206 LAURENCE F. FOSTER 



Experiment XIII. The inhibitory action of acids upon a culture 

 of Streptococcus hemolyticus 



One per cent glucose broth was inoculated as usual with an 

 eighteen-hour actively growing culture and permitted to incubate 

 for eighteen hours. At the end of this interval a portion was 

 filtered with sterile precautions through a Berkefeld candle and 

 another portion was centrifugalized. Ph determinations were 

 then made upon the supernatant and the filtrate. Portions of 

 beef infusion broth containing 0.5 per cent KH2PO4 (to aid in 

 maintaining the reaction) and 0.5 per cent glucose were next 

 adjusted to the Ph levels of the cultures, using the acids indicated 

 in table 12. The supernatant fluid, Berkefeld filtrate, and tubes 

 containing the broth adjusted with acids were inoculated with 

 equal amounts of an eighteen-hour culture in 1 per cent glucose 

 broth. Tests of viability were carried out by streaking one 

 loopful of material on the surface of blood-agar plates at hourly 

 intervals. As' will be seen by reference to table 12 some of the 

 tubes contained 5 per cent horse serum. 



In the cases of (2) and (4) the addition of 5 per cent horse 

 serum caused a change in Ph toward the alkaline side and con- 

 sequently the results in these tubes are not comparable with 

 the others. The rather close agreement in toxicity between 

 lactic (1) and acetic (2) acids at the same Ph is of interest. The 

 mixture of the two acids in molecular proportions killed in 

 twelve hours, but inasmuch as the Ph of this tube was 5.15 as 

 against 5.25 in (1) and (3) the result cannot be considered as 

 evidence of increased toxicity. By comparing (6) with (1), (3), 

 and (5) the protective action of horse serum is strikingly illus- 

 trated. Tube (6) contained viable cells after fifty-four hours 

 contact with an acidity of 5.20; in other words, the streptococci 

 were able to tolerate the same degree of acidity for a period 

 nearly four times longer when in contact with 5 per cent horse- 

 serum. Close agreement between the toxicities of the super- 

 natant and filtrate are apparent ((7) and (8)) though neither 

 proved as toxic as lactic or acetic acids of the same hydrogen-ion 

 concentration. 



