30 PHARMACEUTICAL BACTERIOLOGY. 



Investigators have succeeded in prolonging the life cycle of Paramecium. 

 Normally P. caudatum dies out in about 175 generations; but by applying 

 alcohol (1-5000 to 1-10,000) the cycle has been increased to 860 generations. 

 Very dilute solutions of strychnine gave similar results. If the life cycle or 

 vital impulse of these simple organisms can be prolonged it is probable that 

 similar effects can be produced in higher organisms. Numerous investi- 

 gators have from time to time sought after agents which might inhibit the 

 senile changes in cells and circulatory system (arteriosclerosis) but thus 

 far without conclusive results. It is, however, highly probable that within a 

 comparatively short time means may be fgund to prolong the life of the 

 higher animals from 10 to 20 per cent, and even more. 



Microbes feed upon organic substances generally. Those which feed 

 upon dead organic substances are said to be saprophytic; those feeding upon 

 living substances are said to be parasitic. If they can live on dead organic 

 substances only, they are obligatively saprophytic; if they can feed on both 

 dead and living organic substances, they are facultatively saprophytic, or, 

 vice versa, facultatively parasitic. The great majority of microbic parasites 

 are facultatively so, as is evidenced by the fact that they can be grown in 

 artificial culture media. Many of the microbic saprophytes will develop on 

 living substances under certain conditions, thus showing that they are facul- 

 tatively parasitic. It is no doubt true that no known microbic parasite 

 actually feeds upon the living substances of the various hosts, since the cyto- 

 plasm is in all instances dead before it is taken up and assimilated by the 

 microbe. It would therefore be more correct to say that parasitic microbes 

 are biologically associated with living organisms, while the saprophytes are 

 biologically associated with dead organic substances, and that they all 

 feed upon and assimilate dead organic substances. In certain mutualistic 

 symbioses (as in the root nodules of the Leguminosae) the biological rela- 

 tionship of microbe and host plant is very intimate, but there is no actual 

 interchange of living material. 



All microbes require moisture and warmth (comparatively speaking) 

 for their development, although they are enabled to withstand greater ex- 

 tremes of heat and cold than other organisms. The temperature of liquid 

 air (about — 270° F.) does not kill them at once, and the spores may be boiled 

 for some time without destroying their germinating power. Cold (freezing 

 temperature) promptly checks growth and septation, and so does dryness and 

 excessive warmth, although life may not be destroyed. The majority of 

 microbes develop most actively at a temperature of 25° C, a few species 

 develop more actively at a lower temperature (20° C), and a few others at a 

 higher temperature (38° C). Those which develop at a temperature rang- 

 ing from 0° C. to 30° C. are said to be cold loving (psychrophile), from 10° to 

 45° C, mesophile, from 40° to 70° C, thermophile. Thermophile species are 



