190 STRONG AND TEAGUE. 



tremely difficult to break up the small clumps and to obtain a 

 homogeneous and durable suspension. When grown at 37° C, 

 the bacilli, when collected upon the platinum loop, form a homo- 

 geneous, moist, mucoid mass which readily forms a homogeneous 

 suspension when shaken in saline solution. At 30° C, the growth 

 results sometimes more like the growth at 37° C. just described, 

 at other times more like the growth at the temperature of the 

 ice-box, depending upon the strain employed; most strains cul- 

 tivated at 30° C. produce more mucus than is usually seen in 

 cultures developed in the ice-box and less than is seen in cultures 

 grown at 37° C. 



Another factor, which in our experience has exerted an im- 

 portant influence upon the mucus production of a plague strain, 

 is the length of time it has been cultivated upon artificial media. 

 Freshly-isolated strains, whether from human subjects or from 

 our experimental animals, produce more mucus than strains 

 which have been cultivated on agar for some time. 



We cultivated a number of strains at 32° C. upon sugar-free 

 agar, glucose-agar, saccharose-agar, and starch-agar, and did not 

 observe that these carbohydrates caused an increase in the 

 mucus production. 



The age of the culture is a factor influencing the amount of 

 mucus present. A twenty-four-hour culture will contain less 

 mucus than the same culture several days later. 



We have pointed out that the readiness with which the strains 

 form homogeneous suspensions appears to run parallel with their 

 mucus production and hence the former serves as a good index 

 of the latter. There are, however, other factors which bring 

 about the formation of homogeneous suspensions, notably the 

 presence of alkali. The addition of a few drops of alkali to a 

 suspension of a culture grown at ice-box temperature and shaking 

 quickly brings about the disappearance of the clumps, and a 

 homogeneous suspension results. It is for this reason difficult 

 to determine whether or not the reaction of the culture-medium 

 exerts an influence upon the mucus-production of plague or- 

 ganisms; we can only affirm that this influence, if present, is 

 not marked. 



To sum up, in our opinion the two factors of paramount im- 

 portance with regard to the mucus production of plague bacilli 

 are the temperature at which the cultures are grown and the 

 length of time that the organisms have been cultivated on arti- 

 ficial media since their isolation from the animal host. 



Bearing these facts in mind, we have not observed with regard 



