22 A MANUAL OF BACTERIOLOGY 



and too little attention has hitherto been paid to it. This 

 is the co-existence of two or more species which together 

 bring about certain changes. For example, in the well- 

 known ginger-beer plant, Marshall Ward 1 isolated several 

 yeasts, bacteria, and moulds ; of these, one of the yeasts 

 and one of the bacteria together induce the particular 

 changes in a saccharine fluid to which ginger has been 

 added, which render the mixture like ginger-beer, and 

 these changes do not occur unless both species develop 

 together. 



Another extraordinary feature exhibited by bacteria is 

 the selective action exerted on certain substances which 

 contain isomerides or right- and left-handed modifications 

 of a substance. The Bacillus eihaceticus attacks mannitol 

 but not dulcitol, two alcohols which are very similar in 

 taste and properties and possess the same simple chemical 

 formula. 



By a series of most brilliant researches Emil Fischer 

 succeeded in determining the constitution of the various 

 sugars, and, what is more, has produced them artificially 

 in the laboratory. The natural sugars are all compounds 

 with dissymmetric molecules, powerfully affecting the 

 beam of polarised light, but when prepared artificially 

 they are without action on polarised light, because the 

 artificial product consists of equal numbers of left-handed 

 and right-handed molecules, and the molecules of the one 

 neutralise the molecules of the other, thus giving rise to 

 a mixture which does not affect the polarised beam. 



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. 



1 Phil. Trans. Roy. Soc. Lond., vol. clxxxiii, 1892, p. 125. 



