AGf~ICULTURAL RESEARCH, PUSA.

CUL'lTVA'1'10N Oft' jUIC1W-OIWANIJ..(j'M8 1313
are distributed singly throughout the solid media.
This method is used when the number of viahlc
organisms present in a fluid has to he ascertained,
as in water and milk examinations, and in blood in
cases of septicaemia.
3. By HEATING AND SUBSEQUENT PLATING.­
This method is employed where the organisms to be
obtained in pure culture are more resistant to heat
than the remainder of the bacteria present. This
method applies especially to spore-bearing organisms,
such as the anaerobes (p. 388), the spores of which survive
the heating. The mixture of bacteria is heated
to 80° C. for half-an-hour and then plated. The
spores form individual colonies, which may then be
" picked ofr."
Illate cultures should have the nature of the
material, and also the date, written on the glass of
the Petri dish by means of a grease pencil. Agar
plates are incubated in the illvertcd position-i.e.
the lid of the plate is underneath and the greasepencil
writing should be on the portion of the
dish containing the medium. On the other hand,
gelatin, because it is liquefied by many organisms,
is incubated with the lid uppermost, on which the
necessary pencil notes are made.
Care must be taken in picking off single
colonies, particularly when thcy are very close
to one another, that the point of thc needle cloes
not touch any of the neighbouring colonies. The
plate should first be looked at through the medium
by holding it up to the light. The lid should bc
removed and the dish held round the side by the
thumb and middle finger of the left hand. The
colonies selected should he marked by greasepencil
rings on the hottom of the dish. To pick
off the colony, first sit down with both elbows on
the llCneh. Hold the plate vertically with the IcL't
hand, then grasp the holder Dr the wire like n. pen,
with the fingers quite close to the wire. Steady
the right hand by placing the little finger on the