136 A MANUAL OF BACTERIOLOGY 



alcohols, etc. ; the nature of the gas, if any, formed therefrom, 

 and the H : C0 2 ratio. 



6. The behaviour towards oxygen is it aerobic or anaerobic ? 



7. The range of growth at different temperatures. 



8. The reducing power by growing in litmus broth which 

 becomes decolorised, or by the formation of nitrites in a solution 

 containing nitrates. 



9. The production of indole with or without nitrites. 



10. The production of pigment and the conditions under which 

 it occurs. 



11. The pathogenic action on various animals if it be a disease 

 germ, or the changes which it produces if it be an organism con- 

 nected with other conditions. 



12. The chemical changes which it induces. 



13. The thermal death-point and the action of germicides and 

 antiseptics upon it (see Chapter XXII.). 



14. The serological relationships of the organism. 



For descriptive purposes, " standard " culture media 

 should always be employed, and the reaction of the 

 medium stated (p. 66). 



It must never be forgotten that under cultivation the 

 properties of organisms may be considerably modified, and 

 due allowance must be made for this. For example, patho- 

 genic organisms may lose their virulence more or less 

 completely, pigment production be lost, and fermentative 

 action modified (see also p. 6). 



To obviate these difficulties the organisms should be 

 cultivated under as nearly natural conditions as possible 

 and sub -cultivation avoided so far as can be. No general 

 rule can be given as to the duration of life of cultures on 

 artificial media. Most organisms will retain their vitality 

 for at least three or four weeks without being transferred 

 to a fresh soil, some for many months ; a few must be 

 sub -cultured every week, or even every three to four days, 

 or they will die out ; while there are still a small number 

 which have so far rarely or never been cultivated. On the 

 whole, organisms retain their vitality best on gelatin. 



