128 METHODS OF PURE CULTURE. 



84. Liquefiable Solid Media. 



If a species is represented in a bacterial mixture by a few 

 individuals only, its isolation by the dilution method requires 

 an inconveniently large number of culture vessels. In order to- 

 overcome this difficulty (with others that need not now be touched 

 upon), Robert Koch, utilising a method practised by Schroeter, 

 devised a new method of separation, generally termed plate- 

 culture. The essential part of the method consists in the addition 

 of a gelatinising substance to the nutrient solution, whereby the 

 latter acquires the property of becoming liquid at a moderate 

 warmth but is solid at room-temperature. The medium thus 

 liquefied is inoculated with a little of the bacterial mixture to 

 be separated, and, after being well shaken up, is poured, whilst 

 still fluid, on to sterilised glass plates, on which it sets as a thin 

 film. In this film (under favourable conditions) each one of the 

 cells inoculated therein is held fast and isolated from the others,, 

 and can subsequently multiply, undisturbed, into an aggregation 

 of similar cells known as a colony. 



Gelatin is the substance most frequently used for this purpose, 

 and nutrient media containing it are called by the generic name of 

 nutrient gelatin, a distinction being drawn between wort-gelatin, 

 meat-juice-gelatin, must-gelatin, &c., according to the kind of nut- 

 rient solution used. The amount of gelatin added is about 9 or 10- 

 per cent., and this produces a medium that is liquid above 30 C. 

 and solid below 24 C., so that inoculation can be conveniently 

 performed at 35 C., a temperature exerting no injurious influence 

 on organisms. Bouillon gelatin, often called peptonised bouillon 

 gelatin, is prepared by making up the meat extract prepared as 

 already described to its former volume, i.e. i litre, with distilled 

 water, after boiling, filtering, and mixing it in a glass flask with 

 i per cent, of peptone, 0.5 per cent, of NaCl, and 10 per cent, 

 of gelatin. The flask is carefully warmed in the water-bath or 

 steamer until the gelatin liquefies, and the liquid is then neutralised 

 in the manner prescribed for bouillon. It is next boiled in the 

 steamer for half-an-hour and filtered hot through a moist folded 

 filter, to remove the precipitated albuminoid matters and those 

 thrown down in neutralising. Samples that clarify badly are 

 improved by egg-albumin, since the filtrate has to be perfectly 

 clear and transparent. The liquid is filled, whilst warm and 

 fluid, into vessels for use (e.g. test-tubes holding 5-8 c.c.), and 

 sterilised by intermittent heat, being left for twenty to thirty 

 minutes in the steamer on three consecutive days, as explained 

 in the preceding chapter. In bacteriological treatises frequent 

 mention is made of " nutrient gelatin " pure and simple without 

 any qualifying term ; in such cases the peptonised bouillon gelatin 

 referred to above is always meant. The tubes spoken of in the 

 colloquial language of the bacteriological laboratory as " gelatin 



