AGf~ICULTURAL RESEARCH, PUSA.

132 PRACTICAL BAC1'BRIOLOGY
'rhe beJleh Oil whidt tile hood is fitted should have a gas
supply for the BUllsen burner. 'rhe hood Illay he lighted by
meallS of' an electric lamp sW5penclcd from thc top.
SEPARATION OF MIXED CULTURES
1. 13y l'LATlNG.---The term" plating" is gcnerally
applicd to the inoculation of medium in Petri dishes,
HSllally by successive strokes or :;prcading. The
platinum loop is charged ·with the bacterinl mixture,
pus, fragmellt of tisi:illc, etc., and !)cvel'al strokes in
Herics arc made on the surface of the medium (either in
a Petri dish or sloped in tllhes) without recharging thc
wire (vide p. 332).
An alternative method for Petri dishes is to employ
:l, spreader. This is made by bending a piece of glass
rod 3 mm. diameter at a right. angle in thc blowpipe
Hame, the short limb, used for spreading, being 1 in.
long. A small amount of the bacterial mixture is
placed on the plate with the inoculating loop or
capillary pipette. By means of t.he spreader, previously
sterilised by boiling and then cooled, the material is
evenly dist.ributed over the surface. 'rhe spreaclel' is
thcn transfcrred to a secolld plate, which is similarly
inoculated. 1'1ms the medium in the sccolHl dish is
inoculated merely with the ol'ganisms carl'ied over hy
the sprcadcr from the first.
Bv these methods t.he bacteria. are gradually wiped
off the wire or spTeader so that they are ultimately
deposited singly. Generally from each bacterium an
isolated colony will grow; a single colony may hc
subcultured on fresh media and so yield a pure
growth. In order to enst.t1'e separation, the s'll1jace oj
the rnecli'l.bm must be dry.
2. By I)LATING DECIMAL DILU'l'IONS OF 'rUE
INOCULUM.-A series of tubes of melted agar or
gelatin are inoculated with successive decimal dilutions
of the infected material and then the medium in each
tube is pOUTed into a Petri dish and allowed to
solidify. By dilution, the bacteria are separated from
one another, and on incubation the resulting colonies