RESISTANCE OF BACTERIA 209 



by reducing the thickness of the substrate thus interfering with the 

 diffusion of those products which are inhibitory. 



I have apphed the term "inLxed cultures" to cultures derived from 

 secondary cultures when resistant bacteria and virulent bacteriophage 

 corpuscles co-exist. The appearance of mixed cultures varies in 

 accordance with the degree of resistance of the bacterium. 



With the bacteria possessing but relatively little resistance, and as a 

 consequence having a bacteriophage of but low virulence, the appearance 

 upon a solid medium or in a liquid medium very closely approaches that 

 of a normal bacterial culture. The isolated colonies are often irregular 

 (Kuttner;^'^^) the so-called "flattenformen" of Gildemeister. 



When the degree of resistance on the one hand and the virulence on the 

 other are somewhat higher the bacteria develop in the sediment, the 

 medium remaining more or less clear TGratia-^^). 



When the resistance and the virulence are very high, the bacteria 

 grow in the form of very dense agglutinated masses impossible to 

 dissociate by shaking. None of ^them float in the medium; all are 

 collected on the bottom and on the walls of the tube. Growth is very 

 slow but after some 15 days each of the agglutinated masses may have 

 the size of a small pin-head (d'Herelle^-^- I have observed this appear- 

 ance of mixed cultures, always with refractory bacteria, with the 

 organisms of the colon-typhoid-dysentery group and with staphylococci. 



According to Brutsaert,^"^ secondary cultures always present this 

 appearance in a hypertonic bouillon. 



This very distinctive appearance, characteristic of resistance and 

 maximal virulence, is retained but rarely throughout a series of suc- 

 cessive transfers in bouillon. I have found this appearance to be 

 permanent in only two cases and these two were mixed cultures of 

 Shiga bacilli and the bacteriophage. Usually the virulence of the 

 bacteriophage weakens rapidly, beginning with the second or third 

 transplant and one then finds the ordinary type of mixed culture in the 

 sediment while the supernatant fluid is more or less cloudy. In the two 

 cases of permanently agglutinated mixed cultures it was, however, only 

 necessary to render the conditions favorable for the bacterium (increase 

 in temperature, in acidity, etc.) in order to obtain mixed cultures in 

 the sediment. 



Upon agar, mixed colonies containing bacteria of very high resist- 

 ance and bacteriophage corpuscles of high virulence are usually small, 

 viscous, and of slow growth (d'HereUe''^^). 



Bordet and Ciuca^^ have obtained mixed colonies of B. coli which 



