The flasks were sealed with rubber stoppers supporting
filter paper strips, 2 by 1 cm (on a bent pin or
probe), saturated with a solution of 10% (wt/vol) KOH
and were shaken in a water bath at 37°C at 0.7 Hz.
After 3, 5, and 7 h the filter papers were removed and,
when necessary, replaced with fresh KOH-imnpregnated
filter papers, and the incubation was continued.
The filter papers were dried at 55°C for 0.5 to 1 h, and
radioactivity was counted in vials containing 6 ml of
scintillation fluid with a Packard Tri-Carb liquid scintillation
spectrometer (model 3003). A culture was
considered to be positive for Salmonella when (i) the
ratio of radioactivity measured from flask A to that
from flask B was 21.5:1 and (ii) when the total radioactivity
measured from flask A exceeded 500 cpm.
Routine culture method. The routine culture
method used involved liquid resuscitation where necessary,
and liquid selective enrichment and solid selective
media for the isolation of presumptive Salmonella
and subsequent biochemical and serological confirmation.
With the exception of bismuth sulfite agar
(Difco, Detroit, Mich.), all media were prepared from
Oxoid (London, U.K.) dehydrated products