INFLUENCE OF NUTRITION AND MOISTURE. 139 



soil bacteria none by itself may be pathogenic, but 

 when inoculated into animals in certain combinations 

 produce disease. 



4. Slightly pathogenic species, such as attenuated 

 tetanus bacilli, for example, gain in virulence when 

 cultivated together with the proteus vulgaris. 



Want of Nutrition and Water. When bacteria which 

 require much organic food for their development, and 

 these include most of the pathogenic species, are placed 

 in distilled water they soon die that is, within a few 

 days; even in sterilized well-water their life-duration 

 does not usually exceed eight to fourteen days, and they 

 rarely multiply. Instances, however, of much more 

 extended life under certain conditions are recorded. 

 Want of water affects bacteria in different ways. 

 Upon dried culture media development soon ceases; but 

 in media dried gradually at the room-temperature 

 (nutrient agar, gelatin, potato) they live often for a 

 longtime, even when there are no spores to account for 

 it. A shrunken residue of such cultures in bouillon has 

 often been found, after a year or more, to yield living 

 bacteria. The question as to how long the non-spore 

 bearing forms are capable of retaining their vitality 

 when dried on a cover glass or silk threads has been 

 variously answered. We know now that there are 

 many factors which influence the retention of vitality. 

 The following table of the results obtained by Sirena 

 and Alessi gives some idea of the extent and effect of 

 such influences. In the experiments silk threads were 

 saturated with bouillon cultures or aqueous suspensions 

 of the bacteria, and some then enclosed in tubes con- 

 taining sulphuric acid or calcium chloride, while others 

 were left exposed to various outside influences: 



