C.-E. A. WINSLOW 71 



pass the critical point. In the important contribution of Ayers and Johnson (1910), 

 however, many individual samples show the beginning of a decline after 4 or 5 days 

 even when held at 10° C. 



So far as the temperature effect is concerned we may note that Lane-Claypon 

 (1909) found the end of the period of logarithmic increase for Bact. coli in broth to oc- 

 cur after i\ hours at 37° C, after 8-8^ hours at 30° C, 12-15 hours at 25° C, and 

 20-24 hours at 20° C. In the work of Reed and Reynolds (1916) on the growth of 

 pure cultures of thirteen different species of micro-organisms in milk the average 

 period of maximum growth was 7 days at 35° C. and 20 days at 13° C. 



The explanation of the onset of the phase of crisis must obviously be sought in the 

 changed composition of the medium due to the growth of the bacterial population 

 during the phase of increase. It is clear that there is no essential and invariable life- 

 cycle involved since the results of Penfold and of Barber indicate that transfer of cells 

 from a culture in the early phase of increase to a new tube of the same medium yields 

 a constant and continuous development at a logarithmic rate. The change in the 

 medium which makes it unsuitable for further growth might theoretically be either an 

 exhaustion of essential foodstuffs or the formation of toxic waste products, and both 

 processes may no doubt play a part under certain conditions. In spring-water samples 

 containing but little organic matter it seems very probable that the exhaustion of food 

 may play a predominant role (though even Miquel stressed the conception of toxic 

 products). In rich culture media the formation of waste products is probably the fac- 

 tor of major importance. In milk and other sugar media the production of acid is 

 often by itself entirely adequate to explain cessation of growth. Thus Heinemann 

 (1915) found that a series of pathogenic bacteria were all destroyed when the acidity 

 of milk reached 0.45 per cent and that even Bact. coli died out when the milk reached 

 an acidity of 0.6 per cent, while Bact. coli itself increased the acidity of milk to 0.5 per 

 cent. Cohen and Clark (1919) found the limiting pH values for growth in peptone 

 broth to vary from 4.4 for Bact. aerogenes to 5.5 for Bact. alcaligenes; but they pointed 

 out the extreme complexity of the phenomena involved, as shown by the fact that 

 while fermentative activity seems to cease in a sugar medium at about the pH con- 

 centration known to limit growth, in previously adjusted media growth in the sugar 

 medium seems to stop at a point corresponding to concentration of the acetic acid 

 formed rather than the pH. Furthermore, in a broth medium free from sugar growth 

 ceases much sooner than in the sugar medium and with a total concentration of cells 

 only one-fifth as great as that which is found in the presence of sugar. 



In dealing with the phase of crisis, as in the case of the phase of adjustment, it 

 does not seem particularly profitable to refine our methods of analysis too far until 

 more detailed experimental data are available. If we assume that the cycle of a bac- 

 terial population starts from and returns to a stable state there are really but two 

 fundamental processes involved, an increase from the original stable condition and a 

 subsequent decrease to a second level, with phases of transition before the increase, 

 between the increase and the decrease, and before the period of ultimate stability. 

 Clearly, as a result of various factors, such as food supply, toxic products, and temper- 

 ature, the slope of the curve during the phase of crisis may take any form from a sharp 

 tooth to a flat plateau. 



