514 GEOEGE E. HOLM AND JAMES M. SHERMAN 



this effect is modified by the hydrogen ion concentration of the 

 medium, and that such effects are probably great enough to be 

 given consideration in bacterial culture. 



EXPERIMENTAL 



The organism used was Bad. coli, and the basic medium chosen 

 was a 1 per cent pepton solution to which was added the crystal- 

 line salts in amounts necessary to give the desired concentrations. 

 The media were autoclaved and filtered in case of the formation 

 of a slight precipitate. At this point the pH was adjusted if 

 necessary with HCl or NaOH, and the media tubed and steril- 

 ized. These tubes containing 10 cc. were in each case inoculated 

 with a loopful of a young culture and incubated at 37°C. 



There was some question as what should constitute a measure 

 of bacterial growth and what factor would be constant enough 

 for comparative purposes. The reduction of methylene blue 

 was first tried. In this case a layer of paraffin oil was used to 

 prevent oxidation by the air. Although it worked quite satis- 

 factorily, it was found that reoxidation occurred in the cases 

 where bacterial action was slow, and thus, instead of giving a 

 sharp end point, really increased the time for reduction. The 

 rapidly growing cultures gave a sharp end point. It was noticed, 

 however, that a slight turbidity was apparent in most cases 

 before reduction could be detected. It was decided, therefore, 

 to use the first sign of turbidity as an indication of the rapidity 

 of bacterial growth. 



A few trial experiments indicated that the two methods of 

 detecting growth checked very well, except for the fact that 

 turbidity was first detected and proved a sharper measure than 

 reduction. The first sign of turbidity when the tubes were 

 held against a strong artificial light was therefore used to measure 

 rapidity of growth. This method was further verified by grow- 

 ing the same organism in a medium of 1 per cent pepton con- 

 taining 1 per cent lactose and adjusted to a pH of 7.0. The 

 production of acidity paralleled the results obtained by reduction 

 and visible turbidity. 



