194 BACTERIA IN RELATION TO PLANT DISEASES. 



The absolute sterilization of the surface to prevent the plant from contracting disease, 

 to prevent infection of the soil by suspected seeds, cuttings, etc., or finally to prevent the 

 infection of menstrua and the consequent miscarriage of special pathological, physiological, 

 or chemical experiments is sometimes a matter of considerable importance. Unfortunately 

 our knowledge is still very incomplete. Often no specific advice can be given, preliminary 

 experiments being necessary in particular cases. The best that can be done here is to make 

 a few suggestions and record the results obtained in particular instances. The reader is also 

 referred to the special cases mentioned under particular diseases. 



For holding solutions sterile for short periods without changing their composition the 

 chemists have used chloroform, thymol, toluene and similar substances. To get the full 

 restraining effect of these substances the material to which they have been added should 

 be shaken continuously and shut away from the air; otherwise bacterial growths are likely 

 to appear (see Vol I, p. 74). Glycerin is not germicidal to Bacillus subtilis see p. 149. Con- 

 tinuous exposure at i°C. or at 80° C. will often answer when the nature of the reaction 

 desired, or of the substance to be isolated, will permit it. The growth of aerobes can be 

 held in check by substituting for air some inert gas such as hydrogen or nitrogen. Surfaces 

 can not be washed free from bacteria. 



Inasmuch as pathogenic bacteria are often transmitted from diseased plants to healthy 

 plants by insects, the wholesale destruction of the latter often becomes imperative, and some 

 comments are, therefore, added on the most efficient insecticides. 



In 1908, H. Chick and C. J. Martin published a paper on the standardization of dis- 

 infectants in which they record the following results. 



It is generally recognized that in any method of standardization the temperature, and the com- 

 position of the culture medium should be constant, while the number of bacteria per unit volume and 

 the resistance of the test organisms used should be as constant as possible. The authors adopted 

 20° C. as the temperature most closely approximating the conditions of practical disinfection. Thirty 

 minutes was found the most satisfactory time unit of exposure, as a shorter time was unfavorable 

 to mercury and silver salts, while for phenol and emulsified disinfectants either 10 minutes or 30 

 minutes was satisfactory. 



It was found best to employ a sulphide to neutralize the traces of disinfectant carried over with 

 the test sample. Mercuric chloride required an excess of sulphide to decompose a compound formed 

 between the mercuric salt and the substance of the bacteria which prevented growth. The most 

 satisfactory results were obtained with 0.2 cc. of saturated solution of hydrogen sulphide in distilled 

 water to 10 cc. of broth. 



The efficiency of a disinfectant was found to vary with the organism used. In the case of spores, 

 metallic salts were the most effective germicides. These were effective in very small concentration 

 (1000 times less than phenol). With phenol, sporing forms were from 17 to 25 times more resistant 

 than vegetative forms. Virulent strains were generally more resistant than non-virulent ones. 



Since in practice disinfectants are commonly used in the presence of organic matter it seemed 

 desirable to introduce this factor into the process of standardization. Experiments with this in view 

 showed that 10 per cent blood serum reduces the efficiency of phenol about 12 per cent. A somewhat 

 greater reduction occurs with emulsified disinfectants and a much greater with mercuric chloride. 

 A solution containing 0.5 per cent of the latter was reduced from 0.6 to 0.06 of its original value 

 as the concentration of serum was increased from 5 to 30 per cent. The presence of particulate 

 organic matter (dust, animal charcoal, finely pulverized coagulated albumen, bacteria and faeces) 

 affects the germicidal value of emulsified disinfectants far more than that of phenol. Commercial 

 cresols were reduced in efficiency 30 to 50 per cent by the introduction of such matter. Finer emul- 

 sions are more seriously reduced than coarse ones. This reduction was shown to be principally due 

 to adsorption of the emulsion upon the surfaces of the particles. 



In 1908, Dr. Rideal discussed the question of uniform methods of testing disinfectants. 

 He states what every one knows that it is very difficult and in many cases impossible to 

 compare the results of different workers because no definite standards have been adopted. 

 He cites tests of Koch, Esmarch, Fraenkel, Geppert, Klein, Cash, Wynter Blyth, and 

 Sternberg as examples. 



