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 
long time, 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: 
