70 



THE RISE AND FALL OF BACTERIAL POPULATIONS 



rate of growth of different species in the same medium, the colon bacilH rising more 

 rapidly at first and the streptococci becoming dominant in the later stages of the cycle. 

 Reed and Reynolds (1916) give interesting data as to the period of maximum growth 

 for various types of bacteria inoculated into milk in pure culture, the periods at 37° C 

 varying from i day ior Bad. lactis-acidi and Sarcina lutea to 21 days for M. citrictis 

 and 42 days for Oidium lad is. 



In media which are unfavorable the phase of crisis may set in very early. Chick 

 (191 2) found that the usual relations as to lag period, logarithmic multiplication, and 



DAYS-*-© 7 II 16-17 23 Z7 39-40 52 63 



Fig. 3. — Multiplication of Bad. coli and streptococci in glucose broth culture into which they 

 were simultaneously inoculated. (Average of results reported by Prescott and Baker, 1904.) 



influence of temperature held for the growth of Bad. coli in normal rabbit serum — the 

 only difference being that the period of increase is very brief and leads only to a 

 doubling of bacterial numbers, after which a rapid decline sets in, due to the bacteri- 

 cidal effect of the serum. In rich culture media the initial growth will be rapid, but 

 here too the phase of crisis will usually set in within 24 hours as a result of the forma- 

 tion of inhibitive waste i)roducts. In milk containing mixed cultures of many species 

 of bacteria, the bacteria present may continue to increase for days and may reach 

 enormous figures (several Inllions per cubic centimeter, for example, in samples held 

 at 15.5° C. by Ayers, Cook, andClemmer, 1918). In most studies on milk bacteriology, 

 indeed, the existence of a period of crisis and subsequent decline has not been appar- 

 ent since observations have usually not been continued for a long enough period to 



