AGf~ICULTURAL RESEARCH, PUSA.

EXAlIl1NA'l']ON OF' TVA'l'BR 233
_----------- -'---- _--------ice-boxes
for thc purpose can be obtained, and arc
esscntial where specimens have to be transported
some distancc.
Enurneratlan afviable bactcl'ia.-With a sterile pipette
add 1 c.c. water to 10 c.c. nutrient agar (standardised
to a pH ()'u-7'O), melted and coolerl to .15 0 C.; mix
thoroughly, pour in a Petri dish, and allow to
solidify.
If the wuter is suspectcd of contamination, add a
smaller quantity-e.g. 0·1 c.c.-and in dealing with
specimens of uncertain purity it is advisable to make
a series of plate cultures with varying quantities of
the water. The agar should be as transparent as
possible.
Make duplicate plates in this way and incubate one
at 37° C. for two days, the other at 20° C., for three
days. Nutrient gelatin in place of agar is often w.;ed
for detel'Illinillg the muul)l:r of' OJ'ganisllls viable at
:,w0 C.
Count the colonies that develop in the medium. To
aid counting, divide the plate into sections by ruling
Oll the glass with a grease pencil. Count the colonies
in each section (using a hand lens if' necessary to
detect small colonies) and add the numbers. Each
colony may be taken to represent one \'iable hacterium
in the original specimen.
The number of' viable bactcria in the average
municipal water supply-e.g. aftcr filtration-should
not usually exceed 100 per c.c. A count of 1000
per c.c. or more may arousc suspicion regarding thc
purity of' a water supply, hut Oll the whole the results
of such enumeratioll arc of relatively little valu£' by
thcmselves in cstinlUtillg the hygienic purity of a
watcr.
In ullcontaminated water, since the organisms
prcscnt are mainly saprophytes, the llUlnher of eolonics
developing at 20'; C. is usually much greater than
at 37° C., the ratio heing highcr than 10: 1. A
contaminated ,vater may yield a ratio less than this
-c.g. 10: 3.