294 



BACTERIA IN RELATION TO PLANT DISEASES. 



The thermal death-point is 43° C, the lowest yet recorded for any organism infesting 



plants, and until recently the lowest known. 



In an experiment made in June, 1896, an exposure of one hour to 41° C. killed all, i. e., 



no colonies developed on the agar plate poured from the heated bouillon (i i days incubation) 



whereas the check-plate, i. e., the plate inoculated from the tube before heating yielded 



several thousand colonies per square centimeter. 



Exposure in bouillon for one hour at 40° C. killed three-fourths or more, as determined 



by agar poured-plates. Exposure to 40° C. for one-half hour destroyed about half. 



In many ways B. tracheiphilus is a very sensitive organism, 

 \/ and consequently it is difficult to work with. It is hard to plate out 



J[ owing to its viscidity. It is very sensitive to heat, to dry air, and 



to direct sunlight. Freezings are also harmful. It is sensitive also 

 to its own decomposition products, especially acids. It is not a 

 rapid grower nor a very copious one on culture-media and vigorous 

 organisms crowd it out. It does not, however, lose virulence readily 

 by cultivation on artificial media. 



On most media, transfers must be reasonably frequent to keep 

 it alive. It was alive once on sweet potato after 33 days. It was 

 dead on carrot after 33 days. In one instance on steamed Irish 

 potato, parts of a culture were alive after 26 days. In another 

 instance a potato culture (which did not gray) was dead at the end of 

 16 days (see also plant inoculations, pages 275, 283). It has lived, 

 however, in some of the writer's slant agar-cultiures for several 

 months and in litmus milk for 3 months; it may be kept alive for 

 several months in peptonized beef-bouillon with addition of cane- 

 sugar, if calcium carbonate is also added so as to neutralize the acid 

 as fast as it is formed. Often from agar-stab-cultures a few months 

 old it is recoverable, if at all, only by pouring sterile bouillon into the 

 tubes and incubating for a week or more at 25° C. It is often recov- 

 erable from the interior of the plant only by direct streaks on potato 

 or other suitable medium, or by putting the viscid slime into bouillon 

 for 6 to 24 hours before attempting plates. In this case great care 

 must be used to avoid surface intruders and the second or third 

 dilutions, if they cloud, are best for the plates. In experiments 

 made in May- June, 1895, the organism was dead in the upper part 

 of agar-stabs at the end of six weeks at room temperature. The 

 organism was alive in saccharose-peptone-bouillon after 10 days, 

 but not after 24 days (acid was detected prior to the tenth day). 



RESUMl^ OF SALIENT CHARACTERS. 



Fig. 93.^ 



Positive. 



A bacillus in the vascular bundles of cucurbits causing a wilt- 

 disease; short rods (single, paired, in fours end to end, or in small clumps); motile, peri- 

 trichiate; capsules; pseudozoogloeae ; involution-forms; stains readily; smooth; white; 

 viscid; glistening; slow grower on media; surface colonies small, round, discrete; no 

 growth at 37° C. or at 6° C. (16 days); aerobic; facultative anaerobic (with grape-sugar, 

 cane-sugar or fruit-sugar) ; from these sugars a non- volatile acid, soluble in ether ; grows 

 only in open end of F-tube with dextrine or glycerin, acid from glycerine ; slime on steamed 



*FiG. 93. — Method of making a culture of Bacillus tracheiphilus on slant agar in hydrogen. After thoroughly 

 displacing the air, tubes were sealed in an open flame at the constricted parts. These tubeswere about 9 inches long 

 and I inch or more in diameter. In later experiments Novy jars were used. 



