FORMS OF THE COLON BACILLUS 129 



and the time that half of the cells remaining in the culture have 

 become stained by Congo red. 



The rate of autolysis so determined proved to be lowest in the 

 alkaline culture (17 days), the other cultures in order of increas- 

 ing autolysis rate being the neutral one (13.25 days), the salt medium 

 (6.25 days), the acid agar (4.25 days), and the medium containing 

 calcium chloride (4 days). 



The highest rate of death was shown by the salt culture, the 

 mean daily decrease in the plate counts being 21.8 per cent; in 

 the alkaline medium it was 16.5 per cent, in the acid one 16.4 

 per cent, in the calcium chloride agar 11.9 per cent, and in the 

 neutral culture 9.7 per cent. The period of maximum death rate 

 occurred in the neutral agar between the second and third days, the 

 decrease per day being 60.2 per cent; in the acid medium between 

 the fifth and seventh days, 37.7 per cent; in the medium with sodium 

 chloride, fifth to seventh days, 32.2 per cent; in the calcium chloride 

 agar, second to third days, 26.8 per cent; and in the alkaline culture, 

 tenth to fourteenth day, 23.3 per cent. The order of the cultures 

 with regard to rate of autolysis does not agree with either of the 

 above, and the rate of autolysis is therefore independent of the 

 death rate. 



The medium most favorable to growth, as measured by the 

 maximum yield (plate counts), was the acid agar; the others, in 

 descending order being the alkaline, the neutral, and those contain- 

 ing salt and calcium chloride. The death phase began earliest in 

 the neutral and alkaline media, both showing the maximum plate 

 counts on the second day; in the salt agar and calcium chloride 

 medium on the third day; and in the acid agar on the fifth day. 



The microscopic counts in the death phase are determined not 

 only by the rates of death and autolysis, but also by the degree 

 to which those cells still viable are multiplying; for the marked in- 

 crease in the microscopic counts after the plate counts have reached 

 their maximum can only be explained by the fact that some of 

 the cells are still actively dividing and the progeny then soon dying. 

 This occurred in greatest degree in the neutral medium, the maxi- 

 mum microscopic count showing an increase of 176 per cent over 

 the maximum plate count. In the calcium chloride medium this 



