Studies on the Fire-Blight Organism, Bacillus amylovorus 11 



Similar tests were made with Uschinsky's solution, but no growth 

 was obtained below pH 4.70. 



Smith (1920:372) slates thai 15 per cent of the organisms survive 

 exposure to freezing. He dot's not mention the temperature to which 

 the cultures were exposed. Stewart (1913: 337) reports having exposed 

 bouillon and agar cultures to -14° C. for fourteen days, with occasional 

 temperatures as low as -28° C, without killing the organism. Jones 

 and Morse (1903:235) obtained no growth at 0.5° C, and very slow 

 growth at 3° C. 



While working on the production of toxins by B. amylovorus, the 

 writer had occasion to freeze 25 cubic centimeters of sap, expressed 

 from blighted pear fruits, in liquid air (about -183° C.) for ten 

 minutes. After thawing at 3° C, the sap was tested for viable baeteria 

 and was found to contain them in large numbers. This test was not 

 repeated. 



Jones and Morse (1903) gave the upper thermal death point for the 

 bacteria as 43.7° C. (for ten minutes in recently inoculated bouillon). 

 O'Gara (1908) says that all the bacteria are killed on exposure at 55° C. 

 for ten minutes. D. H. Jones (1909) states that the thermal death point 

 in bouillon lies between 45° and 50° C. Stewart (1913 : 335) says: "The 

 death-point is about 47° C." The writer found, in working with sap of 

 blighted pear fruits, that exposure at 48° C. for ten minutes did not 

 always give sterile cultures. However. 4!)° ( '. for ten minutes gave sterile 

 cultures in every case (approximately 60) . This slight difference between 

 bouillon and pear-sap cultures might be due to difference in heat 

 conductivity. 



Prom the foregoing evidence it appears that B. amylovorus is able 

 to live in an acid medium as low as pH 4.60; to resist extremely low 

 temperatures for a short time in liquid media; and occasionally to 

 survive exposure at 48° C. for ten minutes. 



STRAINS OF THE ORGANISM 



It is well known from the work of bacteriologists and others that 

 strains of bacterial species exist. Variations in cultures of the fire- 

 blight organism were noticed by Stewart (1913:327—328), who remarks: 

 "It should be noted that ;i strain from Colorado, No. 7, always made 

 a slower growth than any of the others; while No. 9, recently isolated 

 from apple ;it Ithaca, New York, was usually the most rapid grower." 

 Stewart uses the term strain as indicating different isolations. Jackson 

 (1915) found cultures of Bacillus amylovorus from prune to be more 

 virulent than cultures from pear. In New Zealand, Waters (1921) 

 repeatedly isolated from medlar cultures of />. amylovorus which showed 



