FILTRATION : SELECTIVE AND INDICATOR MEDIA 355 



B.P.), for a time varying from 5 to 60 minutes according to the thickness of the 

 sputum, and inoculations are then made on to egg medium, the tubercle bacilli 

 will develop in pure culture ; without the antiformin they would be overgrown 

 in 24 hours or less. Fifteen per cent, sulphuric acid is often used for the same 

 purpose. Subcultures should be made at intervals of from 5 to 20 minutes. 



I. Agglutinating Serum.^ — If it is suspected that relatively few organisms of a 

 particular species are present in a bacterial culture or suspension, so that direct 

 plating is unlikely to prove successful, it is sometimes possible to concentrate them 

 by adding a specific high-titre agglutinating serum, incubating for 2 hours, and 

 centrifuging. The organisms, which are clumped together, are easily thrown down, 

 and are present almost exclusively in the deposit. Plates may then be streaked 

 from this directly. This method is sometimes of value in isolating the typhoid 

 bacillus from water. The same principle underlies the method of separating the 

 phases of motile diphasic organisms of the Salmonella group. If, for example, 

 a Craigie tube (see p. 354), containing semi-solid agar to which a small quantity 

 of agglutinating serum active against Phase 1 has been added, is inoculated with 

 the strain under test, the Phase 1 organisms will be agglutinated and rendered 

 non-motile, whereas the Phase 2 organisms will grow down the inner and up the 

 outer tube, and be recoverable from the surface of the agar (see Tulloch 1939). 



J. Filtration. — This method may be used to separate the filtrable viruses from 

 the ordinary bacteria, or if a fairly coarse candle is used, such as a Berkefeld N or 

 V, it may be used to separate very small bacteria like Bad. 'pneumosintes from 

 larger organisms. Many spirochaetes will also pass through Berkefeld filters ; this 

 property is made use of in separating them from the bacteria with which they are 

 often contaminated. 



K. Selective and Enrichment Media. — For the isolation of special organisms 

 from others that are likely to overgrow it in culture, it is common to add to the 

 medium substances having either a stimulating effect on the organism it is desired 

 to cultivate or an inhibitory effect on those it is desired to suppress. If the sub- 

 stance is added to a liquid medium the result is an absolute increase in the numbers 

 of the special organism, which becomes at the same time relatively more numerous 

 to others than in the original material used for the inoculum. Such a liquid 

 medium is known as an enrichment medium. If, on the other hand, the substance 

 is added to a solid medium it acts selectively, enabling a greater proportion of the 

 special organisms to form colonies than would otherwise have been possible. Such 

 a solid medium is known as a selective medium. It is common to use the two 

 types of media in conjunction, inoculating the material first into a liquid enrich- 

 ment medium so as to obtain a relative and absolute increase in the numbers of 

 the special organism, and then plating the culture on to a solid selective medium 

 so as to favour the development of colonies of the special organism at the expense 

 of others. It is important to realize that colonies on a selective medium are not 

 necessarily pure. At the base of the colony other organisms may be present 

 which, though unable to develop on the selective medium itself, will nevertheless 

 grow rapidly when transferred to a non-selective medium. Thus, fermentation 

 results may be misleading if the sugar tubes are inoculated directly from colonies 

 on a selective medium. It is therefore wise to re-plate colonies from a selective 

 medium on to a plain medium to ensure their purity before testing the biochemical, 

 antigenic or other properties of the organism under study. 



