EXPERIMENT STATION BULLETINS. 499 



in the niiniber of organisms introduced tak&s jilace, seemingly irrespec- 

 tive of tlie greater or lesser initial number. 



Experiment IV. 



Further experiments in the filtration of the lactose broth cultures of 

 different lactic organisms were carried on for the comparison of the acid 

 produced by each with regard to their relative germicidal powers. Ow- 

 ing to the length of time which elapsed between the inoculation of the 

 filtrate with B. typhosus and the second and third platings, the time in 

 which the destruction of the typhoid bacteria took place was not suffi- 

 ciently defined. This was remedied by plating at 1, 24, 31, 48, etc., hours 

 after inoculation. 



Organisms. — The lactic organism, Bact. lactis acidi (from starter) is 

 now a weak lactic, while Bact. lactis acidi (from sour milk) represents 

 the tyx>ical lactic; Bact. acidi lactici. No. 2 produces acid and gas in 

 broth also in milk; Bact. lactis acidi, 53 B2 is a lactic acid-producing 

 organism isolated from a sample of milk wliidi contained nearly 2 per 

 cent lactic acid; it also possesses the power at times of producing slimy 

 milk ; Bact. hiihjaricum was isolated from a culture of Yoghourt imported 

 from Holland. 



These microorganisms were taken as a fair representation of the most 

 common diverse types of lactic organisms. 



The outline for this experiment differs somewhat from that in the 

 previous filtration experiment. 



Medium. — In the first two or three tests II/2 P^r cent lactose broth 

 (+38°) was used; broth (+15°) containing 5 per cent lactose was 

 used for the concluding tests as being nearer the lactose content of milk. 



Method. — About 200 cc, lactose broth (+18°) was inoculated with the 

 lactic culture and incubated at room temperature (21^-25°C.). The 

 acidity was tested from time to time, and when the culture had reached 

 40° acid or thereabouts, it was immediately filtered. The filtrate was 

 separated into two parts by means of a sterile pii>ette, and transferred 

 into sterile flasks, every precaution being taken to keep it from being 

 contaminated. 



Both flasks were incubated at 37°C. for 48 hours to give any organisms 

 present a chance to assert themselves.* 



If no growth occurred in either flask, one was inoculated with a known 

 number of typhoid germs, the second flask being sealed and kept for 

 control. 



The time for plating was kept as nearly constant as possible, the time 

 being, 7, 24, 31, 48 hours, etc., after inoculation of the filtrate. Plating 

 was continued as long as the filtrate remained clear or until the typhoid 

 bacilli were all destroyed. 



The plates were kept at 37°C. for 24 hours, then at room temperature 

 for 48 hours before the final count was taken. 



'Check flasks (as in note, p. 489) were not necessary as the lactic organisms were not 

 allowed to produce enough of their own products (i. e., their maximum acid) to inhibit any 

 organisms which might pass through the filter. 



