222 THE DISSEMINATION OF PATHOGENIC ORGANISMS BY FLIES 



From the gut of the fly the intl-ction may be distributed in 

 two ways, either from the mouth or from the anus. In describino- 

 the method of feeding, the habit which flies have of regurgitating 

 the food and depositing the liquid in the form of "vomit spots" 

 was mentioned. Such vomit spots may distribute micro-organisms 

 taken up with the food. Also, during the act of feeding, micro- 

 organisms previousl}^ absorbed when feeding on infected matter 

 may be deposited by the proboscis. Infection from the anus may 

 take place by means of infected faecal matter, so promiscuously 

 deposited as " fly specks." 



A most thorough and valuable series of experiments with 

 a view to ascertaining the possibilities of the aforementioned 

 methods of distributing infection has been conducted by Graham - 

 Smith (1910-13) in connection with the Local Government 

 Board's inquiry. The importance of this aspect of the problem 

 warrants my quoting the results of his experiments somewhat 

 fully. 



The gi-eater possibility of non-spore-bearing bacteria surviving 

 in the gut of the fly than on the exterior and also probable in- 

 fection by the proboscis were shown in his experiments with 

 Bacillus enteritidis (Gaertner). It was shown that B. enteritidis 

 may be present in the contents of the crops and intestines of flies 

 for at least seven days after infection. Flies can infect plates over 

 which they walk for some days (seven days in the experiment) in 

 spite of the fact that the organisms can be seldom isolated from 

 their legs (once in 82 cultures). When walking over the culture 

 plates flies constantly place their proboscides on the medium and 

 in most cases leave imprints on its surface. The colonies of 

 bacteria develop round these marks. As Graham-Smith points 

 out, the infection of the plate is probably due to inoculation by 

 the proboscides of the flies. 



The same investigator in another series of experiments utilised 

 Bacillus prodigiosus, which was chosen on account of its being 

 easily cultivated and identified on plate cultures. Further, it 

 seemed likely that its results would give some information as to 

 the behaviour of other non-spore-bearing organisms. Experiments 

 on the duration of life of B. prodigiosus on the exterior and in 

 the aliuK'ntary tract of flies showed that this organism may remain 



