EPIDEMIOLOGICAL AND BIOLOGICAL PROBLEMS 19 



at Eh values between -350 and -400 millivolts. This strongly reducing con- 

 dition is apparently provided by the growth activity of the accompanying 

 bacterial flora. In another communication, the author (1945) pointed out 

 the importance of ligating the rectum in the establishment of infection in 

 kittens of 2 strains of E. histolytica which had lost much of their infectivity 

 through prolonged in vitro cultivation, and felt that the ligation of the 

 rectum favored the development of pathology not so much because of the 

 retention of the inoculum, but rather because of changes induced in the 

 intestine after ligation, particularly changes in the bacterial flora. 



It is beyond the scope of the present paper to discuss in detail the 

 problem of pathogenesis of E. histolytica. There should not be any doubt 

 that E. histolytica is a pathogenic protozoa. However, parasitism in the 

 human intestine by a pathogenic protozoon may not always be accompanied 

 by pathological changes. Haemolytic streptococci are unquestionably patho- 

 genic bacteria; but they are frequently found in the throat of healthy 

 individuals. The same thing has been shown to be true of many other patho- 

 genic bacteria. 



Whatever may be the actual mechanism by which the bacterial flora 

 affects the pathogenic activity of E. histolytica, it is probably a more com- 

 plex phenomenon than the mere damaging of the tissue to favor invasion 

 by the amoebae. The provision of anaerobic conditions by the bacterial 

 flora, in the author's opinion, may not only favor the survival and multipli- 

 cation of amoebae but may also play a leading role in facilitating the tissue 

 invasion by the latter. In one experiment (unpublished data) in which the 

 author fed culture-induced cysts of a strain of amoeba that had lost much 

 of its infectivity for kittens to 6 kittens, 2 animals had the rectum ligated; 

 2 had the rectum ligated and also received 10 ml of a rich culture of 

 Clostridium perfringens in gelatin; and 2 were control. The first 2 animals 

 became sick after the 5th day and were sacrificed on the 9th day. At 

 autopsy, the caecum of each animal was punctured with a platinum elec- 

 trode and a capillary pipette containing saturated KCl in 2% agar, con- 

 nected to a potentiometer. The potentials were measured against a calomel 

 reference cell and the En values of the caecal contents of both animals were 

 found to be -275 and -282 millivolts respectively. Both caeca showed a few 

 patches of superficial ulcers of the mucosa with many trophozoites in the 

 ulcers but outside the basement membrane. The second two animals that 

 had received the Clostridium culture were sick on the 3rd day and dying 

 on the 6th day. They were sacrificed. At autopsy, the Eh values of the 

 caecal contents were recorded as -375 and -392 millivolts respectively, and 

 both caeca showed several typical amoebic ulcers with numerous tropho- 

 zoites in the submucosa. The control animals remained uninfected and 

 were sacrificed 7 days after feeding. The Eh values of their caecal contents 

 were -185 and -198 millivolts respectively and no lesion was noticed at 

 autopsy. Although the number of animals used was small, the unique results 

 provide some basis for explaining the establishment of amoebic infection. 

 It is anticipated that the ingestion of cysts of E. histolytica by some indi- 



