequent yield. This decrease in growth rate before the final yield 

 is attained with increasing concentration of lactate, also holds for 

 strain 322. It appears, therefore, that high lactate concentrations 

 interfere to some extent with the utilization of glucose. Hence, the 

 phase of subsequent slow growth and the secondary rise in growth 

 rate may be due to an adaptation to glucose utilization in the pres- 

 ence of lactate, i.e., to the development of a tolerance to lactate as 

 well as to the utilization of lactate. Numerous other changes must 

 occur in the medium which may impede the rate and also in spe- 

 cific instances may give preference to the rapid increase of mutant 

 types in the cultures. 



