Distribution of coliform organisms in milk and the accuracy of the presumptive coliform test

Abstract

1. The presumptive coliform test was carried out by each of three workers, at each of four dilutions, on seventeen sets of five tubes for each of seven samples of 18-hour-old afternoon milk, held overnight in the ice chest. 2. With the exception of three slightly anomalous results, the data are consistent with the hypothesis that the chance of a tube remaining sterile is constant for all tubes inoculated from the same sample of milk at any one dilution. 3. The assumption of a Poisson distribution of organisms in parallel tubes is accurate enough, with these data, to give a good idea of the order of magnitude of the number of bacteria per ml. 4. There is, however, a tendency, more marked in our data in autumn than in spring, for the estimated count to rise at the higher dilutions. This may plausibly be attributed to the breaking up of clumps.