The ratio of radioactivity from flask A to that
from flask B was 5.8:1 (Fig. lb). A ratio of this
magnitude was only obtained if pure cultures of
Salmonella were used for the inoculum or if
there were no other organisms which fermented
dulcitol in a mixed-culture inoculum. In the enrichment
cultures from most of the food samples
tested, some bacteria other than Salmonella
were present which fermented dulcitol. Two examples
of this are shown in Fig lc. One meat
sample (1A, IB) was found to be contaminated
with S. typhimurium and the other meat sample
(2A, 2B) was negative for Salmonella. S. newport
cells were added with the homogenate of
the latter sample to give 106 cells per flask (3A,
3B). The ratios of 1A to 1B and 3A to 3B were
1.5:1 and 1.8:1, respectively, and complied with
the chosen ratio of >1.5:1 for a sample to be
positive for Salmonella.