12 HISTORICAL OUTLINE 



problem of soil fertility, without arriving at any very helpful conclusions. One 

 curious phenomenon was the reaccumulation of nitrates in the soil, in spite of 

 their constant removal by the washing action of the rain. It was suspected that 

 these nitrates might be derived in some way from the decomposition of organic 

 material, and in 1877 Schloesing and Miintz, acting on a suggestion made by 

 Pasteur in 1862, showed by experiment that the formation of nitrates was due to 

 the action of living organisms. Warington, at Rothamsted, confirmed these 

 results in 1878 and 1879, and showed that two stages were involved, a preliminary 

 conversion of ammonia to nitrites, and a subsequent oxidation of nitrites to nitrates. 

 He believed that these two stages were carried out by different organisms, but 

 failed to isolate or identify them. This problem was solved by Winogradsky in 

 1890, who isolated and described both the nitrite- and nitrate-forming organisms. 

 In 1888 Hellriegel and Willfarth described the nitrogen-fixing bacteria which caused 

 the formation of nodules on the roots of leguminous plants. Later Winogradsky 

 described a free-living anaerobic organism which was able to fix atmospheric 

 nitrogen, and Beijerinck, some ten years later, described a large, free-living, 

 nitrogen-fixing aerobic bacterium, which he named Azotobacter, and which has 

 since been extensively studied. The bacteriology of the soil thus became an 

 important part of agricultural science. 



In the early years of the bacteriological revolution it had been demonstrated 

 that bacteria attacked plants, as well as animals. In 1878 Burrill described the 

 organism of pear blight, and in 1883 Wakker discovered the bacillus which causes 

 the " yellows " of the hyacinth. This branch of bacteriology has been pursued 

 energetically during the last thirty years, especially by Erwin Smith and his 

 colleagues in America. 



The demonstration by Pasteur of the essential nature of fermentation led, as 

 a natural consequence, to the entry of the bacteriologist into the commercial 

 sphere. His help was required in dairy farming, in brewing, in the preservation 

 of foods, and in all those commercial processes in which bacterial activity was 

 desired or feared. 



Fig. 3, which sets out in diagrammatic form the time relations of the more 

 important discoveries associated with the work of Pasteur, Koch and Lister, may 

 be of some service in enabling the student to follow the development of our know- 

 ledge down to the end of the nineteenth century. 



This brief summary will indicate with sufficient clearness to how great an extent 

 the bacteriologist has been occupied with applied problems. He has, by way of 

 description, usually been satisfied if he could determine, for any given bacterium, 

 a number of characters sufficient to differentiate it from the organisms with which 

 he considered that it was most likely to be confused. It is in this way that our 

 knowledge of bacterial characters has slowly grown and it is not surprising that 

 the results should be an arbitrary and rather odd assortment of differential criteria. 

 In such a bacterial group as that comprising the colon and typhoid bacilli, 

 and certain nearly related organisms, it has been demonstrated that fermentation 

 reactions form a reliable method of differentiation, and such reactions have been 

 extensively studied. In another group morphological differences may be more 

 distinctive, or the production of specific toxins may be a well-marked feature in 

 certain species. The soil-bacteriologist employs methods which differ in important 

 respects from those used by other workers. It is the inevitable result that 

 systematic bacteriology has been very generally neglected, and it is only in recent 



