MICROBIAL DISEASES OF INSECTS 913 



The intestinal content of locusts attacked with this disease shows 

 microscopically practically a pure culture of this bacillus. The intes- 

 tinal contents of healthy locusts are poor in bacteria, sometimes seem- 

 ingly aseptic. Among the saprophytes found, the most common is a 

 motile, Gram-positive coccobacillus which causes death of locusts by 

 injection but never by ingestion. It is distinguishable from the specific 

 coccobacillus by the disagreeable odor which it produces in the locusts 

 or in culture media. Sometimes staphylococci are found, rarely B. 

 suUilis; only one saprophyte per hundred of the specific bacillus ren- 

 ders the isolation of the latter very simple. 



All of the tissues of the locust are invaded by this bacillus as has 

 been proved microscopically. A pure culture can be obtained from the 

 blood at the same time that the intestinal contents are attacked, thus 

 B. acridiorum produces a veritable septicemia. 



CAUSAL ORGANISM. B. acridiorum, the causal organism of the Mexican locust 

 epizootic is a short, slightly ovoid bacillus, decidedly polymorphous; in the same 

 culture coccus forms of about 0.6^1 are found beside of forms plainly bacilli, 0.4^ to 

 0.6/x by o.g/i to i-5/x; actively motile possessing peritrichic flagella; Gram-negative 

 but stains readily with anilin dyes; the bacillus takes the stain most deeply at the 

 poles, especially if Ziehl's carbol-fuchsin is used for one to two seconds. 



Facultative anaerobe; cultures grow readily from 16 to 43 in all ordinary media, 

 even in Raulin's medium. It develops very rapidly at 37 in broth, turbidity appear- 

 ing at about the fourth hour. A delicate membrane is formed on broth which clears 

 only after three weeks, leaving a heavy sediment. Young agar colonies are cir- 

 cular and have a waxy appearance; they grow rapidly, being plainly visible after 

 twelve hours; after eighteen hours, they are 2-3 mm. in diameter. Subsurface 

 colonies are small, spherical, whitish and opaque. Gelatin is not liquefied. 

 Milk is coagulated and rendered strongly alkaline. Grows abundantly on potato 

 having a creamy appearance; the culture in the water at the bottom of the tube is 

 so dense that the liquid becomes sirupy and has a strong alkaline reaction. 

 Dextrose, levulose, maltose and galactose are fermented; the inoculated medium 

 containing one of these sugars becomes acid at first, then alkaline. This alkalinity 

 is due to the formation of ammonia. B. acridiorum has lived over two years in 

 sealed tubes. 



METHODS OF INFECTION. Natural.- -There are several natural 

 methods by which the epizootic is spread. Sick locusts or nymphs 

 leave their infectious liquid dejecta on the vegetation, the other locusts 

 eat the contaminated herbage, contract the disease and in turn infect 

 new plants thus continuing the cycle. With certain species of locusts, 

 the Schistocerca for example, another very important mode of contagion 



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