28 A MANUAL OF BACTERIOLOGY 



By the action of micro-organisms, however, on such an 

 inactive mixture the one set of molecules is sought out by 

 the microbes and decomposed, leaving the other set of 

 molecules untouched, and the latter now exhibit their 

 specific action on polarised light, an active sugar being 

 thus obtained. 



Fructose was one of the principal artificial sugars pre- 

 pared by Fischer ; it is inactive, being a mixture of 

 dextro- and Isevo-rotary Isevulose. Lsevo-rotary Isevulose 

 occurs in nature, while dextro-rotary Isevulose, so far as 

 is known, does not. Now, on putting brewer's yeast 

 into a solution of fructose, the inactive artificial product, 

 the yeast organisms attack the Isevo-rotary Isevulose 

 molecules and convert them into alcohol and C0 2 , while 

 the dextro-rotary Isevulose is left untouched. 



Pressure, unless very great, has little effect on bacteria. 

 Roger investigated the effects of high pressure on certain 

 organisms in bouillon cultures. Pressures of 200 to 

 250 kilos, per square centimetre had no effect ; by raising 

 the pressure to 3,000 kilos, per square centimetre one- 

 third of streptococci were killed, and of anthrax without 

 spores a good many ; while sporing anthrax, Micrococcus 

 pyogenes, var. aureus, and the colon bacillus were 

 unaffected. 1 



Our countrymen Downes and Blunt first called atten- 

 tion to the injurious effect of light upon bacteria. If plate 

 cultures be prepared and exposed to sunlight, a portion 

 of the plate being protected from its action, as by sticking 

 on a letter cut out of black paper, and the preparation 

 afterwards incubated, it will be found that the colonies 

 develop at the protected portion only, those parts which 



1 Bacteria being so minute, the actual pressure on a bacterial cell, even 

 with these high pressures, is small. If. for example, a bacterium measures 

 1 /j. by 5 ju, a pressure of ] ,000 kgrm. per square centimetre would be but 

 0-05 grm. (f grain) on the cell. 



