V49-U MANUAL OF METHODS FOR PURE CULTURE STUDY 



some time ago because of the toxic properties of these salts; and 

 Hunter and CreceHus (1938) show the superiority of bismuth media 

 over iron media. ZoBell and Feltham (1934), moreover, have 

 shown distinct advantages from the use of lead acetate test strips, 

 without any of these metallic salts in the media. The advantage of 

 the test strip technic is that it is more sensitive and does not intro- 

 duce the possibility of inhibiting the bacterial growth if the con- 

 centration of metallic salt in the medium is too great. It is important, 

 as emphasized by Hunter and Crecelius, that the indicator and 

 method employed be stated when results are given. Untermohlen 

 and Georgi (1940) suggest use of nickel or cobalt salts, but specially 

 emphasize the variations in results with different media and indica- 

 tors. 



When using the test strip technic the bacteria may be grown in ordinary broth, 

 peptone sokition alone, or a peptone agar suitable to the organism in question. One 

 must be certain that the peptone contains available sulphur compounds. This can be 

 determined by running a check tube inoculated with a slow hydrogen sulfide producer. 

 For this procedure the test strip should be prepared by cutting white filter paper 

 into strips approximately 5 x 50 mm., soaking them in a saturated solution of lead 

 acetate, sterilizing them in plugged test tubes and drying in an oven at 120°C. One 

 of these strips should be placed in the mouth of the culture tube before incubation in 

 such a position that one-quarter to one-half of the strip projects below the cotton plug. 

 These tubes should be incubated at about the optimum temperature of the organism 

 under investigation and examined daily to notice whether or not blackening of the test 

 strip has occurred. 



Because of the inconvenience of the test strip technic, media in 

 which iron salts are incorporated are now generally preferred. A 

 dehydrated medium of such composition is available and has been 

 found quite satisfactory. 



Quantitative methods for determining hydrogen sulfide produc- 

 tion are given in Leaflet VI. 



Liquefaction of Gelatin 



The conventional method of determining liquefaction, which has 

 been given with but slight modification in all the reports on methods 

 is as follows: 



Make a gelatin stab (plain 12% gelatin) and incubate 6 weeks at 

 20°C., provided the organism under investigation will grow at that 

 temperature. Care must be taken to observe whether the organisms 

 produce rapid and progressive liquefaction or merely slow liquefaction 

 not extending far from the point of inoculation. In the latter case 

 the liquefaction may be due merely to endo-enzymes that are re- 

 leased from the cell after death and may not be what is generally 

 called "true liquefaction" (that is, the process resulting from the 



