EFFECTS OF RADIATION ON BACTERIA 1139 



apparatus needed in obtaining the wave-lengths of radiation required 

 should be the best procurable for the purpose, as should the installation 

 for energy measurements. The exposure vessels or culture procedure 

 used may vary widely, depending upon the nature of the problem. The 

 agar-plate technique, whereby a suspension of bacteria of known con- 

 centration is washed over the hardened agar surface and then selected 

 areas of this surface exposed to radiation, commends itself on account of 

 simplicity. Probably this is reasonably accurate, but many have found 

 difficulty in obtaining an equal distribution of the bacteria, and this 

 difficulty is increased when the organism is produced in chains or adheres 

 in groups. The accuracy of colony counts depends upon the concentra- 

 tion, and there are narrow time limits within which the work may be 

 satisfactorily carried out. The agar-surface technique is not applicable 

 when the period of exposure is greater than the division time of the cells, 

 and it is unsuitable for work with viruses and enzymes. 



The suspension method (the bacteria being suspended in physiological 

 salt solution) may be used with a high degree of accuracy but not without 

 careful study of each organism used. It permits a wide variation in the 

 concentration of organisms, since plating out from suspensions may be 

 made after any interval of time. The progressive rate of killing may be 

 followed relatively accurately with any range in concentration (through 

 the dilution plate series) desired. Ideally, washed bacteria might be 

 employed, but ordinarily washing promotes clumping and apparently 

 increases the percentage of inactive or dead cells, so that caution is 

 necessary. A source of error with the suspension method is to be found 

 in the amount of scattered hght in the preparation. Scattering is not 

 excluded in the agar preparations but is no doubt at a minimum. In 

 this connection reference should be made to the work of Vies (152) who 

 has considered scattering in detail in relation to the radiation of bacterial 

 suspensions. The purified suspension also offers less opportunity for the 

 influence of any indirect effects through changes in the medium induced 

 by long exposure or high intensity. It is the only technique available 

 for comparative work with viruses and enzymes. 



Fluorescence in Relation to Bacteria. — Studies on the fluorescence of 

 bacteria are relatively recent. In relation to the general problem of 

 fluorescence much information has been brought together by Radley 

 and Grant (124). Burge and Neill (22) reported fluorescing bacteria 

 much less sensitive to light than those which are nonfluorescing. The 

 investigation made by Gassul and Zolkovic (57), using X3650 A, led to 

 the claim that types of bacteria might be distinguished by characteristic 

 fluorescent properties. This work was not supported by the observations 

 of Danielson (30), who found this property insufficiently definite for 

 application in species diagnosis. The more extensive observations of 

 Lasseur, Dupais, and Lecaille (94) included 33 species of bacteria and 



