20 MASS. EXPERIMENT STATION BULLETIN 319 



for each strain. However, it was apparent that daily transfer for a period of two 

 weeks, and also aging in broth for this period, did not markedly alter the colonial 

 appearance of strains that appeared typical at the outset. 



After further aging of these cultures for a period of two months, Strains I and 

 II had partially lost their rough colonial characteristics, while Strain IV exhibited 

 a typical smooth colony and numerous extremely small colonies which appeared 

 very irregular in shape. Meat extract broth tubes which were inoculated with the 

 small colonies did not reveal as heavy a turbidity of growth as is usually observed 

 with typical S. pullorum. Stained smears prepared from the small colonies con- 

 tained cells that were highly pleomorphic. A peculiar characteristic of this 

 variant (designated IV-a), was the production of very small "pin-point" colonies. 

 The size of these colonies increased but little with prolonged incubation. Some of 

 the widely separated colonies attained a size of one millimeter, and after selection 

 and further cultivation of these larger colonies it was observed that they retained 

 the peculiar characteristic small size. The pleomorphic cellular material consisted 

 mostly of thick, short rods which appeared almost coccoid in shape. Filaments, 

 large rods, club-forms and other "aberrant" types were observed. 



As a matter of interest, the small type colony was transferred to hormone 

 agar. The response of the organism led to the formation of a very irregular colony 

 which possessed numerous projections and resembled the appearance of a tick. 

 The structure of the colony appeared partly granular and partly or entirely 

 striated. Stained smears from the striated portions showed mostly a mass of 

 filamentous forms, whereas smears from the granular portion revealed a mixed 

 pleomorphic picture. 



In an attempt to develop a larger type of colony by artificial selection it was 

 found that the majority of the colonies that developed resembled the small type 

 in both colonial and cellular form. This process of colony selection was carried 

 on for a period of two years. At the present writing both variants (IV-a — small 

 type and IV-b — large type) exhibit characteristics which are essentially the same, 

 but appear different from the original isolated variants. While the characteristic 

 small size of the colony has been retained, the peculiar irregular colony shape 

 of the original variant has been lost to a very marked degree. The colony appears 

 almost typical, except for its extremely small size. The cellular morphology has 

 been altered slightly in that the filamentous forms are not as numerous as in the 

 original variant. 



Other properties of these variants will be discussed in the latter part of this 

 section (See Table 4). 



Experiment II. — In this experiment the procedure was identical with that of 

 Experiment I, except that 15 different strains of 5. pullorum were employed. 

 The history of the strains is as follows: — 



Strain No. History 



I) 



II f Isolated from a chick in 1933 



III) 



IV Isolated from a hen in 1932 



(Same as Strain IV in Exp. I) 



v ) 



VI J Isolated from a chick in 1933 



VII Old laboratory strain which fermented 



maltose 

 Villi 

 IX J Isolated from a chick in 1933 



