TYPHOID FEVER 129 



is improbable that infected food will be found. It is therefore 

 difficult to prove the connection between infected flies and cases 

 of the disease. 



Finally it is by no means an easy task to isolate and identify 

 B. typJios2is from the fly. In the fly's intestine there are very 

 frequently present many non-lactose fermenting bacilli (see 

 Table 21), apparently well adapted to life in this situation, 

 closely resembling B. typhosus. Not only do the colonies which 

 many of them produce on the original plate cultures resemble 

 those of B. typhosus, but some of these bacilli cannot be 

 distinguished with certainty from B. typhosus even when isolated 

 and cultivated in suitable media. Such species can only be 

 distinguished by means of agglutination and absorption tests 

 with known immune sera. The occurrence of these non-lactose 

 fermenting bacilli probably often leads to failure in isolating 

 B. typhosus even when it is present, and in any case renders its 

 detection a long and laborious undertaking. 



Experimental evidence. 



Celli (1888) was apparently the first observer to make 

 experiments on flies with B. typhosus. He fed flies with pure 

 cultures and examined their faeces and contents and came to the 

 conclusion that these bacilli could be found in the excreta. 

 His experiments are of doubtful value owing to the inadequate 

 means of differential diagnosis at that time. 



Firth and Horrocks (1902) carried out experiments in 



"a large box measuring 4 ft. by 3 ft. by 3 ft. One side of it was occupied by a pane 

 of glass and at one end was a large circular aperture closed by means of a muslin 

 funnel. This muslin had an opening through it, which was readily closed either by a 

 tape or by a clamp. Through this muslin-closed opening flies were introduced, and 

 also a bottle containing larva; and chrysalids of flies ; these gradually developing, 

 maintained a steady supply of flies within the box. When a sufiicient number of flies 

 were in the box, a small dish containing a rich emulsion in sugar made from a 

 twenty-four hour agar slope of Bacillus typhosus recently obtained from an enteric stool, 

 and rubbed up with some fine soil, was introduced ; also a small pot containing some 

 honey, the margin of which was smeared with honey and scrapings from a fresh agar 

 slope of Bacillus typhosus. 



" At the same time some sterile litmus agar plates and also some dishes containing 

 sterile broth were exposed and placed at a spot some distance away from the infected 

 soil and infected honey. A sheet of clean paper was also placed in the box. The 



G.-S. 9 



