1 88 MEDICAL BACTERIOLOGY 



MEDIA FOR SUBCULTURES 



There is probably no one factor, with the possible exception of temperature, 

 that has more to do with irregularities in results than the media for subcultures. 

 Where the typhoid bacillus is used for the test organism, as in the Rideal- Walker 

 method, and the method proposed by us, it is of paramount importance that the 

 media have a reaction of just +1.5. A reaction greater than this exerts a 

 decided inhibiting action upon the growth of the transplanted organism. This 

 is an important point, for if the transplant is made from a test dilution which is 

 just under the killing strength of the disinfectant, the inhibiting action of the 

 media may be sufficient to prevent growth, thus giving a false result. In the 

 hands of different workers a difference in the reaction of the media may result 

 from the degree to which the color reaction in titration is carried. We always 

 carry it to the point where the pink color is distinctly perceptible, but even 

 then there seems to be at times a slight difference in various lots of our media. 



It is a noteworthy fact that the influence of the reaction of the subculture 

 media upon the growth of the exposed organism was decidedly more pronounced 

 after it had been exposed to phenol than to any of the other disinfectants tried. 



It was found, too, that a more vigorous growth and a growth from stronger 

 solutions were obtained when the exposed organisms were planted in meat 

 broth than when they were planted in extract broth. It is therefore evident 

 from the above that the reaction and character of the subculture media has an 

 important bearing upon the results obtained in determining the phenol coeffi- 

 cient of disinfectants. However, as extract broth is more uniform in composi- 

 tion, more easily prepared, and cheaper than meat- broth, we recommend that 

 extract broth be always used and when it is not, that the fact be so stated. 



The amount of media in the tubes for subculture should be sufficient to 

 prevent any antiseptic action, due to the transferred disinfectant. With some 

 substances, such as bichloride of mercury, this is often an important point. 



It may be stated here that in our work with some disinfectants, particularly 

 those containing coal-tar products, the disinfectant carried over in making the 

 inoculations of the subcultures caused a distinct cloudiness in the media; but 

 after 48 hours' incubation this always cleared up so that there was no difficulty 

 in making out the presence or absence of growth. 



MacConkey's bile-salt medium was given a limited trial with B. coli com- 

 munis. We found that after exposing the B. coli communis to the action of a 

 i per cent, solution of carbolic acid and planting in MacConkey's medium and 

 extract broth, respectively, every 2^ minutes for 15 minutes, and incubating 

 for 48 hours, there was a growth in all the tubes of the extract broth, but only in 

 a 2%-minute tube of MacConkey's medium. This condition or result was 

 more marked with carbolic acid than with any other disinfectant tried. 



When using the B. typhosus the possibility of contamination in the tubes of 

 broth that show a growth at the end of 48 hours can be determined by the use of 

 antityphoid serum. 



