DISCOVERY 



155 



organisms were common in other soils, and, secondly, 

 which of the various bacteria isolated were of chief 

 importance in causing these antiseptics to decompose 

 in the soil itself. To settle this first question, about 

 150 soil samples were collected, with precautions to 

 exclude outside contamination, from a wide area in 

 Great Britain, and the phenol, cresol, and naphthalene 

 bacteria were isolated from them. It was found that, 

 although some 200 strains of antiseptic-decomposing 

 bacteria were obtained from over this wide area, thcj- 

 fell into four main grovips, three of which had already 

 been found in Rothamsted soil. 

 These groups were : 



A. A number of Pseudomonads similar to the 

 common organism Ps. fluorescens-liquefaciens. 



B. A faw varieties of small rod-like bacteria. 



C. A number of large rod-like forms that produced 

 spores. 



D. A group perhaps related to the tubercle bacillus. 



These latter organisms undergo remarkable changes in 

 appearance under cultural conditions. (See Fig. 2.) 



Geographical Range of Bacteria 



The firit three groups appear to be widely distri- 

 buted over Great Britain, but the last group has an 

 ■nteresting and so far unaccountable geographical 

 range, since it is apparently not evenly distributed 

 over the area searched, being abundant in the south- 

 east and middle of England and in the Edinburgh and 

 Glasgow area, but very rare in the North of England 

 and in the rest of Scotland. 



Phenol and naphthalene organisms are also known 

 to occur in soils from other countries, having been 

 found in soils from Norway, the Tyrol, and Gough 

 Island in the South Atlantic. As it appears, therefore, 

 that they are of world-wide occurrence, there is reason 

 to suppose that the results of the study of phenol, 

 cresol, and naphthalene decomposition at Rothamsted 

 will be applicable to other parts of the world. 



The second question — namely, as to which group of 

 organisms ii chiefly effective in decomposing these 

 antiseptics in the soU itself — cannot be considered as 

 finally settled. If a dose of phenol be added to manured 

 soil, it produces at first a slight fall in the numbers of 

 bacteria followed by a very rapid increase, the numbers 

 sometimes reaching astonishing proportions. This 

 quick rise does not take place, nor does the phenol 

 disappear so rapidly, in soil from which the phenol 

 bacteria are absent. The bacteria which are involved 

 in this great multiplication are all of the Pseudomonad 

 type of phenol-destroying organisms. There is thus 

 reason to suppose that this is the type of organism 

 which is of chief importance in the soil. On the other 



hand, the increase in the organism does not begin until 

 about half the added phenol has disappeared, as can be 

 seen from the chart (Fig. 3). There is still the possi- 

 bility, then, that another group of micro-organisms 

 that are not revealed by the counting technique at 

 present employed may be attacking the phenol. The 

 work on these bacteria is not, however, as yet com- 

 pleted, so that a full account of it cannot be given, 

 but will appear elsewhere in due course. 



The addition to the soil of such organic antiseptics as 

 these we are considering produces an important effect 

 beside that of destroying plant pests. In normal soil 

 not treated with antiseptics, the bacteria which produce 

 available plant food are in a state of equilibrium 

 with the population of soil protozoa. It has been 



to 



i^ i* ^ IS u 



JJnrs. 

 FIG. 3.— INCREASE IN NUMBER OF BACTERIA IN SOU, TREATED 

 WITH PHENOL. 



f jund by the Protozoology Department at Rothamsted 

 that, in a field soil, increases in the numbers of actively 

 feeding amoeba; are accompanied by decreases in 

 bacterial numbers, and vice versa. Now, the addition 

 of a mild antiseptic to the soil disturbs this equili- 

 brium between Protozoa and Bacteria, enabling the 

 latter to increase. Thus Russell and Hutchinson 

 found in 1909 that the addition of toluene to soil 

 caused an increase in bacterial numbers and a corre- 

 sponding increase in ammonia and nitrate available 

 to the plant as food. It is thus possible to increase 

 the fertility of the soil by the addition thereto of a 

 mild antiseptic. 



The study of the effect of adding these organic 

 antiseptic compounds to the soil is therefore of import- 

 ance, not only with a view to their use for destroying 

 soil pests, but also because it promises to throw light. 



