1. Gastro-enteritis caused by certain varieties of Bact. coli is a highly infectious disease among babies, and it has been suspected that the infectivity is associated, partly at least, with the very large numbers of pathogens excreted in the faeces. Very large numbers were found in the faeces, but large numbers of pathogens were also found in dysentery, salmonella food-poisoning and in some cases of paratyphoid fever.

In cases of gastro-enteritis the dilution of faeces containing 5 to 50 pathogenic Bact. coli per ml. was usually 10−8; in Sonne dysentery and salmonella food-poisoning the corresponding dilution was 10−6. It is doubtful if even this difference can account for the differences in infectivity.

2. When normal Bact. coli outnumber pathogenic Bact. coli, shigellae or salmonellae by as little as 10: 1 it is difficult to identify the pathogens unless a selective culture medium can be used.

3. Ordinary nutrient broth is generally able to act as an enrichment medium for the isolation of pathogens from faeces.