8o THE AMOEBAE LIVING IN MAN 



Food. — E. colt, as already noted, is a voracious and omnivorous 

 feeder. In its food vacuoles may be found bacteria of all sorts, and 

 every kind of vegetable debris from the contents of its host's colon 

 (cf. fig. 17, PI. II). Individuals may often be found containing starch 

 grains and other plant remains. They also sometimes ingest the cysts 

 of other protozoa such as lamblia (Giciidia intestiiiaiis},' coccidia 

 {Isospora liotninis),'\ and E. histolytica :+ and they may even eat unen- 

 cysted protozoa, according to some observers. Grassi, and Casagrandi 

 and Barbagallo, found free trichomonads and Giardia within them, 

 and O'Connor (19 19) has confirmed the latter observation. In my 

 cases I have seen only the cysts of this flagellate inside £. coli. § 



Red blood corpuscles and tissue cells appear to be about the only 

 things which E. coli will not eat. It is true that various observers — for 

 example, Craig (191 1) — note the occasional presence of such elements 

 in the vacuoles of this species ; but nobody has yet adduced sufficient 

 evidence to place this observation beyond doubt. It is highly probable, 

 indeed, that the individuals with ingested red corpuscles were really 

 £. histolytica and not E. coli, the cases studied having been infected with 

 both species. Mixed infections are extremely common ; but £. coli 

 containing red corpuscles — if they really do occur — are so uncommon 

 that I have never encountered them. My observations agree in this 

 respect entirely with those of Wenyon and O'Connor (19 17). These 

 workers also attempted to cause E. coli to ingest red corpuscles i>i vitro, 

 but with negative results. There can be no doubt that it is sufficiently 

 correct for all practical purposes to state that E. coli does not ingest red 

 corpuscles ; and that if an intestinal amoeba|i is found containing these, 

 then it belongs almost certainly not to this species, but, with the greatest 

 probability, to E. histolytica. This fact is of the greatest service in 

 diagnosis. It is also of interest biologically, for it marks the profound 

 difference in food-habits between the two species — a difference clearly 

 correlated with the great difference in their mode of life and patho- 

 genicity. 



It is now unnecessary to insist upon the fact that E. coli is a harmless 

 commensal, possessed of no pathogenic powers whatever — so far as 

 evidence is available: for only the inexperienced or prejudiced hold 

 a contrary opinion. The early views of Cunningham, Grassi and Caian- 

 druccio, Casagrandi and Barbagallo, and many others, have, in this 

 respect, been completely vindicated. It may be added that some of the 

 Italian workers — especially Casagrandi and Barbagallo (1897) — have even 

 gone so far as to maintain that E. coli is beneficial to its host ; for it 

 removes and destroys bacteria and waste matters of all sorts, and so 

 acts as a useful scavenger of the large intestine. 



Habitat. — All the evidence available goes to show that E. coh lives in 

 the large intestine. The active forms live and multiply in the soft or 



* Vide Grassi (1888 a), Casagrandi and Barbagallo (1897), O'Connor (1919). 



t r/V/<f O'Connor (1919). 



X Vide Wenyon and O'Connor (1917). 



§ Capt. O'Connor has, however, shown me stained preparations of E. coli containing 

 unencysted lamblia. 



II The body in question must, of course, be really an amoeba. Endothelial cells in 

 human stools — especially in cases of bacillary dysentery — are often mistaken by the 

 inexperienced for E. histolytica when they happen to contain red corpuscles. 



