44 METHODS OF CULTIVATION OF BACTERIA 



haze. The haze in the case of b. coli is due to the ready production of 

 acid from the lactose causing a precipitate of the taurocholate. Any other 

 organism capable of producing acid from lactose will give a similar 

 reaction, and the haze can be readily cleared up by floating a drop of 

 ammonia on the surface of the medium. MacConkey also used with a 

 similar object a 5 per cent glucose bile-salt bouillon tinted with neutral 

 litmus as in Drigalski's medium. 



With reference to MacConkey's fluid media, organisms are divided into 

 (1) those which produce both acid and gas ; (2) those producing acid only ; 

 (3) those growing but not producing either acid or gas ; (4) those 

 incapable of growing. B. coli belongs to the first group and b. typhosus 

 to the second, and to these groups also belong most ordinary organisms 

 growing in faeces, practically none of which are found in the third and 

 fourth classes. Thus if any growth takes place on this medium when 

 inoculated with, say, water, the probability is that the bacteria have been 

 derived from faeces, but of course their identification might present some 

 difficulty. With the neutral-red solid media the colonies of any organism 

 giving rise to acid will be of a beautiful rose red colour. 



Petruschky's Litmus Whey. The preparation of this medium, which 

 is somewhat difficult, is as follows : Fresh milk is slightly warmed, 

 and sufficient very dilute hydrochloric acid is added to cause precipita- 

 tion of the casein, which is now filtered off. Dilute sodium hydrate 

 solution is added up to, but not beyond, the point of neutralisation, and 

 the fluid steamed for one to two hours, by which procedure any casein 

 which has been converted into acid albumin by the hydrochloric acid 

 is precipitated. This is filtered off, and a clear, colourless, perfectly 

 neutral fluid should result. Its chief constituent, of course, will be 

 lactose. To this sufficient Kubel-Tiemann solution of litmus is added, 

 the medium is put into tubes and then sterilised. After growth has 

 taken place, the amount of acid formed can be estimated by dropping 

 in standardised soda solution till the tint of an uninoculated tube is 

 reached. 



Media for growing Trichophyta, Moulds, etc. 1. Beer Wort Agar. 

 Take beer wort as obtainable from the brewery and dilute it till it has an 

 s.g. of 1100. Add 1'5 per cent of powdered agar and heat in the Koch 

 till it is dissolved (usually about two hours are necessary). Filter 

 rapidly and fill into tubes. Sterilise in the Koch for twenty minutes on 

 three successive days. If the medium is heated too long it loses the 

 capacity of solidifying. 



2. Sabouraud's medium (modified). Take 40 grammes maltose and 

 10 grammes Witte's peptone and dissolve these in one litre of water, then 

 add 13 grammes of powdered agar. Heat in the Koch till the agar is 

 dissolved, filter and fill into tubes, sterilise in the autoclave for twenty 

 minutes at 120 C. 



To use these for isolating, say, the Tinea tonsurans, pick out an 

 infected hair, wash in absolute alcohol for a few seconds, then wash in 

 changes of sterile water and stab the hair into the surface of the medium 

 in a number of places ; incubate at 24 C. Usually it is sufficient to 

 stab the hair as it is picked from the skin into the medium. 



Potatoes as Culture Material. 



(a) In Potato Jars. The jar consists of a round, shallow, 

 glass vessel with a similar cover (vide Fig. 9). It is washed 



