FLIES AND DISEASE 465 



specific serum obtained with another variety. A positive reaction 

 must also be controlled, as the colon bacillus tends to be agglu- 

 tinated by normal serum. 



" Coliform " Organisms 



Organisms are frequently met with in faeces, manure, sewage and 

 polluted water which resemble the typical B. coli in many of their 

 characters, but which differ from it in certain particulars. Thus 

 the colonies on gelatin, instead of being smooth, may be wrinkled ; 

 milk may be but slowly curdled (three to eight days) ; acid or gas 

 production, or both, in sugars may be less marked than usual. 

 These forms, termed " coliform " organisms, are generally regarded 

 as varieties of the B. coli, and are perhaps derived from typical 

 B. coli. There is, however, little evidence that B. coli can be 

 transformed into such varieties, or that these varieties can be 

 reconverted into typical B. coli, though Revis (loc. cit.) has pro- 

 duced considerable alterations of fermentive power, and in the 

 characters of the colonies of certain coliform organisms. 



A number of other organisms, which have been given distinctive 

 names, are allied to B. coli (consult table of fermentation 

 reactions). 



Flies as Carriers of Infection 



Flies and other " insects " may convey infection (1) by acting as 

 "porters" carrying the virus and infecting food, etc., (2) by 

 direct inoculation in the case of "biting" species, (3) by inocu- 

 lation after a cycle of development in which case the carrier 

 is more or less specific ; e.g. anopheline mosquitoes in malaria. 

 In the first method the organisms are generally bacteria, occa- 

 sionally ova of worms ; in the second, bacteria or protozoa ; 

 in the third, generally animal organisms, e.g. protozoa, filarise, 

 etc. 



The ordinary domestic fly, the blue-bottle and other similar 

 flies (of which there are many) have no biting proboscis, but 

 undoubtedly directly convey infection to food, etc., by carrying 

 organisms upon various parts of their body, or by the organisms 

 being regurgitated from the stomach or passing through the 

 digestive tract and infecting the food with the faeces. In this 

 way, typhoid and paratyphoid fevers, bacillary dysentery, B. 



M.B. 30 



