BIOGRAPHICAL MEMORANDA 



considered to be the potential characteristic of alcoholic fermentation. 



In similar experiments with a large variety of homofermentative 

 lactic acid bacteria a redox potential of — 1 60 mV was found in 

 every case, which thus appeared to define the specific potential of a 

 lactic acid fermentation. Here, too, some anomalies had to be faced, 

 such as the fact that some Streptococcus species, although typical lactic 

 acid bacteria in all respects, cannot reduce methylene blue, whose 

 normal potential is surely high enough to cause its complete reduction 

 in an environment with a redox potential as low as —160 mV. This 

 contention was obviously supported by the rapid and complete re- 

 duction of methylene blue in cultures of other lactic acid bacteria. 

 Furthermore, some species differ markedly from others in their ability 

 to reduce litmus and indigocarmin, a finding that seemed incompat- 

 ible with the establishment of identical potentials in cultures of all the 

 lactic acid bacteria containing the 'universal indicator mixture'. 



The experience gained in the studies on the redox potentials of 

 micro-organisms carrying out an alcoholic fermentation led, how- 

 ever, to a ready explanation of these anomalies. First of all, it was 

 found that the electrode potentials of cultures of various lactic acid 

 bacteria not containing the indicator mixture were by no means iden- 

 tical, and might even differ by as much as 200-300 mV. This sug- 

 gested that some species do, in fact, excrete redox systems with much 

 lower potentials than others. Secondly, this differential behaviour also 

 made it obvious that the latter types cannot reduce indigocarmin; 

 this dye does not ordinarily penetrate into living cells, so that its re- 

 duction can be accomplished only under the influence of excreted 

 redox systems at a sufficiently low potential. And thirdly, the failure 

 of some species to reduce methylene blue could be attributed to the 

 fact that this substance is quite toxic to these bacteria; they cannot 

 grow and metabolize in media with the usually employed concen- 

 tration of this dye, which accounts for the fact that no reduction can 

 be observed. 



Initially these studies on the redox potentials in suspensions of 

 metabolizing micro-organisms may well have appeared to hold out 

 promise for the subsequent development of a quantitative theory of 

 biocatalysis, and they were extended to a number of other metabolic 

 processes. Thus Kingma Boltjes [1935], while studying the nitrifying 

 bacteria in Kluyver's laboratory, determined the potentials in cul- 



