Possible explanations for the inhibitory action of boric acid lactose broth at 43 C on fecal coliform bacteria.

It has been shown (Njoku-Obi and Skinner, 1957) that the most probable number of coliforms and of Escherichia species in human feces is consistently higher when counts are made by dilution extinction methods in standard lactose broth at 35 C than when boric acid lactose broth (Vaughn et al., 1951) at 43 C is used. This is an example of a general phenomenon repeatedly noted in the literature. Another example is the work of Skinner and Brown (1934). When the most probable number of coliforms from fresh human feces was determined by the dilution extinction method, the same sample usually yielded a much higher count of coliforms and of Escherichia species with lactose broth incubated at 37 C than with glucose broth at 46 C according to the original Eijkman method. There have been various theories advanced to explain such phenomena but little or no data which would tenid to substantiate the theories. Actually, in most cases, they are merely postulations or assumptions with no proof whatever. The present paper will attempt to subject some of these postulations to proof with the idea of narrowing down the possibilities to one or two, which can then be investigated themselves.