A Study of Coliform Organisms in samples of “Certified Milk.”

Abstract

The experiments herein described show that:(1) Of the 268 coliform organisms isolated from 32 samples of certified milk, 65.7 per cent, fall within the true B. coli group and 18–6 per cent, in groups X and Y of Stewart.(2) 54–1 per cent. of the organisms, i.e. those of the B.faecalis alkaligenes group and those in subgroups 1, 2 and 3 of MacConkey which fermented lactose, gave a negative V. and P. and a positive M.R. reaction, can be assumed to be of faecal origin.(3) 45–9 per cent., namely those organisms of the B. proteus group, those of subgroup 4 of the B. coli group and those of groups X and Y are of the type generally found in soil and water.(4) All of the 32 samples examined contained coliform organisms in a dilution of 1 in 10 as shown by bile salt agar plates. None of these samples, however, gave a positive reaction with the presumptive test in two out of three tubes of 1/10 c.c. dilution. The presumptive test cannot, therefore, be relied upon to give a true indication of the presence of coliform organisms in milk. The inaccuracy of the test may be due in part to the structure of the ordinary Durham's tube which fails to ensure the collection of the gas produced.(5) Although the majority of the coliform organisms present in milk ferment lactose other members of the group which are not lactose fermenters occur and consequently their presence is not demonstrated by the presumptivetest.