EXPERIME^NT STATION BULLETINS. 549 



tion around tlie cell of an acid coagulnm of albnmin. In the study of 

 tnbercnle bacteria in naturally infected milk van der Slnis (21) ob- 

 served that a temperature of SO'' C. was necessary to insure destruction 

 of this bacterium. He also observed that artificial cultures in media 

 containing milk acquired an ability to withstand a slightly higher 

 temperature than the Bact. tuhcrculofiis normally present in milk. 

 Schultz and Ritz (19) in stud^-ing the thermal resistance of young and 

 old cells of a strain of B. coli observed that cultures three to six hours 

 old were killed by an exposure for twenty -five minutes at 52° C. while 

 cultures eight to twenty-four hours old survived and believe that the 

 rapidly reproducing cells are less resistant than older individuals. At- 

 tention is called by Ayers and Johnson (1) to a lactic acid bacterium 

 isolated from milk, the thermal death point of which is 74.4° C. in 

 bouillon and 75.0° C. in milk when exposed in Sternberg bulbs for thirty 

 minutes. When lieated in milk for ten minutes the thermal death point 

 is 77.8° C. 



Nearly every investigator who has worked with pasteurized milk and 

 cream believes that members of the B. coli and Bact. lactis acidi groups 

 survive pasteurization and too, that members of these groups exist whose 

 thermal death point is higher than the temperature commonly emploj^ed 

 in commercial pasteurization. Many are of the opinion that the thermal 

 death point determined in bouillon or in milk is the same, especially if 

 the milk is sealed to prevent the formation of a surface pellicle or mem- 

 brane; while others have discovered that the killing temperature of a 

 number of microorganisms is higher in milk than in bouillon. 



EXPERIMENTAL WORK. 



METHODS. 



Thermal Death Point. The thermal death point of a bacterium is 

 that temperature which causes death of the vegetative cells during an 

 exposure of ten minutes in bouillon unless otherwise stated. (As a jnat- 

 ter of comparison the exposure was lengthened to twenty minutes, but 

 when this is done specific mention of the fact will be made.) 



Methods of Dctennination. One cubic centimeter of a twenty-four 

 liour bouillon culture was introduced into test tubes containing about 

 10 c.c. each of the sterile medium in which the thermal death point is to 

 be determined. Duplicate tubes were placed in a water bath at the de- 

 sired temperature, which was not allowed to vary over 0.5° C. for ten 

 minutes, then they were removed immediately and cooled below 25° C. 

 Special precaution was taken not to permit the bouillon culture to come 

 in contact with the walls of tlie tube above the medium and to have the 

 surface of the medium in the tube below tlie level of the water in the 

 bath. The tubes were incubated at a temperature optimum for the bac- 

 terium under consideration (25 to 30° C. for Bact. lactis acidi and 37° 

 C. for B. coli). The bacteria in the duplicate tubes, when neither shows 

 growth within five days, are regarded as having been killed and are re- 

 corded negative for that specific temperature. 



