68 THE RISE AND FALL OF BACTERIAL POPULATIONS 



half by the addition of 0.17 per cent glucose. The literature is full of comparative 

 studies of media for water and milk analysis or for the isolation of specific organisms 

 which bear upon this point. As an example of such studies we need only cite the recent 

 work on media for milk analysis which has indicated the widely different results ob- 

 tained with various brands of peptone (Shrader, 1926). Davis and Ferry (1919) note 

 that both the growth and the toxin production of the diphtheria bacillus in a beef- 

 infusion medium are dependent on the particular types of amino acids present and 

 suggest that other accessory factors, perhaps of the nature of vitamines, are also es- 

 sential. The presence of growth hormones was held to be necessary for meningococci 

 and gonococci by Lloyd (191 6) and Cole and Lloyd (19 17); and Wildiers (1901) 

 claimed that a hypothetical substance called "bios" was necessary for the fullest 

 growth of yeast in a synthetic medium. 



Devereux and Tanner (1924) and Werkman (1927) have recently reviewed va- 

 rious aspects of this subject and conclude that the evidence as to the influence of true 

 vitamins or growth-promoting substances (other than those of a nutrient character) 

 is very doubtful.' 



In addition to those nutrient materials which are directly essential for the up- 

 building of bacterial protoplasm the rate of multiplication of bacteria is also governed, 

 like many other biological processes, by the regulative action of mineral salts. This 

 subject has been admirably reviewed by Falk (1923), and we need only point out here 

 that Hotchkiss (1923) and others have shown that a wide variety of salts stimulate 

 bacterial growth in low concentration and inhibit it in a higher concentration. Even 

 such toxic salts as HgCla may stimulate growth when present in a dilution of one- 

 millionth of a molar concentration (and inhibit it entirely in a concentration of one- 

 hundred-thousandth molar) while NaCl and KCl stimulate in .25-M concentration 

 and inhibit in 2-M concentration (under the conditions of the Hotchkiss study). It 

 seems possible that the multiplication of water bacteria in a sample bottle may in part 

 be due to the stimulant action of minute traces of salts dissolved from the glass during 

 sterilization, since Kohn (1906) has shown that the increase is most marked in bottles 

 of the more soluble types of glass. 



Finally, the dissolved gases in a medium affect bacterial multiplication in far- 

 reaching ways which we are as yet far from comprehending. Wolffhiigel and Riedel 

 (1886) found that multiplication of bacteria in a flask stoppered with cotton was 

 greater than in one closed with a rubber stopper, and Whipple (1901) observed that it 

 was greater when a bottle was only partly filled than when it was filled more nearly to 

 the top. Curiously, however, agitation and artificial aeration seemed unfavorable to 

 growth in WTiipple's experiments. One of the most important contributions to bacte- 

 riology in recent years has been the demonstration by Valley and Rettger (1927) that 

 a small percentage of carbon dioxide is essential to bacterial growth and that when 

 this gas is entirely removed growth ceases completely. 



4. THE PHASE OF CRISIS 



After a lapse of time, varying with the nature of the organism, the medium, and 

 the temperature, the period of logarithmic increase draws to a close, and after an in- 

 termediate phase of crisis a phase of decrease supervenes. This is of course a phenom- 



' Cf. chapter xxxvii in this vohime. 



